Oxidation Number Calculator
Find the oxidation number of a target element in a species from the standard rules for hydrogen (+1), oxygen (−2) and the overall charge.
How it works
(n_target × ox_target) + (n_H × +1) + (n_O × −2) = overall charge · ox_target = (charge − known) / n_target
Oxidation numbers are assigned by a fixed set of rules: hydrogen is usually +1, oxygen usually −2, and the sum of all oxidation numbers equals the species’ overall charge (zero for a neutral compound). With those two elements fixed, the oxidation number of the remaining target element is whatever makes the total balance. This is the standard bookkeeping used to identify what is oxidized or reduced.
Worked example
Sulfate, SO₄²⁻: one S (target), four O at −2, no H, overall charge −2. Known contribution = 4 × (−2) = −8. So S = (−2 − (−8)) / 1 = +6. Sulfur is in the +6 oxidation state.
Sources: Oxidation state — Wikipedia
Frequently asked questions
When do the +1 and −2 rules fail?
Hydrogen is −1 in metal hydrides (e.g. NaH), and oxygen is −1 in peroxides (H₂O₂) or +2 in OF₂. For those cases override the H or O counts, since this tool assumes the usual values.
How do I handle a neutral compound?
Set the overall charge to 0. The oxidation numbers of all atoms must then sum to zero, and the target element takes whatever value balances H and O.
What if other elements are present?
Fold their known oxidation contributions into the picture manually. This helper covers the very common H/O case; for halogens or metals with fixed values, add their total to the “known” contribution mentally before reading off the target.
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This calculator is for educational purposes. Double-check important results before acting on them.