Oxidation Number Calculator: 5 Free Modes, Rules Shown

An oxidation number calculator assigns a signed number to every atom in a chemical formula — the charge that atom would carry if every bond were completely ionic. Type KMnO₄ and it tells you that manganese is +7; type Fe₃O₄ and it tells you that iron averages +8/3 and why; type Cr₂O₇²⁻ and it gives chromium +6 with the algebra written out. That number is the bookkeeping device behind every redox reaction, every Roman numeral in a compound’s name, every n-factor in a titration and every d-electron count in coordination chemistry.

The free oxidation number calculator above does all of that in five modes. It applies the standard rules in their proper order, solves the unknown element algebraically, recognises polyatomic ions so that mixed-valence salts such as NH₄NO₃ come out right, detects peroxides, superoxides and hydrides, analyses a whole redox equation, computes equivalent weights, writes Stock names and works out unpaired electrons and magnetic moments — with every step shown so you can reproduce it in an exam.

This guide explains the rules and the order in which to apply them, walks through the classic examples and the awkward ones (H₂O₂, OF₂, Fe₃O₄, Na₂S₂O₃, CrO₅), shows how oxidation numbers identify oxidizing and reducing agents, and connects them to equivalent weights, nomenclature and transition-metal chemistry. Ten practice problems and ten FAQs close it out.

What Is an Oxidation Number?

The oxidation number (or oxidation state) of an atom in a compound is the charge it would have if all its bonds to different elements were 100 % ionic — that is, if every shared electron pair were handed entirely to the more electronegative partner. It is a formalism, not a measured charge. In water the oxygen does not really carry two full negative charges, but assigning it −2 and each hydrogen +1 gives a consistent bookkeeping system that makes electron transfer visible.

IUPAC’s current definition (2016) says exactly this: the oxidation state of an atom is the charge of that atom after ionic approximation of its heteronuclear bonds. Bonds between atoms of the same element are split evenly, which is why the oxygen atoms in H₂O₂ are −1 rather than −2 and why every free element is 0.

Three consequences follow at once, and an oxidation number calculator relies on all of them:

• The sum rule. The oxidation numbers of all atoms in a species add up to its charge: zero for a neutral compound, −2 for sulfate, +1 for ammonium. This is the equation that lets you solve for the one unknown element.
• Redox is a change in oxidation number. If an element’s number goes up, it has been oxidized (lost electrons); if it goes down, it has been reduced (gained electrons). No change anywhere means the reaction is not redox.
• Averages are allowed. Because the number is assigned per formula, atoms of one element in different environments can produce a fractional average (Fe₃O₄: +8/3). The fraction is a signal, not an error.

The terms “oxidation number” and “oxidation state” are used interchangeably in general chemistry. Strictly, IUPAC writes oxidation states as signed Arabic numbers (+7) and oxidation numbers, as used in Stock nomenclature, as Roman numerals (manganese(VII)). Either way the value is the same, and this oxidation number calculator reports both.

How to Use the Oxidation Number Calculator

Pick a mode with the pill tabs at the top of the oxidation number calculator, fill the fields and press Calculate. Each mode has example chips that fill everything with a realistic case, and each has its own Reset button.

1. Type the formula the way you would write it. KMnO4, K2Cr2O7, Ca(OCl)2, K4[Fe(CN)6], CuSO4·5H2O and NH4NO3 are all understood. Subscripts as plain digits are fine; brackets, dots for hydrates and nested groups are parsed.
2. Add a charge at the end for ions. Write SO4^2-, Cr2O7^2-, Fe^3+, NH4+ or MnO4-. A single trailing sign is a charge of one; use the caret for larger magnitudes. Plain Fe3+ is also recognised as iron(III).
3. Optionally name the element you care about. Enter “Mn” and the headline answer becomes “oxidation number of Mn in KMnO₄ = +7”; the table still lists every element.
4. Fix a known state when the rules cannot decide. For compounds with two variable elements — CuFeS₂, for example — enter Fe=+3 in the third box and the oxidation number calculator solves the rest.
5. Read the answer, then the steps. The blue panel gives the values and confirms that they sum to the charge; the steps show which rule fixed each element and the algebra for the unknown; the table lists atoms, values and reasoning; the bar chart shows positive values in blue and negative in red.
6. Export or share. Copy the text, download CSV or TXT, print a report or share by WhatsApp, Facebook, X or the native share sheet. The last ten calculations stay in the history list until you clear them.

The Five Modes Explained

Modes 2 to 5 are all built on Mode 1. The redox analyzer runs the oxidation number calculator on every species in the equation and compares the results; the n-factor mode runs it on the reactant and the product; the naming mode uses the result to choose a Roman numeral; the d-electron mode starts from the oxidation state you supply.

The Rules, in Order of Priority

The rules below are the ones every oxidation number calculator and every textbook uses. The order matters: when two rules conflict, the one higher in the list wins. That single idea resolves almost every “exception”.

Two general principles sit behind the list. First, the more electronegative atom gets the negative number; when the rules run out, the oxidation number calculator falls back on electronegativity to decide who is negative. Second, an element’s oxidation number is bounded by its group: the maximum is the group number for main-group elements (S +6, Cl +7, Mn +7) and the minimum is group number minus eight for non-metals (N −3, O −2, Cl −1). A result outside those bounds means a rule has been misapplied — almost always a peroxide oxygen counted as −2.

Worked Example: KMnO₄ Step by Step

Potassium permanganate is the example every oxidation number calculator is tested on, because it uses three rules and a line of algebra.

The calculator also shows the split into K⁺ and MnO₄⁻, then solves within the permanganate ion: x + 4(−2) = −1 ⇒ x = +7. Both routes give the same answer; the ion route is the one that scales to salts such as (NH₄)₂Cr₂O₇ where the whole-formula method gets clumsy.

More Worked Examples

Each of the following is a chip in the oxidation number calculator; here is the reasoning behind the answer.

Peroxides, Superoxides, Hydrides and OF₂

The “exceptions” students memorise are not exceptions at all; they are what happens when a higher-priority rule overrides oxygen’s or hydrogen’s usual value. An oxidation number calculator handles them by applying the rules in order and by refusing to let a fixed-valence element take an impossible value.

Hydrogen peroxide, H₂O₂. If oxygen were −2 the sum would be 2(+1) + 2(−2) = −2, not 0. Hydrogen is bonded to oxygen, so it must be +1 (rule 5 outranks rule 6); therefore each oxygen is −1. Structurally, the O–O bond is split evenly and each oxygen gets only one electron from its hydrogen. The same applies to Na₂O₂ and BaO₂: sodium and barium have fixed values, so oxygen must be −1.

Potassium superoxide, KO₂. Potassium is +1 (rule 4), so the two oxygens share −1: −½ each. The O₂⁻ ion has an odd electron and the fraction is real. Ozonides such as KO₃ give −1/3.

Oxygen difluoride, OF₂. Fluorine is always −1 (rule 3), so oxygen must be +2: the only common compound in which oxygen is positive. In O₂F₂ it is +1.

Metal hydrides. In NaH, CaH₂, LiAlH₄ and NaBH₄, hydrogen is bonded to something less electronegative than itself, so it takes −1. Sodium is +1, so in NaH the hydrogen must be −1 for the sum to be zero; in LiAlH₄, Li +1 and Al +3 force the four hydrogens to −1 each. That −1 hydrogen is exactly the hydride that these reagents deliver in organic reductions.

Chromium peroxide, CrO₅. With all five oxygens at −2, chromium would be +10 — impossible for a group 6 element. The molecule contains two peroxo groups: four oxygens at −1 and one at −2, giving chromium its normal +6. The oxidation number calculator spots the impossible value, tests the peroxide hypothesis and reports the correct assignment with a note. Peroxymonosulfuric acid H₂SO₅ (S +6, one peroxo pair) and peroxydisulfate S₂O₈²⁻ (S +6) work the same way.

Pyrite, FeS₂. Iron would need to be +4 if sulfur were −2. The mineral contains the disulfide ion S₂²⁻, so sulfur is −1 and iron is +2 — the sulfur analogue of a peroxide.

Fractional Oxidation Numbers

A fractional result from an oxidation number calculator means one thing: atoms of the same element in the formula are not all equivalent, and the number you see is their average. The classic cases are worth knowing by heart.

Averages are perfectly usable for redox bookkeeping. In the iodometric titration Na₂S₂O₃ → Na₂S₄O₆, sulfur goes from +2 to +2.5 on average; two sulfur atoms per thiosulfate × 0.5 = one electron per formula unit, which is exactly the n-factor of 1 that the titration uses. The oxidation number calculator reports the average, adds a note about the real distribution when it knows it, and Mode 3 turns the average into the correct n-factor.

Polyatomic Ions and Mixed-Valence Salts

Whole-formula algebra fails on a compound like NH₄NO₃: with H +1 and O −2 you get 2x + 4 − 6 = 0 ⇒ N = +1, which is true as an average and useless as chemistry. The nitrogen in ammonium is −3 and the nitrogen in nitrate is +5; the “+1” belongs to neither.

The fix is to recognise the ions first. An oxidation number calculator that knows sulfate, nitrate, ammonium, permanganate, dichromate, cyanide and sixty other groups splits NH₄NO₃ into NH₄⁺ and NO₃⁻, solves each ion on its own and reports both nitrogens. The same approach handles:

• (NH₄)₂Cr₂O₇ — ammonium N −3, dichromate Cr +6; on heating, the −3 nitrogen reduces the +6 chromium (the “volcano” demonstration).
• Ca(OCl)₂ — hypochlorite Cl +1; and bleaching powder CaOCl₂, where one chlorine is −1 and one is +1 (average 0).
• NH₄NO₂ — N −3 and N +3, which comproportionate to N₂ on heating.
• [Cu(NH₃)₄]SO₄ — neutral ammine ligands (N −3), sulfate −2, therefore Cu +2.
• KAl(SO₄)₂·12H₂O (alum) — K +1, Al +3, two sulfates, twelve neutral waters.

When you use the oxidation number calculator on a salt, look at the “Part” column of the table: it names the ion each atom belongs to and gives the charge that was used for it. If you disagree with the split, fix an element’s value in the “known oxidation states” box and the calculator will honour it.

Reference Table: 50 Common Species

Every value below was produced with the oxidation number calculator and checked against the rules. The “key element” is the one the question is usually about; the other elements take their standard values (H +1, O −2, alkali metals +1, halides −1) unless the note says otherwise.

Oxidation Numbers and Redox Reactions

Oxidation numbers exist to make electron transfer countable. Assign them to every element on both sides of an equation and four things fall out immediately — which is exactly what Mode 2 of the oxidation number calculator reports.

• Oxidation is an increase in oxidation number (loss of electrons). Reduction is a decrease (gain of electrons). “OIL RIG” — Oxidation Is Loss, Reduction Is Gain.
• The reducing agent is the species that contains the element being oxidized; it gives electrons away. The oxidizing agent contains the element being reduced; it takes them. Students invert this constantly — the oxidizing agent is itself reduced.
• Electrons lost must equal electrons gained. Multiply the change per atom by the number of atoms (including coefficients) on each side; the totals match in a balanced equation, and that equality is what the ion-electron method exploits.
• No change anywhere means no redox. Acid–base neutralisation, precipitation and double displacement leave every oxidation number untouched.

Example: Zn + CuSO₄ → ZnSO₄ + Cu. Zinc goes 0 → +2 (oxidized; Zn is the reducing agent). Copper goes +2 → 0 (reduced; CuSO₄ is the oxidizing agent). Sulfur (+6) and oxygen (−2) are spectators. Two electrons move per zinc atom.

Example: MnO₄⁻ + 5Fe²⁺ + 8H⁺ → Mn²⁺ + 5Fe³⁺ + 4H₂O. Manganese +7 → +2 gains five electrons; each iron +2 → +3 loses one, and there are five of them. Five lost, five gained; the oxidation number calculator confirms the equation is balanced in atoms and charge and reports 5 electrons transferred as written. That 1 : 5 ratio is the whole basis of permanganate titrations of iron.

Disproportionation is the special case where one element in one species is simultaneously oxidized and reduced. Cl₂ + 2NaOH → NaCl + NaClO + H₂O: chlorine goes from 0 to both −1 and +1. 2H₂O₂ → 2H₂O + O₂: oxygen at −1 goes to −2 and to 0. The analyzer flags both. The reverse, comproportionation, brings two states together: 2Fe³⁺ + Fe → 3Fe²⁺.

Because the oxidation number calculator uses the coefficients you type, an unbalanced equation gives mismatched electron totals and a “NOT balanced” flag — useful as a check on your own balancing before you commit it to paper. To balance from scratch, use the stoichiometry calculator; this page tells you which species change and by how much.

n-Factor and Equivalent Weight

In a redox titration what matters is not moles of reagent but moles of electrons. The n-factor of an oxidant or reductant is the number of electrons one formula unit accepts or donates in the reaction at hand; the equivalent weight is the molar mass divided by the n-factor; and normality is molarity times the n-factor. All three depend on the change in oxidation number, which is why Mode 3 of the oxidation number calculator starts by running Mode 1 on the reactant and the product.

The same reagent can have different n-factors in different media, and the classic illustration is permanganate:

A 0.02 M KMnO₄ solution is therefore 0.1 N in acid but only 0.06 N in neutral solution — the normality label is meaningless without the reaction. For the acid–base and precipitation analogues of equivalent weight, see the equivalent weight calculator; for converting between molarity and normality in solution preparation, the molarity calculator.

Stock Names and Roman Numerals

The Roman numeral in iron(III) chloride is an oxidation number. Alfred Stock proposed the system in 1919 to replace the old –ous/–ic suffixes (ferrous/ferric, cuprous/cupric), which tell you that there are two states but not what they are. The rules, which Mode 4 of the oxidation number calculator follows, are short:

• Ionic compounds: cation name, then anion name. Add a Roman numeral in parentheses only when the metal has more than one common oxidation state (Fe, Cu, Co, Ni, Mn, Cr, Sn, Pb, Hg, Au, Pt, Ti, V …). Sodium, potassium, calcium, magnesium, aluminium, zinc and silver never get one.
• The numeral equals the oxidation number, not the number of anions: FeCl₃ is iron(III) chloride; Fe₂O₃ is also iron(III) oxide; Fe₂(SO₄)₃ is iron(III) sulfate.
• Mercury(I) is the dimer Hg₂²⁺, so Hg₂Cl₂ is mercury(I) chloride, not mercury(II).
• Mixed valence: Fe₃O₄ is iron(II,III) oxide, or triiron tetroxide.
• Hydrates: append a Greek prefix and “hydrate”: CuSO₄·5H₂O is copper(II) sulfate pentahydrate.
• Acids: –ate ions give –ic acids (sulfate → sulfuric, +6), –ite ions give –ous acids (sulfite → sulfurous, +4); hypo–/per– extend the series for the halogens (hypochlorous +1, chlorous +3, chloric +5, perchloric +7).
• Binary molecular compounds of two non-metals use Greek prefixes instead of numerals: N₂O₄ is dinitrogen tetroxide, P₄O₁₀ tetraphosphorus decoxide, SO₃ sulfur trioxide.

Naming is a good test of whether you have the oxidation number right. If your Roman numeral does not make the compound neutral — “copper(I) sulfate” for CuSO₄ would need two coppers — the assignment is wrong. The formula weight calculator goes the other way, from name and formula to mass.

d-Electron Count and Magnetic Moment

For a transition metal the oxidation number does one more job: it fixes how many d electrons the ion has, and therefore its colour, magnetism and preferred geometry. The rule is simple — d-electron count = group number − oxidation state — because in forming an ion the metal loses its s electrons first and then d electrons.

Mode 5 of the oxidation number calculator writes the configuration of the atom and the ion, fills the five d orbitals for the environment you choose — free ion, octahedral high- or low-spin, tetrahedral or square planar — counts the unpaired electrons and computes the spin-only moment μ = √[n(n+2)] Bohr magnetons. It is the fastest way to see why [Fe(CN)₆]⁴⁻ (Fe²⁺, d⁶, strong-field cyanide, low-spin) is diamagnetic while [Fe(H₂O)₆]²⁺ (same d⁶, weak-field water, high-spin) has four unpaired electrons.

Oxidation Numbers of Carbon in Organic Compounds

Organic chemists rarely quote oxidation numbers, but they use them constantly under other names: “oxidation level”, “reduction”, “oxidative addition”. Assigning a number to each carbon individually — H +1, O −2, N −3, halogens −1, and 0 for a bond to another carbon — makes the familiar functional-group ladder quantitative:

Each rung is a two-electron oxidation. Ethanol to acetaldehyde to acetic acid: the functional carbon goes −1 → +1 → +3, and the oxidation number calculator gives the whole-molecule averages −2, −1 and 0 for C₂H₆O, C₂H₄O and C₂H₄O₂. Glucose C₆H₁₂O₆ has carbon at 0 on average; respiration takes all six carbons to +4 in CO₂, releasing 24 electrons per molecule to oxygen — that is the redox accounting behind the 2870 kJ/mol. Photosynthesis runs it backwards. In a titration context the classic organic reductant is oxalic acid, carbon +3 → +4, n-factor 2.

Averages hide detail (acetic acid’s “0” is really −3 and +3) and the calculator’s note says so when it recognises the molecule. For the hydrogen-deficiency side of organic formulas — rings and double bonds rather than electron count — use the degree of unsaturation calculator.

Who Uses an Oxidation Number Calculator

• Secondary and first-year students checking homework on oxidation states, identifying oxidizing and reducing agents and naming compounds — the three most common exam questions on this topic.
• Teachers generating worked solutions with every rule labelled, and awkward examples (CrO₅, Fe₃O₄, Na₂S₄O₆) that expose whether a class has understood the rule hierarchy or merely memorised it.
• Analytical chemists converting between molarity and normality for permanganate, dichromate, iodine and thiosulfate titrations, where the n-factor depends on the medium.
• Inorganic and coordination chemists reading off d-electron counts, predicting spin states and magnetic moments, and checking that a proposed complex is charge-consistent.
• Geochemists and environmental scientists tracking Fe(II)/Fe(III), Mn(IV)/Mn(II), Cr(VI)/Cr(III) and As(V)/As(III) speciation, where the oxidation number decides toxicity and mobility.
• Battery and corrosion engineers for whom every cell reaction is an oxidation-number ledger: LiCoO₂ cycles cobalt between +3 and +4; rust is iron 0 → +3.

Full Walkthrough: A Titration Problem

Problem. 25.00 mL of an iron(II) solution needs 18.40 mL of 0.0200 M KMnO₄ in dilute sulfuric acid to reach a permanent pink end-point. Find the concentration of Fe²⁺ and the mass of FeSO₄·7H₂O in one litre of the solution.

Every number in that chain rests on the two oxidation numbers found in Step 1. Get manganese wrong (+5 instead of +7 is a common slip) and the n-factor, the mole ratio and the final answer all shift by the same factor. Running the equation MnO₄⁻ + 5Fe²⁺ + 8H⁺ → Mn²⁺ + 5Fe³⁺ + 4H₂O through Mode 2 of the oxidation number calculator confirms the 1 : 5 ratio before you touch a burette. The molar mass of hydrates calculator supplies the 278.01, and the grams to moles calculator handles the last conversion.

A Short History of the Concept

“Oxidation” originally meant combination with oxygen — Lavoisier’s 1770s chemistry of combustion and calcination — and “reduction” meant winning a metal back from its ore. The idea that both are electron transfers, and that they can happen without any oxygen at all, came only after the electron was discovered in 1897 and Lewis’s shared-pair bond appeared in 1916.

Alfred Stock introduced Roman-numeral nomenclature in 1919 so that names would state the metal’s valence unambiguously. Wendell Latimer’s Oxidation States of the Elements and Their Potentials in Aqueous Solutions (1938) organised the whole of inorganic redox chemistry around the oxidation number, and his “Latimer diagrams” still line the walls of inorganic lecture rooms. The set of rules that every oxidation number calculator now follows was codified in general-chemistry textbooks in the 1940s and 1950s.

The formal definition kept being argued over — is it a bond-based or an electronegativity-based quantity? — until IUPAC’s 2016 technical report (Karen, McArdle and Takáts) settled on the ionic approximation of heteronuclear bonds, with the electronegativity scale of Allen to decide which atom wins each bond. That definition is what this page and this calculator use; for the everyday compounds in the reference table it agrees with the classical rules in every case.

Limits: What the Number Does Not Mean

• It is not a real charge. Manganese in MnO₄⁻ does not carry seven positive charges; computed partial charges are around +1 to +2. The oxidation number is a bookkeeping limit, and its power lies in being consistent, not in being physical.
• Averages hide structure. Fe₃O₄‘s +8/3 and thiosulfate’s +2 describe no actual atom. The oxidation number calculator adds a note for the well-known cases, but for an unfamiliar formula a fraction should send you to the structure.
• Formal conventions differ at the margins. In metal carbonyls (Ni(CO)₄, Ni 0), nitrosyls and organometallics the assignment depends on how the ligand is counted; textbooks disagree about the central nitrogen of azide. The calculator follows the most common convention and says so.
• It cannot tell you whether a reaction happens. Oxidation numbers show that a reaction would be redox and by how many electrons; whether it is spontaneous is a question for standard potentials and Gibbs energy.
• Electronegativity ties. C–S, P–H and a few other bonds join atoms of nearly equal electronegativity; the assignment there is by convention (P +3 in PH₃ by electronegativity, though it behaves like a hydride donor). When the rules alone cannot separate two variable elements, the calculator asks you to fix one rather than guess.

Practice Problems With Answers

Try each one by hand, then check with the oxidation number calculator. Click a problem to reveal the answer.

Tips for Accurate Results

• Work down the rule list in order. Fixed elements first, oxygen and hydrogen next, the unknown last, exactly as the oxidation number calculator does. Every “exception” is just a higher rule beating a lower one.
• Split salts into their ions before you start. NH₄NO₃, (NH₄)₂Cr₂O₇ and K₄[Fe(CN)₆] give nonsense as single formulas and clean answers as ions (the oxidation number calculator splits them for you).
• Sanity-check against the group. Main-group maximum = group number; non-metal minimum = group number − 8; Mn stops at +7, Cr at +6, Fe at +6 (ferrate) and normally +3. A value outside the range means a peroxide, a disulfide or a typo.
• Treat fractions as a prompt, not a result. Ask which atoms are different (Fe₃O₄, S₄O₆²⁻) before writing the average in an answer — the oxidation number calculator shows the distinct atoms in its steps.
• For redox, tabulate every element on both sides. It takes thirty seconds by hand and one click in the oxidation number calculator’s redox mode, and it catches the reactions where two elements change (KClO₃ decomposition) or where none does.
• Name the agent by the species, not the atom. “MnO₄⁻ is the oxidizing agent”, not “Mn is”.
• Always state the product when quoting an n-factor. KMnO₄ is 5, 3 or 1 depending on the medium. Mode 3 of the oxidation number calculator asks for both reactant and product for exactly this reason.
• Keep the working. Copy or print the steps from the oxidation number calculator; examiners award marks for the equation with x, not for the number alone.

Common Mistakes

These are the errors the oxidation number calculator sees most often in the formulas people type, together with what goes wrong and how to fix it.

How This Calculator Is Different

Most online oxidation number calculator pages return a bare list of numbers. The comparison below shows what this one adds.

This oxidation number calculator is one of a family of free tools on molecularweightcalculator.org. Start with the molecular weight calculator for any formula’s molar mass, the atomic mass calculator for individual elements and the formula weight calculator for salts. Redox titration work continues in the equivalent weight calculator, the molarity calculator, the stoichiometry calculator, the grams to moles calculator and the molar mass of hydrates calculator. Formula determination lives in the empirical formula calculator, the molecular formula calculator, the percent composition calculator, the combustion analysis calculator and the degree of unsaturation calculator. Molar mass from physical measurements: the vapor density calculator, the molar mass from gas density calculator, the freezing point depression calculator, the boiling point elevation calculator, the osmotic pressure calculator and the van der Waals equation calculator. Mass spectrometry: the exact mass calculator, the monoisotopic mass calculator, the isotopic mass calculator and the guide to average vs monoisotopic mass. Mixtures and polymers: the average molecular weight of a mixture, number average, weight average, polydispersity index and molecular weight from viscosity.

Where the Numbers Come From

The oxidation number calculator needs no data beyond the rule hierarchy and, for tie-breaks, the Pauling electronegativity scale (F 3.98, O 3.44, Cl 3.16, N 3.04, Br 2.96, I 2.66, S 2.58, C 2.55, H 2.20 …). The definition follows the IUPAC 2016 recommendation. Polyatomic-ion charges are the standard ones (sulfate 2−, nitrate 1−, phosphate 3−, ammonium 1+, and so on). The molar masses and equivalent weights quoted on this page use the IUPAC 2021 standard atomic weights with the conventional value for interval elements: H 1.008, C 12.011, N 14.007, O 15.999, S 32.06, Cl 35.45, K 39.098, Mn 54.938, Fe 55.845, Cr 51.996, Cu 63.546, I 126.90. Magnetic moments are spin-only values √[n(n+2)] in Bohr magnetons; measured moments for many ions are slightly higher because of orbital contribution.

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 oxidation number calculator reading:

• IUPAC Gold Book — oxidation state (iupac.org): the formal definition and the ionic-approximation rule
• LibreTexts — Oxidation States (Oxidation Numbers) (libretexts.org): rules with worked examples
• IUPAC CIAAW — Standard Atomic Weights (ciaaw.org): the atomic weights behind every molar mass and equivalent weight here
• Wikipedia — Oxidation state (wikipedia.org): overview, history and the list of oxidation states of the elements

Exam Checklist

Run through this list before every answer; each line matches a step in the oxidation number calculator’s working.

Frequently Asked Questions

10 Facts Worth Knowing

A few things the oxidation number calculator’s reference data turns up that rarely make it into the textbook chapter.

1. Nitrogen shows every oxidation number from −3 (NH₃) to +5 (HNO₃) in common compounds — nine states, more than any other light element.
2. The highest oxidation number ever confirmed is +9, in the iridium tetroxide cation IrO₄⁺ (2014); +8 occurs in OsO₄, RuO₄ and XeO₄.
3. The lowest is −4, for carbon in methane and silicon in silane; no element goes below its group number minus eight.
4. Oxygen is positive in only a handful of compounds, all with fluorine: OF₂ (+2) and O₂F₂ (+1).
5. Fluorine has never been found in any oxidation state other than −1 and 0.
6. The colours of transition-metal compounds map onto oxidation numbers: vanadium runs yellow (+5), blue (+4), green (+3), violet (+2) as it is reduced — a classic demonstration with zinc.
7. Permanganate is an excellent self-indicator because Mn(VII) is intensely purple and Mn(II) is nearly colourless: one drop of excess titrant is visible.
8. Every cell in your body runs on oxidation-number changes: glucose carbon goes from 0 to +4 and oxygen from 0 to −2, 24 electrons per glucose.
9. Prussian blue, Fe₄[Fe(CN)₆]₃, owes its colour to electron hopping between Fe(II) and Fe(III) — a mixed-valence compound in which the two oxidation numbers really are different atoms.
10. “Ferrous/ferric” and “cuprous/cupric” survive in everyday language, but IUPAC has preferred Stock’s Roman numerals — iron(II)/iron(III) — since 1940.

Final Thoughts

The oxidation number is a fiction that tells the truth. No atom in permanganate carries seven positive charges, yet assigning manganese +7 predicts exactly how many electrons it will take from iron, how much dichromate does the same job, what the compound is called and why its d⁰ ion is coloured only by charge transfer. Eight rules, applied in order, and one line of algebra give you all of that for any formula you will meet, and the oxidation number calculator applies them the same way every time.

The places students stumble are always the same: the forgotten subscript, the ion treated as neutral, the peroxide oxygen counted as −2, the salt averaged instead of split, the oxidizing agent named backwards, and the n-factor quoted without its reaction. The worked examples, the reference table and the practice problems above are designed to make each of those reflexes automatic. When you want to check a formula, an equation or a titration, the free oxidation number calculator at the top of this page will do the bookkeeping and show its working — bookmark it and share it with your study group.