Oxidation Number Calculator
Find the oxidation state of every element in a formula or ion. The standard rules are applied in order, each element shows the rule that fixed it, fractional averages such as +8/3 are explained, and Redox mode finds what is oxidised, what is reduced and how many electrons move.
How to Use
- Choose Formula / ion to find the oxidation states in one substance, or Redox reaction to compare the two sides of an equation.
- Type the formula with correct capitals (KMnO4, Fe3O4, CuSO4.5H2O). For an ion add the charge with a caret: SO4^2-, Cr2O7^2-, NH4+.
- For a reaction type reactants = products, for example Cu + HNO3 = Cu(NO3)2 + NO + H2O. Coefficients are optional: an unbalanced equation is balanced first.
- Read the state of each element and the rule that fixed it. The number line shows every element at its oxidation state; a fraction such as +8/3 is an average and the note explains the mixed valence behind it.
- In Redox mode read what is oxidised and reduced, the oxidising and reducing agents and the electrons transferred; Show Work lists the states of every species.
SO4^2- NH4+ Fe^3+ I3^- · hydrates: CuSO4.5H2O · capitals matter: Co is cobalt, CO carbon monoxideFe3+ + e-Worked Example
Manganese in potassium permanganate, KMnO₄. Potassium is a group 1 metal, so +1; oxygen is −2. The states add up to the charge, 0: (+1) + x + 4 × (−2) = 0, so x = +7, the highest state manganese has.
Iron in magnetite, Fe₃O₄. With oxygen −2, 3x + 4 × (−2) = 0 gives x = +8/3. No single iron atom can be +8/3: this is an average. Magnetite contains one Fe²⁺ and two Fe³⁺ per formula unit, and 2 + 3 + 3 = 8.
The common mistake: forgetting how many atoms there are. In potassium dichromate, K₂Cr₂O₇, the sum is 2 × (+1) + 2x + 7 × (−2) = 0, so 2x = +12. Stopping there gives chromium +12, which is impossible (chromium is in group 6). Dividing by the two chromium atoms gives the right answer, +6.
Show Work
Formulas
From “Combining with Oxygen” to Electron Bookkeeping
Antoine Lavoisier, in the 1770s and 1780s, showed that burning and rusting are combinations with oxygen, and “oxidation” first meant exactly that. Once the electron was known, the word was widened to mean any loss of electrons, with reduction the gain, so that iron is oxidised by chlorine just as it is by oxygen.
The oxidation state is the bookkeeping that makes this countable: every shared electron pair is given to the more electronegative atom, and the charge each atom is left with is its oxidation state. Alfred Stock proposed writing it as a Roman numeral after the name in 1919, giving names such as iron(III) chloride, and Wendell Latimer’s 1938 book The Oxidation States of the Elements and Their Potentials in Aqueous Solutions organised inorganic chemistry around it. IUPAC published a comprehensive definition in 2016 (Karen, McArdle and Takats). The rules this tool applies are the textbook shortcuts that give the same answer for most compounds; the group numbers, electronegativities and lists of common oxidation states it uses come from the PubChem periodic table (NIH).
About This Tool
This calculator applies the oxidation-number rules in their usual order and tells you which rule fixed each element. When oxygen is the last element left it lets the sum decide, so peroxides, superoxides and oxygen fluorides come out right instead of being forced to −2. Ionic compounds made of known polyatomic ions are split into their ions, so ammonium nitrate shows nitrogen at both −3 and +5 rather than only the average of +1. Fractions are kept exact and explained, and values outside what an element can have (sulfur +8 in H₂SO₅) are flagged as a sign of a hidden O–O bond.
When the rules leave two or more elements undetermined, as for the carbon and nitrogen in CH₃NH₂, the tool says so instead of guessing: those need the bonding structure. Redox mode balances the equation if needed, compares every element on both sides and counts the electrons. Everything runs in your browser; nothing is sent anywhere.
Related tools: Chemical Equation Balancer, Nernst Equation Calculator, and Lewis Structure & VSEPR Shape Calculator.
Frequently Asked Questions
How do you find the oxidation number of an element in a compound?
Fix the elements that have rules (F −1, group 1 +1, group 2 +2, H +1, O −2), then use the fact that all the oxidation states add up to the charge. In KMnO₄: (+1) + x + 4 × (−2) = 0, so manganese is +7. In the sulfate ion SO₄²⁻: x + 4 × (−2) = −2, so sulfur is +6.
Why can an oxidation number be a fraction?
The rules give the average over all the atoms of an element. In Fe₃O₄, 3x + 4 × (−2) = 0 gives x = +8/3: magnetite really contains one Fe²⁺ and two Fe³⁺ (2 + 3 + 3 = 8). In the tetrathionate ion S₄O₆²⁻ sulfur averages +5/2 because its four sulfur atoms are bonded differently.
When is oxygen not −2?
In peroxides, which have an O–O bond, each oxygen is −1 (H₂O₂: 2 × (+1) + 2x = 0). In superoxides it is −½ (KO₂: +1 + 2x = 0). Bonded to fluorine it is positive: in OF₂, x + 2 × (−1) = 0 gives +2. A peroxide hidden in a larger formula shows up as an impossible value, such as sulfur +8 in H₂SO₅, which this tool flags.
When is hydrogen −1?
In hydrides, where hydrogen is bonded only to metals (or to boron, silicon or germanium, which are less electronegative than hydrogen). In NaH sodium is +1, so hydrogen is −1; in CaH₂, +2 + 2x = 0 gives −1; and in sodium borohydride, NaBH₄, hydrogen is −1 and boron +3. With non-metals, as in H₂O, CH₄ and PH₃, hydrogen is +1.
How do you identify the oxidising and reducing agents?
Find the element whose oxidation state rises (oxidised) and the one that falls (reduced). In MnO₄⁻ + 5Fe²⁺ + 8H⁺ → Mn²⁺ + 5Fe³⁺ + 4H₂O, Mn goes from +7 to +2 and gains 5 electrons, while each Fe goes from +2 to +3 and loses 1, five times. MnO₄⁻ is reduced, so it is the oxidising agent; Fe²⁺ is oxidised, so it is the reducing agent.
How do I use the Oxidation Number Calculator?
Just type your numbers. The answer shows up right away — there is no button to press. Change anything and it updates by itself.
Does it cost anything or need an account?
No. The tool is completely free, there is no account to create, and it keeps working offline after the page first loads.
Is anything I type uploaded?
No. The tool works entirely on your device, so the values you enter never leave your browser.
Common Use Cases
Balancing redox equations
In 3Cu + 8HNO₃ → 3Cu(NO₃)₂ + 2NO + 4H₂O copper loses 6 electrons and only 2 of the 8 nitrogen atoms are reduced, from +5 to +2.
Titrations
Dichromate titrations rely on Cr going from +6 to +3: Cr₂O₇²⁻ + 6Fe²⁺ + 14H⁺ → 2Cr³⁺ + 6Fe³⁺ + 7H₂O moves 6 electrons.
Iodometry
In I₂ + 2S₂O₃²⁻ → 2I⁻ + S₄O₆²⁻ iodine is reduced from 0 to −1 and sulfur goes from an average of +2 to +5/2, for 2 electrons.
Batteries
In a lead–acid cell, Pb + PbO₂ + 2H₂SO₄ → 2PbSO₄ + 2H₂O: lead goes from 0 up to +2 and from +4 down to +2, so 2 electrons flow per reaction.
Disproportionation
Chlorine in hot alkali, 3Cl₂ + 6OH⁻ → 5Cl⁻ + ClO₃⁻ + 3H₂O, goes from 0 to both −1 and +5, with 5 electrons transferred.
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