What an oxidation number tells you
An oxidation number (or oxidation state) is the hypothetical charge an atom would have if all of its bonds were fully ionic. Oxidation numbers are used to track electron transfer in redox reactions and to name compounds.
How this calculator works
The core rule of oxidation numbers is that they must sum to the overall charge of the species: the sum of every atom’s (oxidation number × how many atoms of it there are) equals the compound’s or ion’s overall charge (0 for a neutral compound). Enter the overall charge and the combined contribution of every atom whose oxidation number you already know (using standard rules, e.g. oxygen is usually -2, hydrogen is usually +1, group 1 metals are +1), along with how many atoms of the unknown element are present, and this tool solves for that element’s oxidation number: (charge − known sum) ÷ count.
Note: You must apply the standard oxidation-number rules yourself when determining the “known” atoms’ contributions (e.g. peroxides, metal hydrides, and elemental forms are exceptions to the usual O and H rules). This tool is for general chemistry education.