Oxidation State Calculator
Oxidation states track how electrons are notionally distributed in a compound and are governed by a fixed set of rules. The key one is that the oxidation states of every atom, weighted by how many of each are present, add up to the overall charge of the species: zero for a neutral molecule, or the ion charge for an ion. If you know the oxidation states of all but one element, you can solve for the last one. This calculator takes the overall charge, the summed contribution of the known atoms, and the count of the unknown element, then returns the unknown oxidation state.
Oxidation state formula
sum of all (oxidation state * atom count) = overall charge
unknown contribution = overall charge - sum of known contributions
unknown oxidation state = unknown contribution / count of unknown atoms
The count of the unknown element's atoms must be greater than zero. For sulfate, with charge -2, two known... oxygens at -2 each give -8 known; with one sulfur the example below solves to +6.
Oxidation state context
- The oxidation states of all atoms sum to the overall charge of the species.
- Oxygen is usually -2 and hydrogen usually +1, with known exceptions.
- Group 1 metals are +1 and group 2 metals are +2.
- The summed known contribution is each known state times its atom count, added together.
- An average oxidation state can be fractional in mixed-valence compounds.
Oxidation state: frequently asked questions
What is an oxidation state?
An oxidation state, or oxidation number, is the charge an atom would have if all its bonds were fully ionic, with shared electrons assigned to the more electronegative atom. It tracks electron transfer in redox reactions and is set by a defined set of rules.
How do I find an unknown oxidation state?
Use the rule that the sum of all oxidation states times their atom counts equals the overall charge of the species. Subtract the contribution of the known atoms from the overall charge, then divide by the count of the unknown element.
What are common fixed oxidation states?
Group 1 metals are +1 and group 2 metals are +2. Oxygen is usually -2 (except in peroxides, where it is -1) and hydrogen is usually +1 (except in metal hydrides, where it is -1). Enter the known total so the calculator can solve for the unknown element.
Why must oxidation states sum to the overall charge?
Oxidation states partition the species' total charge among its atoms. For a neutral compound they sum to zero; for an ion they sum to the ion's charge. This conservation rule is what lets you solve for a single unknown oxidation state.
Can an oxidation state be a fraction?
An average oxidation state can be fractional when atoms of the same element occupy different environments, as in Fe3O4. The rules give a whole number for each individual atom, but the calculator returns the average per atom, which may be a fraction.
Official sources
- IUPAC Gold Book: oxidation state and oxidation number rules.
- NIST: periodic table of the elements.
Reviewed by the CalculatorHub team, edited by James Graham, 17 June 2026. See our methodology.