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Redox · Chemical formulas

Oxidation Number Calculator & Rule Checker

Find one element’s oxidation number from a formula, total charge, and known states of the other elements, then verify the charge balance.

Formula shown Runs locally Reviewed Aug 11, 2026
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Equation guide

Formal charge balance for a neutral compound or polyatomic ion

Charge balance

Σ nᵢxᵢ = Q

Unknown state

xₜ = (Q − Σknown nᵢxᵢ)/nₜ

Symbols

n: atom count; x: oxidation numberQ: total species charge
Oxidation-state bookkeeping

Solve and verify one unknown oxidation number

SO4S: 1O: 4
Entry rules

Use Q = 0 for a neutral compound and the signed whole-number ion charge otherwise. List every non-target element once as symbol=value. Oxidation-state inputs and displayed results use at most three decimal places.

Σ nᵢxᵢ = Q

Choose the target element and state the assumptions for every other element.

Using the model

Symbols, assumptions and limitations

Interpretation notes

Oxidation numbers are formal electron-accounting values, not measured ionic charges. The compound or ion charge Q equals the atom-count-weighted sum of all oxidation numbers. Common rules have exceptions, especially for oxygen in peroxides, superoxides, and oxygen fluorides.

Worked examples

See the method in practice

01

Sulfur in sulfate

For SO₄²⁻, assigning oxygen −2 gives x(S) + 4(−2) = −2, so sulfur is +6.

02

Manganese in permanganate

For MnO₄⁻, oxygen at −2 gives x(Mn) + 4(−2) = −1, so manganese is +7.

Questions

Frequently asked

Why must I specify the ion charge?

The weighted sum is zero only for a neutral compound. For a polyatomic ion it must equal the ion’s signed charge.

Why are the known oxidation states entered explicitly?

Rules such as O = −2 and H = +1 have exceptions. Explicit values make the chemical assumption visible and checkable.

Can the result be fractional?

An average oxidation number may be fractional in mixed-valence compounds. For a single equivalent site, an unexpected fraction often signals an incorrect formula, charge, or assumed state.

Sources & review

Equations and examples are checked against the references below. Results are educational and should be independently verified for safety-critical work.

OpenStax Chemistry — Oxidation–Reduction Reactions

Written by the STEM Hub editorial team · Reviewed August 11, 2026 · Review methodology