Oxidation Number Calculator
This oxidation number calculator works from a chemical formula and its charge: KMnO4 gives Mn +7, SO3 with charge −2 gives S +4. It applies the standard guidelines for H, O, F and the alkali metals one after another and finds the last element from the rule that all oxidation numbers add up to the charge.
How it is calculated
The oxidation numbers of all atoms in a molecule or ion add up to its charge. OpenStax Chemistry defines the oxidation number as the charge an atom would have if the compound were ionic, and gives four guidelines: an element in elemental form has 0; a monatomic ion has its charge; hydrogen is +1 with nonmetals and −1 with metals, oxygen −2 in most compounds, fluorine always −1; and the sum equals the charge. The calculator sets the known values and solves for the one element that is left.
Worked examples
| Formula | Element | Oxidation number |
|---|---|---|
| H₂S | S | −2 |
| SO₃²⁻ | S | +4 |
| Na₂SO₄ | S | +6 |
| KNO₃ | N | +5 |
| AlH₃ | Al | +3 |
| NH₄⁺ | N | −3 |
| H₂PO₄⁻ | P | +5 |
| KMnO₄ | Mn | +7 |
| H₂O₂ | O | −1 |
| Fe₃O₄ | Fe | +8/3 |
The first seven are the examples in OpenStax. For SO₃²⁻ the sum rule reads −2 = 3 × (−2) + x, so x = +4. In KMnO₄ potassium (group 1) is +1 and oxygen −2: +1 + x + 4 × (−2) = 0 gives x = +7.
Order of the guidelines
- Fluorine −1, then group 1 metals +1 and group 2 metals +2 (they form 1+ and 2+ ions).
- Hydrogen +1 if a nonmetal is present, −1 if all other elements are metals (AlH₃, NaH).
- Oxygen −2, other halogens −1 if there is no oxygen and no other halogen.
- As soon as only one element is open, it follows from the sum. That is why H₂O₂ gives oxygen −1 (peroxide) and OF₂ gives oxygen +2 – exactly the cases OpenStax names.
If two elements remain, as in CuSO₄, the guidelines are not enough: enter one value yourself (Cu +2 gives S +6) or calculate the ion SO₄ with charge −2. A fraction such as +8/3 is an average over several atoms. The element symbols are in the periodic table; the mass of the compound follows from the molar mass calculator.
Frequently asked questions
How do you find an oxidation number?
Give the known elements their usual values (H +1, O −2, F −1, group 1 metals +1, group 2 metals +2) and use the rule that the sum equals the charge. In H₂SO₄: 2 × (+1) + x + 4 × (−2) = 0, so S is +6.
What is the oxidation number of Mn in KMnO₄?
+7: potassium +1, four oxygens −8, so Mn must be +7 for the sum 0. In the permanganate ion MnO₄⁻ (charge −1) Mn is also +7.
Why is oxygen −1 in H₂O₂?
Hydrogen is +1 with the nonmetal oxygen. The two oxygens must then carry −2 together, i.e. −1 each. OpenStax calls these compounds peroxides.
Can an oxidation number be a fraction?
Yes, as an average. In Fe₃O₄ the three iron atoms share +8, so each has +8/3 on average; OpenStax also names −½ for oxygen in superoxides such as KO₂.
Why does the calculator say “not unique”?
When two elements are left without a fixed rule, as Cu and S in CuSO₄, the sum rule alone cannot separate them. Enter one known value or split the compound into its ions.
What is the difference between charge and oxidation number?
OpenStax writes a charge with the number first (2+) and an oxidation number with the sign first (+2) to keep them apart.
Sources and legal basis
- OpenStax Chemistry 2e, 4.2 Classifying Chemical Reactions – guidelines for assigning oxidation numbers (element 0, monatomic ion = charge, H +1/−1, O −2/−1/−½, F −1, other halogens −1 except with O or other halogens, sum = charge); Example 4.5 H₂S, SO₃²⁻, Na₂SO₄; KNO₃, AlH₃, NH₄⁺, H₂PO₄⁻ (read 10.10.2026)
- OpenStax Chemistry 2e, 2.6 Ionic and Molecular Compounds – alkali metals (group 1) form 1+ cations, alkaline earth metals (group 2) 2+ cations (read 10.10.2026)
- PubChem Periodic Table (NCBI/NLM), GroupBlock – metal, nonmetal, metalloid; OxidationStates – highest listed value (read 10.10.2026)
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