Oxidation Number Calculator
Solve for one element's oxidation state from the formula.
Work out Oxidation Number. Solve for one element's oxidation state from the formula. Kelvin conversion handled for you.
Case sensitive, as always: Co is cobalt, CO is carbon monoxide.
The one whose oxidation state varies — the transition metal, or the central atom.
0 for a neutral compound, −1 for MnO4⁻, −2 for Cr2O7²⁻.
Oxidation number of S
+6
In H2SO4, overall charge 0
Two assignments here have exceptions worth knowing. Hydrogen is taken as +1, which is wrong in metal hydrides such as NaH where it is −1. Oxygen is taken as −2, which is wrong in peroxides like H₂O₂ where it is −1, and in OF₂ where fluorine outranks it and oxygen goes positive. Everything else assigned here — the group 1 and 2 metals, fluorine, aluminium — genuinely does not vary in ordinary compounds.
How the Oxidation Number Calculator works
Type a formula, name the element you want, and give the overall charge. The page assigns every other element the state the rules fix outright and solves for the one left over. Where two elements are both variable it says so instead of guessing.
Also known as: oxidation state calculator · how to find oxidation number · oxidation number of sulfur in h2so4 · oxidation number rules
Frequently asked questions
How do I find an oxidation number?
Assign the fixed states — group 1 is +1, group 2 is +2, fluorine −1, oxygen usually −2, hydrogen usually +1 — then set the total equal to the overall charge and solve for the unknown.
What is the oxidation number of sulfur in H2SO4?
+6. Two hydrogens contribute +2, four oxygens contribute −8, and the compound is neutral, so sulfur must be +6.
Can an oxidation number be a fraction?
Yes, as an average. In Fe₃O₄ the iron is genuinely a mixture of Fe(II) and Fe(III), and the calculation returns +8/3 — which is correct rather than an error to round away.
When is oxygen not −2?
In peroxides such as H₂O₂ it is −1, in superoxides −½, and in OF₂ it is +2 because fluorine is the more electronegative. Hydrogen is likewise −1 in metal hydrides such as NaH.
Put this calculator on your own site
Free to use, on any site, commercial or not. Paste this where you want it to appear. It is a plain iframe, so it works in WordPress, Squarespace, Wix, Webflow, Ghost and anything else that accepts HTML.
<iframe src="https://www.thecalclibrary.com/embed/oxidation-number-calculator" width="100%" height="640" style="border:1px solid #e2e8f0;border-radius:12px" loading="lazy" title="Oxidation Number Calculator"></iframe>
<p style="font:13px/1.5 system-ui,sans-serif;margin:6px 0 0;color:#64748b">Powered by <a href="https://www.thecalclibrary.com/oxidation-number-calculator" style="color:#64748b">Oxidation Number Calculator</a> from The Calc Library</p>The only condition is that the credit line below the frame stays in place. That one line is what pays for the tool being free — it is how anyone else finds it.
Related calculators
Formal Charge Calculator
FC = valence − lone electrons − bonds, per atom.
OpenStandard Cell Potential Calculator
E°cell, ΔG° and K from two reduction potentials.
OpenMolar Mass Calculator
Any formula, with brackets, hydrates and percent composition.
Open