Oxidation Number Calculator
Oxidation numbers track how electrons are distributed in a compound or ion. The governing rule is conservation: the sum of every atom's oxidation number, weighted by how many of that atom are present, must equal the overall charge (zero for a neutral compound, the ion charge otherwise). This calculator solves for the oxidation number of one unknown element. You provide the total charge, the combined contribution of all the known atoms (each count times its known state), and the number of atoms of the unknown element. The tool then solves the resulting linear equation, including non-integer averages where the formula demands.
Oxidation number formula
Conservation: sum(count * oxidation number) = total charge
knownSum + (unknownCount * x) = totalCharge
x = (totalCharge - knownSum) / unknownCount
Check = knownSum + unknownCount * x
For sulfate (charge -2, four oxygens at -2 give -8, one sulfur): x = (-2 - (-8)) / 1 = +6. The check confirms the contributions sum back to the total charge.
Common oxidation-state conventions
- Group 1 metals are +1; group 2 metals are +2 in their compounds.
- Fluorine is always -1; oxygen is -2 except in peroxides (-1) and superoxides.
- Hydrogen is +1 bonded to non-metals and -1 in metal hydrides.
- The oxidation number of a free element is 0; a monatomic ion equals its charge.
- Non-integer averages (e.g. +8/3 for iron in Fe3O4) are valid for mixed-valence formulas.
Oxidation number: frequently asked questions
What is an oxidation number?
An oxidation number is the hypothetical charge an atom would have if all bonds to atoms of different elements were fully ionic. It is a bookkeeping device used to track electron transfer in redox reactions. The sum of oxidation numbers across all atoms in a neutral compound is zero, and in an ion it equals the ion charge.
How does this calculator find the unknown oxidation number?
It uses the conservation rule: the sum of (atom count times oxidation number) over every element equals the overall charge. You enter the total charge and the combined contribution of all the known atoms, plus the count of the unknown element. The calculator solves the linear equation for the unknown oxidation number.
What are the common fixed oxidation states?
Group 1 metals are +1, group 2 metals are +2, fluorine is always -1, oxygen is usually -2 (but -1 in peroxides), and hydrogen is +1 with non-metals and -1 in metal hydrides. These conventions let you compute the contribution of known atoms before solving for the unknown.
Why might the answer be a fraction?
Some species give a non-integer average oxidation number, such as +8/3 for iron in Fe3O4, because the formula contains atoms of the same element in different real oxidation states. The calculator reports the average value, which is mathematically correct for the formula as written.
Does this work for polyatomic ions?
Yes. Enter the ion charge as the total charge. For example, in the sulfate ion the total charge is -2, the four oxygens contribute -8, the sulfur count is 1, so sulfur solves to +6.
Official sources
- IUPAC: IUPAC definitions of oxidation state.
- NIST: NIST Chemistry WebBook.
Reviewed by the CalculatorHub team, edited by James Graham, 16 June 2026. See our methodology.