Molecular Formula Calculator: 5 Free Modes, Every Step Shown

A molecular formula calculator answers the question that elemental analysis alone cannot: not just the ratio of atoms in a compound, but how many of each atom sit in one actual molecule. Analysis tells you glucose is CH₂O — one carbon, two hydrogens, one oxygen — and it tells you exactly the same thing about formaldehyde, acetic acid, lactic acid and ribose. What separates them is the molar mass. Divide 180.16 g/mol by the 30.03 g/mol of one CH₂O unit and you get 6; multiply every subscript by 6 and the molecule appears: C₆H₁₂O₆. That single division, n = M / EFM, is the heart of every molecular formula problem, and it is the step students most often forget to finish.

The free molecular formula calculator above covers the whole problem in five modes: molecular formula from an empirical formula and a molar mass; molecular formula straight from percent composition or element masses plus molar mass, so the empirical step is done for you; molecular formula from combustion analysis (CO₂, H₂O, optional N₂ and SO₂) plus molar mass; molar mass from gas data — the ideal gas law, vapour density or mass and moles — carried straight through to the formula; and a formula analyser that returns molar mass, percent composition, empirical formula, n and the degree of unsaturation for anything you type. Every mode prints the empirical formula mass, the quotient n before and after rounding, a table of the multiples it considered and a warning whenever n is not close to a whole number.

Below the tool is a complete guide: what a molecular formula is and how it differs from the empirical and structural formulas, the n = M / EFM rule and why it must give an integer, where molar masses come from in real problems, worked examples for glucose, vitamin C and caffeine, a reference table of common compounds, the combustion and gas-data routes, the degree of unsaturation as a check, ten practice problems with hidden answers and a frequently-asked-questions section. All numbers use IUPAC 2021 standard atomic weights, the same values the molecular formula calculator uses internally.

What Is a Molecular Formula?

The molecular formula of a compound states how many atoms of each element are present in one molecule. Water is H₂O: two hydrogen atoms and one oxygen atom per molecule. Glucose is C₆H₁₂O₆: six carbons, twelve hydrogens, six oxygens. Hydrogen peroxide is H₂O₂, benzene C₆H₆, caffeine C₈H₁₀N₄O₂. Because the formula counts atoms in a real molecule, its molar mass is the molar mass of the substance itself, and that is the property that makes it measurable.

The empirical formula, by contrast, gives only the simplest whole-number ratio of the atoms. It is what elemental analysis delivers, because analysis measures masses of elements, not molecules. H₂O₂ has the empirical formula HO; C₆H₆ is CH; C₆H₁₂O₆ is CH₂O; caffeine is C₄H₅N₂O. The molecular formula is always a whole-number multiple of the empirical formula — twice it for hydrogen peroxide and caffeine, six times for benzene and glucose, and exactly once for water, ammonia, ethanol and any compound whose subscripts share no common factor. Our companion empirical formula calculator covers that first half of the story in depth; this page is about the second half, and about doing both halves in one pass.

There is a third level, the structural formula, which shows how the atoms are connected. It matters because a molecular formula does not identify a single substance: C₂H₆O is both ethanol and dimethyl ether; C₄H₁₀ is butane and isobutane; C₆H₁₂O₆ is glucose, fructose, galactose and a dozen other sugars. Isomers share a molecular formula and differ in structure. Spectroscopy (NMR, IR, mass-spectral fragmentation) settles the structure; the molecular formula is the essential first step that tells the spectroscopist what to look for, and the degree of unsaturation derived from it narrows the options before any spectrum is recorded.

For ionic and network solids the term molecular formula does not strictly apply, and a molecular formula calculator has to be read with that in mind, because there are no molecules to count — sodium chloride is a lattice, not a collection of NaCl pairs. Their formulas are formula units and are always the empirical ratio; the mass of one such unit is the formula weight, handled by our formula weight calculator. Mode 5 of the molecular formula calculator will still parse Na₂SO₄ or CuSO₄·5H₂O and return molar mass and percent composition, but the multiplier n and the degree of unsaturation are only meaningful for molecular substances.

How to Use This Molecular Formula Calculator

Pick a mode with the pill tabs at the top of the molecular formula calculator: From Empirical Formula, From % Composition, From Combustion, From Gas Data or Analyse a Formula. Each mode has example chips that load a textbook problem, so you can see the expected input before entering your own.

1. Enter what the problem gives you. In Mode 1 type the empirical formula (CH₂O, NO₂, C₃H₄O₃; brackets and pasted Unicode subscripts are fine) and the molar mass. In Mode 2 type up to six elements with their percentages or masses, plus the molar mass. In Mode 3 type the sample mass and the masses of CO₂, H₂O and, if present, N₂ and SO₂. In Mode 4 choose how the molar mass is to be found and fill the matching boxes.
2. Press Calculate. The headline shows the molecular formula with real subscripts; beneath it the empirical formula mass, the quotient n = M / EFM to three decimals, the rounded n, the molecular mass and, for organic formulas, the degree of unsaturation.
3. Read the steps. The steps box shows the empirical formula mass being added up, the division, the rounding decision with its deviation, and the multiplication of each subscript. A table lists the multiples (EF)₁, (EF)₂, (EF)₃ … with their masses and the percentage gap from your molar mass, and marks the one chosen; the bar chart shows the same comparison visually.
4. Heed the warnings. If the formula you typed is not the simplest ratio, the tool says so and names the true empirical formula. If n is more than 0.15 from a whole number (adjustable), the result is flagged rather than silently rounded — that flag almost always means a wrong molar mass or a mis-copied percentage.
5. Copy, download, print or share. The toolbar copies the working as text, downloads a CSV or TXT, prints a clean page, or shares via WhatsApp, Facebook, X or the native share sheet. The last ten calculations stay in the history panel until you press Clear All.

Leave the molar mass blank in Mode 2 or Mode 3 and the molecular formula calculator stops at the empirical formula, lists the first six possible molecules (EF)_n with their masses, and tells you exactly what value of M would select each — useful when the molar mass is the next thing you are about to measure.

The Five Modes Explained

Mode 1 — Molecular formula from empirical formula and molar mass

The core molecular formula calculator problem. Type the empirical formula and the molar mass, and the tool computes the empirical formula mass from IUPAC atomic weights, forms n = M / EFM, rounds it and multiplies the subscripts. It checks that the typed formula really is the simplest ratio (C₂H₄O₂ triggers a note that CH₂O is the empirical formula) and reports the degree of unsaturation of the result as a plausibility check.

Mode 2 — Molecular formula straight from percent composition

The full textbook problem in one run, and the reason most people open a molecular formula calculator. Enter the elements and their mass percentages (or masses in grams — only ratios matter) and the molar mass. The tool converts each entry to moles, divides by the smallest, multiplies by the smallest integer that makes every ratio whole, then applies n = M / EFM. Oxygen can be taken by difference when only C, H and N were measured. Without a molar mass the run ends at the empirical formula with a list of candidate molecules.

Mode 3 — Molecular formula from combustion analysis

Enter the sample mass and the masses of CO₂, H₂O and optionally N₂ and SO₂ collected, in grams or milligrams. The molecular formula calculator converts each product to the element it carries, takes oxygen by difference if asked, finds the empirical formula, and — with the molar mass — the molecular formula. A mass-balance check refuses data in which the recovered carbon and hydrogen exceed the sample mass, the commonest sign of a typing error or a g/mg mix-up.

Mode 4 — Molar mass from gas data, then the molecular formula

Many problems give the empirical formula and then hide the molar mass inside gas data. Choose the ideal gas route (a known mass of vapour occupying a known volume at a known temperature and pressure: M = mRT/PV), the vapour density route (M = 2 × VD relative to hydrogen), or simply a mass and an amount in moles (M = m/n). The molecular formula calculator finds M, shows the arithmetic, then finishes with n and the formula. The same physics underlies our dedicated molar mass from gas density and vapor density tools.

Mode 5 — Analyse a molecular formula

The reverse check, which turns the molecular formula calculator into a verification tool. Type any formula and get its molar mass, the mass percent of each element, the empirical formula, the multiplier n between the two, the number of atoms per molecule and the degree of unsaturation (rings plus π bonds). Add a sample mass to get the grams of each element in it. This is the mode for confirming a result, preparing the theoretical percentages for an analysis report, or checking a formula copied from a paper.

The Rule: n = Molar Mass ÷ Empirical Formula Mass

Because a molecule is n empirical units stuck together, its mass is n times the mass of one unit — the single fact every molecular formula calculator is built on. Mass is additive, so:

The empirical formula mass (EFM) is simply the sum of the atomic masses in one empirical unit: CH₂O = 12.011 + 2(1.008) + 15.999 = 30.026 g/mol; CH = 13.019; NO₂ = 46.005; C₃H₄O₃ = 88.062; C₄H₅N₂O = 97.097. The molar mass M is the experimental value for the compound. Divide, and n must come out as a whole number — 6.00 for glucose, 6.00 for benzene, 2.00 for N₂O₄, 2.00 for vitamin C, 2.00 for caffeine — because you cannot have 2.4 empirical units in a molecule.

That integer requirement is more than a formality; it is the built-in error check of the whole method. If n comes out as 3.3 or 1.6, something upstream is wrong: the molar mass was misread, a percentage was mistyped, an atomic mass was taken for O₂ instead of O, or the “empirical” formula was not actually reduced. The molecular formula calculator therefore reports n to three decimals before rounding, states the deviation, and warns when it exceeds the tolerance (0.15 by default) instead of quietly rounding to the nearest integer and printing a wrong formula with a straight face.

Small deviations in n are normal and harmless, and a molecular formula calculator should say so rather than hide them. A molar mass from freezing-point depression is good to perhaps 2–5%, so for glucose n might come out as 5.8 or 6.2; the rounding to 6 is safe because the neighbouring candidates (n = 5, 150 g/mol; n = 7, 210 g/mol) are 17% away. Trouble starts only when the empirical formula mass is small relative to the molar mass: with CH₂ (14.03) and a molar mass measured as 84 ± 4 g/mol, n could be 5.7–6.3 and C₆H₁₂ is still safe; at 140 ± 7 g/mol, n ranges 9.5–10.5 and C₁₀H₂₀ is no longer certain. The multiples table the tool prints makes this spacing visible at a glance.

Where the Molar Mass Comes From

In an examination the molar mass is usually given. In the laboratory, and in the better exam questions, it has to be found, and knowing the routes tells you which mode of the molecular formula calculator to open.

• Mass spectrometry. The molecular ion peak gives the molecular mass directly, often to four decimal places in high-resolution instruments, which fixes the formula outright. Our exact mass calculator and monoisotopic mass calculator explain the difference between the average molar mass used here and the monoisotopic mass a spectrometer reports.
• Gas density and the ideal gas law. Vaporise a weighed sample and measure its volume, temperature and pressure: M = mRT/PV. This is the Dumas method, and Mode 4 performs it with R = 0.082057 L·atm/(mol·K).
• Vapour density. The density of a gas relative to hydrogen at the same conditions equals M/2, so M = 2 × VD. A vapour density of 46.0 for a gas with empirical formula NO₂ means M = 92 and the molecule is N₂O₄. Mode 4 again.
• Colligative properties. Dissolving a known mass in a solvent and measuring the freezing-point depression, boiling-point elevation or osmotic pressure gives the moles of solute and therefore M = m/n. Enter that mass and amount in Mode 4’s third method.
• Titration. For an acid or base, the volume of standard titrant gives the moles of the sample; M = m/n follows, or the equivalent weight if the number of acidic protons is unknown.

Each of these routes delivers M to the molecular formula calculator with a different precision — mass spectrometry to 0.01%, gas density to about 1%, cryoscopy to a few percent — and that precision decides how confidently n can be rounded. The molecular formula calculator’s deviation readout is where that judgement is made visible.

Worked Examples

Example 1 — Glucose from empirical formula and molar mass (Mode 1)

Example 2 — Vitamin C from percent composition (Mode 2)

Example 3 — Caffeine, four elements (Mode 2)

Caffeine shows why the molar mass is indispensable: C₄H₅N₂O looks like a perfectly reasonable formula, and nothing in the percentages says it is half a molecule. Only the 194 on the mass spectrum doubles it.

Reference Table: Empirical vs Molecular Formulas

The compounds below appear in nearly every problem set. Empirical formula masses, molar masses and percentages were computed with IUPAC 2021 atomic weights (C 12.011, H 1.008, N 14.007, O 15.999, S 32.06, P 30.974) — the same values the molecular formula calculator uses — and rounded as shown. Use them to test the molecular formula calculator, set your own problems or check an answer.

Notice how much the table repeats itself in the first two columns: five compounds are CH₂O, three are CH, three are CH₂. Analysis alone cannot tell glucose from formaldehyde or benzene from acetylene. The n column is where the molar mass earns its keep, and it is the number the molecular formula calculator is really computing. You can regenerate any row with Mode 5, and any percent-composition entry can be pushed through Mode 2 with the molar mass to recover the molecular formula in the sixth column.

From Percent Composition to Molecular Formula in One Run

The complete textbook problem has two halves, and Mode 2 of the molecular formula calculator does both. The first half converts percentages into the empirical formula: assume 100 g so percentages become grams; divide each mass by the atomic mass to get moles; divide every mole value by the smallest; and if a ratio ends in .5, .33, .67 or .25, multiply the whole set by 2, 3 or 4 until every ratio is a whole number. The second half is n = M / EFM and the multiplication of the subscripts.

Two refinements matter in practice, and both are options in the molecular formula calculator. First, if the percentages sum to noticeably less than 100 and oxygen was not reported, the shortfall is oxygen — CHN analysers do not measure it — and the tool’s “assign the remainder to oxygen” option handles this. Second, the rounding tolerance decides when a ratio such as 1.45 is accepted as 1.5 (×2) rather than rounded to 1; textbook data never need more than 0.05, but real analyses with ±0.3% error may need 0.15. The steps box names the multiplier used and the largest deviation so you can judge.

From Combustion Analysis to Molecular Formula

Organic compounds are analysed by burning a weighed sample in excess oxygen: all the carbon becomes CO₂, all the hydrogen becomes H₂O, nitrogen is collected as N₂ and sulfur as SO₂. The products are weighed, and the element masses follow from the fixed fraction of each product that is the element:

Burn 0.2500 g of an ester and collect 0.4995 g CO₂ and 0.2045 g H₂O. Carbon: 0.4995 × 0.27292 = 0.1363 g (0.01135 mol). Hydrogen: 0.2045 × 0.11191 = 0.02289 g (0.02270 mol). Oxygen by difference: 0.2500 − 0.1363 − 0.0229 = 0.0908 g (0.005675 mol). Divide by 0.005675: C 2.00, H 4.00, O 1.00 → C₂H₄O (44.05). With M = 88.11 g/mol, n = 2.00 and the molecular formula is C₄H₈O₂ — ethyl acetate, or one of its several isomers. Mode 3 of the molecular formula calculator prints each of those lines and carries straight on to the molecular formula when M is supplied.

The molar mass for a combustion problem usually comes from a separate measurement quoted in the question — a mass spectrum, a vapour density, or a freezing-point depression. When it is missing, the tool stops at the empirical formula and lists the candidates (C₂H₄O, C₄H₈O₂, C₆H₁₂O₃ …) with the molar mass each would require, so a single later measurement settles the choice.

From Gas Data and Vapour Density

Before mass spectrometers, the molar mass of any volatile compound was measured by weighing its vapour. Vaporise a known mass m in a bulb of volume V at temperature T and pressure P; the ideal gas law gives the amount, n = PV/RT, and therefore M = m/n = mRT/PV. Cannizzaro used exactly this reasoning in 1858 to settle the atomic weights, and it remains the fastest hand calculation of a molar mass from first principles.

Take the molecular formula calculator’s own example, a hydrocarbon of empirical formula CH₂: 1.000 g of its vapour occupies 0.436 L at 25 °C (298.15 K) and 1.00 atm. n = (1.00 × 0.436) / (0.082057 × 298.15) = 0.01782 mol, so M = 1.000 / 0.01782 = 56.1 g/mol. EFM of CH₂ is 14.027, n = 56.1 / 14.027 = 4.00, and the molecular formula is C₄H₈. Mode 4 of the molecular formula calculator runs this with the temperature converted from °C automatically. Vapour density is even quicker: it is the mass of a volume of gas divided by the mass of the same volume of hydrogen, and since hydrogen is 2.016 g/mol, M ≈ 2 × VD. A vapour density of 141.9 for phosphorus(V) oxide vapour gives M = 283.8 and, from P₂O₅ (141.94), n = 2 → P₄O₁₀.

Gas data carry a caveat: real vapours near their boiling points are not ideal, and the molar mass from PV = nRT can be low by 1–3%. That is why n from a gas measurement often reads 5.9 or 2.05 rather than a clean integer, and why the tool’s tolerance exists. For a rigorous treatment of the correction see our van der Waals equation calculator; for the gas-density calculation in isolation, the molar mass from gas density page.

Degree of Unsaturation: The Built-in Sanity Check

Once a molecular formula calculator has produced a formula, one more piece of arithmetic tells you whether it can be a real neutral molecule and hints at its structure. The degree of unsaturation (also called double-bond equivalents, DBE, or the index of hydrogen deficiency) counts rings plus π bonds:

A saturated acyclic hydrocarbon C_nH_2n+2 has DBE 0; each ring or double bond removes two hydrogens and adds 1; a triple bond adds 2; a benzene ring counts 4 (one ring plus three double bonds). Glucose C₆H₁₂O₆: 6 + 1 − 6 = 1. Benzene C₆H₆: 6 + 1 − 3 = 4. Caffeine C₈H₁₀N₄O₂: 8 + 1 + 2 − 5 = 6. Aspirin C₉H₈O₄: 9 + 1 − 4 = 6 (a benzene ring, two C=O). Nicotine C₁₀H₁₄N₂: 10 + 1 + 1 − 7 = 5 (a pyridine ring, 4, and a pyrrolidine ring, 1).

The check is powerful because DBE must be a non-negative whole number for any neutral closed-shell molecule. If the molecular formula calculator reports DBE = 2.5 or −1, the formula cannot be a stable molecule: an odd number of hydrogens with no nitrogen means a radical or an ion, and a negative value means too many hydrogens for the carbons — both signs of a wrong n or a mistyped element. The tool prints DBE for every carbon-containing result and flags any that fails the test, which catches the classic error of multiplying by the wrong n before the formula ever reaches a report.

One Molecular Formula, Many Molecules

A molecular formula is a count, not a structure, and the number of distinct molecules sharing a count grows explosively with size. C₄H₁₀ has two isomers (butane and isobutane), C₆H₁₄ five, C₁₀H₂₂ seventy-five, and C₂₀H₄₂ more than 366,000. Add heteroatoms and functional groups and C₆H₁₂O₆ covers glucose, fructose, galactose, mannose and their ring and chain forms — all identical to a molecular formula calculator, all sharply different to a living cell.

That is why the output of a molecular formula calculator is the beginning of an identification, not the end. It fixes the molar mass, the elemental composition and the degree of unsaturation; from there, infrared spectroscopy reveals the functional groups, NMR the carbon–hydrogen framework, and mass-spectral fragmentation the way the pieces are joined. In modern practice a high-resolution mass spectrum often gives the molecular formula directly, because an exact mass such as 180.0634 fits C₆H₁₂O₆ and no other reasonable combination of C, H, N and O — a calculation our average vs monoisotopic mass guide explains — but the classical route through the empirical formula and molar mass is still what every chemistry course teaches, examines and uses when no spectrometer is to hand.

Who Uses a Molecular Formula Calculator?

• High-school and first-year university students meet the empirical-to-molecular calculation in the stoichiometry unit of every syllabus — IGCSE, A-level, AP, IB, JEE, NEET, MDCAT — usually as a two-part question worth the most marks on the paper.
• Organic and medicinal chemists combine a CHN analysis with a mass spectrum for every new compound; the calculated percentages for the proposed molecular formula (Mode 5) must match the found values within 0.4% before a journal accepts the structure.
• Analytical laboratories characterising unknowns from combustion data and GC–MS molecular ions, where Mode 3 and Mode 1 reproduce the working an auditor wants to see.
• Teachers and tutors who generate problems in Mode 5 (formula → percentages and M), then check student answers in Mode 2 with the printed steps as a marking scheme.
• Anyone reading a label or a paper who wants the molar mass, composition or degree of unsaturation of a formula quickly, with the empirical formula and n thrown in.

Step-by-Step Walkthrough in the Tool

Suppose a liquid is found by combustion to contain 52.14% C and 13.13% H, with the remainder oxygen, and its vapour density is 23.0 — a typical exam problem for a molecular formula calculator. Here is the whole problem in the molecular formula calculator.

1. Open Mode 2. Type C and 52.14 on the first row, H and 13.13 on the second. Set “If the percentages sum to less than 100%” to assign the remainder to oxygen; the tool assigns 34.73% to O.
2. The molar mass from vapour density is 2 × 23.0 = 46.0 g/mol; type 46.0 in the molar mass box. (Or open Mode 4 later to have the tool do the doubling.)
3. Press Calculate. Moles: C 4.341, H 13.026, O 2.171 → divided by 2.171: 2.000, 6.000, 1.000 → empirical formula C₂H₆O (46.069). n = 46.0 / 46.069 = 0.999 → 1 → molecular formula C₂H₆O; DBE = 2 + 1 − 3 = 0, a saturated acyclic molecule.
4. Read the multiples table: n = 1 (46.07, −0.1%), n = 2 (92.14, +100%), n = 3 (138.21). The chosen row is unambiguous.
5. Now the chemistry: C₂H₆O with DBE 0 is ethanol or dimethyl ether. A boiling point of 78 °C or an O–H stretch in the IR settles it as ethanol.
6. Press Copy for the working or CSV for the per-element table; the result is also stored in the history strip.

Switch to Mode 5, type C2H6O and confirm the molar mass 46.069 g/mol and the same percentages. Forward and reverse agreeing is the cheapest confidence check there is.

A Short History: From Ratios to Molecules

For the first half of the nineteenth century chemists could measure ratios but could not agree on molecules — there was, in effect, an empirical formula calculator but no molecular formula calculator. Dalton’s atomic theory (1808) explained the fixed proportions Proust had measured, and Berzelius’s combustion analyses gave reliable empirical formulas by the 1820s, but without a way to count atoms per molecule, formulas were ambiguous by whole-number factors: water was written HO by some and H₂O by others, acetic acid appeared in the literature with at least nineteen different formulas, and atomic weights differed by factors of two from one textbook to the next.

The resolution came from gases. Avogadro’s hypothesis (1811) — equal volumes of gases at the same temperature and pressure contain equal numbers of molecules — implied that gas density is proportional to molecular mass, but it was ignored for decades. Jean-Baptiste Dumas built the vapour-density apparatus in 1826 and Liebig made combustion analysis routine in 1831, yet it took Stanislao Cannizzaro’s pamphlet at the Karlsruhe Congress of 1860 to show that combining the two measurements fixes molecular formulas unambiguously. Within a decade the modern formulas of water, benzene and acetic acid were settled, Mendeleev had the atomic weights he needed for the periodic table, and the calculation that any molecular formula calculator performs today — empirical ratio times the integer from the molar mass — had become the standard first step in characterising any new compound. The mass spectrometer, from Aston’s first instrument in 1919 to modern high-resolution machines, changed only how the molar mass is measured, not the logic.

Limitations and Sources of Error

• The molar mass carries the risk in any molecular formula calculator. A 5% error in M is harmless when EFM is large relative to M (n small), but for CH₂ with M near 140 it makes n = 10 ± 0.5 and the formula uncertain. Look at the spacing of the multiples table.
• Errors in the empirical formula propagate. A ratio wrongly rounded (1.33 → 1) gives an empirical formula mass a third too small and an n that is a non-integer by the same factor; the tool’s warning is the safety net, but it cannot repair mistyped percentages.
• Non-ideal gases. Vapour-density molar masses are typically 1–3% low near the boiling point; a very light empirical unit combined with a heavy molecule may need the tolerance raised.
• Formula, not structure. C₂H₆O is two compounds, C₆H₁₂O₆ dozens; the molecular formula calculator cannot distinguish isomers, only constrain them through the degree of unsaturation.
• Ionic and polymeric substances. Salts have no molecular formula, and polymers have a distribution of molar masses rather than one value (see our number average and polydispersity pages); for both, n is not meaningful.
• Atomic weights. Using C = 12, O = 16 changes EFM by up to 0.1% — invisible in exams, visible against real analyses. The tool uses IUPAC 2021 conventional values throughout.

Practice Problems (With Answers)

Work each problem by hand first, then verify with the free molecular formula calculator above. Answers are hidden until you click.

Tips for Accurate Results

• Always compute the empirical formula mass with at least four significant figures in the atomic masses (C 12.011, H 1.008, O 15.999, N 14.007); with C = 12 and H = 1, CH₂ becomes 14.00 and n drifts.
• Report n to two decimals before rounding, as the molecular formula calculator does. A value of 5.98 is a confident 6; a value of 5.6 is a question, not an answer.
• Make sure the formula you divide into M is really the empirical formula. If you start from C₂H₄O₂ instead of CH₂O, n halves and you may still get a correct molecule — but you will get the wrong one for glucose (n = 3 gives C₆H₁₂O₆ only by luck of the even multiple).
• After multiplying, check three things: n × EFM equals M; the percent composition is unchanged; and the degree of unsaturation is a whole number ≥ 0.
• In combustion problems convert CO₂ and H₂O to C and H first, take oxygen by difference from the sample mass, and keep grams and milligrams consistent.
• When the molar mass comes from gas data, convert °C to kelvin and use R in the units of your pressure and volume (0.082057 L·atm/mol·K, or 8.314 J/mol·K with Pa and m³).
• For vapour density, remember M = 2 × VD, not VD itself — the factor of two is the molar mass of H₂.
• Keep the molecular formula calculator’s tolerance at the default for textbook data; raise it only when you know the molar mass is approximate.

Common Mistakes

Every row corresponds to a safeguard in the molecular formula calculator: it insists on the reduced empirical formula, reports n before rounding, flags deviations, doubles vapour density, converts temperature, and computes the degree of unsaturation for each result.

How This Calculator Is Different

This molecular formula calculator belongs to a family of free tools on molecularweightcalculator.org. Its natural partner is the empirical formula calculator, which treats the first half of the problem in depth. For the masses that feed every calculation start with the molecular weight calculator and the atomic mass calculator; use the formula weight calculator for ionic solids and the molar mass of hydrates calculator for salt·xH₂O; turn to grams to moles, molarity, stoichiometry and equivalent weight for the rest of the mole concept; find molar masses experimentally with the freezing point depression, boiling point elevation, osmotic pressure, vapor density, gas density and van der Waals equation tools; read the mass-spectrometry side in the exact mass, monoisotopic mass, isotopic mass and average vs monoisotopic mass guides; and for mixtures and polymers see average molecular weight of a mixture, number average, weight average, polydispersity index and viscosity molecular weight.

Where the Numbers Come From

The atomic weights in this molecular formula calculator are the IUPAC 2021 standard atomic weights, using the conventional value for elements published as an interval: H 1.008, C 12.011, N 14.007, O 15.999, S 32.06, Cl 35.45, and five significant figures for the others. Combustion-product molar masses follow directly (CO₂ 44.009, H₂O 18.015, N₂ 28.014, SO₂ 64.064 g/mol). The gas constant is R = 0.082057 L·atm·mol⁻¹·K⁻¹, derived from the 2019 SI exact value 8.314462618 J·mol⁻¹·K⁻¹. Reference-table values were computed from these same constants, so the tool and the table agree to the last digit; textbooks that use C = 12.01 and O = 16.00 differ in the second decimal.

If you spot a discrepancy worth checking, our corrections policy explains how reports are handled, and the contact page is the fastest route to us. More about the site and its author is on the about page.

Trusted external references for further molecular formula calculator reading:

• IUPAC Gold Book — molecular formula (iupac.org): the authoritative definition
• IUPAC CIAAW — Standard Atomic Weights (ciaaw.org): the atomic weights used in every calculation on this page
• LibreTexts — Empirical and Molecular Formulas from Analyses (libretexts.org): textbook treatment with combustion examples
• Wikipedia — Chemical formula: molecular formula (wikipedia.org): overview, conventions and history

Exam Checklist

Frequently Asked Questions

10 Facts Worth Knowing

1. Before Cannizzaro’s 1858 pamphlet, acetic acid had been written with at least nineteen different formulas, because chemists could measure its composition but not its molecular mass.
2. Avogadro’s hypothesis of 1811 — the key to turning gas density into molecular mass — was ignored for almost fifty years, largely because Dalton refused to believe that molecules of an element could contain two atoms.
3. Benzene’s molecular formula C₆H₆ was established from vapour density in the 1830s, three decades before Kekulé proposed the ring; its empirical formula CH had been known even longer.
4. Five common compounds — formaldehyde, acetic acid, lactic acid, ribose and glucose — share the empirical formula CH₂O; only the molar mass (30, 60, 90, 150, 180 g/mol) separates them.
5. C₂₀H₄₂ has 366,319 structural isomers, all with the same molecular formula and molar mass of 282.55 g/mol.
6. High-resolution mass spectrometry can read a molecular formula directly: the exact mass 180.0634 fits C₆H₁₂O₆ and no other plausible combination of C, H, N and O within 5 ppm.
7. The “nitrogen rule” is a consequence of the degree-of-unsaturation formula: a neutral molecule with an odd nominal molecular mass must contain an odd number of nitrogen atoms.
8. Phosphorus pentoxide keeps its historical name although the molecule is P₄O₁₀; the name records the empirical formula P₂O₅ found in the 1800s before vapour-density measurements doubled it.
9. Sulfur vapour just above the boiling point is S₈, but at higher temperatures the rings break down and the average molecular formula drifts through S₆ and S₂ — a case where n depends on temperature.
10. Journals typically require calculated and found elemental percentages to agree within 0.4% before accepting a molecular formula — a standard set in Fritz Pregl’s micro-analysis era a century ago.

Final Thoughts

The molecular formula is where a ratio becomes a molecule. Everything before it — percentages, combustion masses, mole ratios — describes a compound only up to a whole-number factor; the single division n = M / EFM supplies that factor, and the requirement that it be an integer is the method’s own proofreader. Learn to compute the empirical formula mass carefully, to read n before rounding it, and to check the result with the degree of unsaturation, and every version of the problem — from a two-line exam question to a CHN report in a research paper — falls to the same short routine.

Bookmark this molecular formula calculator page for your next problem set or analysis, share it with a classmate before the stoichiometry exam, and explore the rest of our free chemistry tools — from the molecular weight calculator homepage to the empirical formula calculator that begins where this page ends. If a result surprises you, read the steps: the reason is almost always in n.