Oxidation numbers track how electrons are formally distributed in a compound or ion, and they are the backbone of balancing redox reactions and naming inorganic compounds. This calculator applies the standard rule hierarchy automatically so you can check a formula in seconds instead of working through the rules by hand.
How the Oxidation Number Calculator works
The calculator parses your formula into element/count pairs, then walks the standard priority order of assignment rules: a single-element formula is a free element (oxidation number 0 for every atom); otherwise fluorine, the Group 1 and Group 2 metals, hydrogen, oxygen, and the halogens each get their standard fixed value (with the usual exceptions for peroxides, superoxides, metal hydrides, and oxyanions). Whatever element is left without a fixed rule — typically a transition metal or another nonmetal — is solved so the whole formula's oxidation numbers sum to the overall charge you entered.
This mirrors exactly how the rules are taught in general chemistry: assign what you know first, then use charge balance to find what you don't.
Inputs and what they mean
Chemical formula accepts standard notation with element symbols, digit counts, and parentheses (e.g. KMnO4, Ca(OH)2, (NH4)2SO4). Overall charge is 0 for a neutral compound or the ion's charge for a polyatomic ion (e.g. −2 for SO4²⁻). Special case lets you flag a peroxide, superoxide, or metal hydride when the auto-detect lookup doesn't recognize your specific formula. Target element just picks which element's number is headlined as the main result — the All Elements tab always shows every element regardless of which one is selected as the target.
Limits and edge cases
This tool solves formulas with exactly one undetermined element — the vast majority of general-chemistry problems. If a formula has two elements that both lack a standard fixed rule (for example, a metal sulfide like FeS, where neither Fe nor S has a fixed oxidation number), the calculator cannot uniquely solve for both and reports an error instead of guessing.
Some real compounds have mixed oxidation states for the same element in different structural positions (for example, Fe3O4 is really a mix of Fe²⁺ and Fe³⁺). This calculator reports the single averaged oxidation number the charge-balance equation produces, which is the standard convention taught in introductory chemistry, even though it may not equal any single atom's true, structurally-resolved oxidation state.