What is the Oxidation Number Calculator?
Calculate oxidation numbers (oxidation states) for elements in a chemical formula using standard general-chemistry rules. Enter a compound like H₂SO₄ or an ion like SO₄ with charge −2, and get each element's oxidation state with the reasoning applied. Supports manual overrides for tricky cases. Runs instantly in your browser with no signup.
How to use the Oxidation Number Calculator
- Enter the chemical formula (e.g. H2SO4, KMnO4, Cr2O7).
- Set the overall charge — 0 for neutral molecules, −2 for sulfate, −1 for permanganate, etc.
- Review oxidation numbers in the results table and the charge-balance equation.
- Add a manual override if multiple elements would otherwise stay unknown.
- Copy the summary for notes or redox homework.
Common use cases
- Finding sulfur's +6 oxidation state in sulfuric acid
- Balancing redox equations with MnO₄⁻ or Cr₂O₇²⁻
- Checking oxidation states before assigning electrons in half-reactions
Frequently asked questions
- What is an oxidation number?
- It is the hypothetical charge an atom would have if bonds were completely ionic. In H₂O, oxygen is −2 and hydrogen is +1. They sum to zero in a neutral molecule.
- How do I enter an ion like SO₄²⁻?
- Use the formula without the charge in the formula field (SO4) and enter −2 in the overall charge field.
- What rules does this calculator use?
- Standard rules: free elements are 0; group 1 is +1, group 2 is +2; Al is +3; F is −1; O is usually −2 (peroxides −1); H is usually +1 (hydrides −1); Cl/Br/I are −1 in binary halides without oxygen.
- Why does S₂O₃²⁻ show one sulfur oxidation state?
- Thiosulfate has sulfur atoms in different environments. This tool reports one oxidation state per element symbol (an average) unless you add manual overrides.
- Can it handle fractional oxidation states?
- Yes. Magnetite (Fe₃O₄) gives an average iron oxidation state of +8/3 when oxygen is assigned −2.
