Find the oxidation number of any element in a compound or ion from its formula and charge, with the rule-by-rule contributions and atom counts shown.
Results
Calculated
Oxidation number
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Of the element you chose
Other atoms subtotal
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Sum of the fixed-rule contributions
Charge on target atoms
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Must balance the overall charge
Atoms read from formula
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Check for typing mistakes
Ready
Type a formula, choose the element to solve for and the overall charge, then press Calculate.
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What this calculator finds
An oxidation number is the charge an atom would have if all its bonds were treated as fully ionic. It is the bookkeeping tool behind redox reactions: it shows which element is oxidized (number rises) and which is reduced (number falls). Type a formula, name the element you want, give the overall charge, and this calculator applies the standard rules and returns that element's oxidation number.
It handles nested brackets such as Fe2(SO4)3 and reports the atom counts it read, so you can catch typing mistakes.
The method
Sum of all oxidation numbers = overall charge of the species.
Known elements get their fixed values: group 1 = +1, group 2 = +2, Al = +3, F = −1, H = +1, O = −2 (with the exceptions in the FAQ).
The target element gets whatever value makes the sum work: ox = (charge − sum of the others) / number of target atoms.
Worked example
Potassium permanganate, KMnO4, solving for manganese with charge 0, the default inputs. The formula has one K, one Mn and four O.
Potassium contributes 1 × (+1) = +1. Oxygen contributes 4 × (−2) = −8. The others total +1 − 8 = −7. Charge minus that is 0 − (−7) = +7 for the single Mn atom, so manganese is +7. The calculator shows +7, the other-atoms subtotal of −7 and a “consistent oxidation state” banner.
Common mistakes and how to read the result
Forgetting the charge. For sulfate SO4 alone the charge is −2, not 0; leaving 0 gives S = +8 instead of the correct +6.
Peroxides. H2O2 needs the peroxide setting or oxygen is treated as −2 and hydrogen comes out wrong.
Case-sensitive symbols. Co is cobalt but CO is carbon and oxygen.
Covalent compounds. Oxidation numbers are a formal convention and need not match real atomic charges.
Frequently Asked Questions
What rules does the calculator use?
In order: atoms in an element have oxidation number 0; the numbers must add to the overall charge; group 1 metals are +1 and group 2 metals +2; fluorine is -1; hydrogen is +1 (or -1 in metal hydrides); oxygen is -2 (or -1 in peroxides, -1/2 in superoxides, +2 with fluorine); the other halogens are -1 unless bonded to oxygen or fluorine.
Why can it only solve one element at a time?
With a single unknown, the charge balance gives one equation and one answer. If two elements both have variable oxidation numbers, such as Fe and Cr in FeCr2O4, the sum alone cannot separate them, so the calculator asks you to supply one of them in the optional known-values box (for example S=6 for Fe2(SO4)3).
What does a fractional answer mean?
It is an average. In Fe3O4 the iron atoms average +8/3 because two are Fe(III) and one is Fe(II). A fraction can also signal a wrong oxygen type, for example a peroxide entered as an oxide.
How do I enter ions?
Type the formula without the charge and put the charge in the overall-charge box. For dichromate enter Cr2O7 with a charge of -2 and solve for Cr, which gives +6.
Oxidation Number Calculator is most useful when the inputs reflect the situation you are actually planning around, not a best-case estimate. Treat the result as a decision aid: it gives you a structured way to compare assumptions, spot outliers, and decide what to verify next. For Chemistry work, the most important review lens is units, concentration, limiting assumptions, temperature, precision, and significant figures.
Start with a baseline run using values you can defend. Then change one assumption at a time and watch which output moves the most. If one input dominates the result, spend your verification time there first. If several inputs have similar influence, use a conservative scenario and an optimistic scenario to create a practical range instead of relying on a single exact number.
Before acting on the result, verify inputs against lab notes, reagent labels, and the expected reaction or solution model. This is especially important when the calculator supports a purchase, project plan, performance target, or operational decision. The calculator can make the math consistent, but the quality of the conclusion still depends on current data, clear units, and assumptions that match your real constraints.
When the output looks surprising, slow down and inspect each input in order. A small change in one high-leverage field can move the final number more than several low-leverage fields combined. For Oxidation Number Calculator, that means you should first confirm the value with the greatest scale, then confirm the value with the greatest uncertainty, then rerun the calculator with conservative and optimistic assumptions. This sequence turns the calculator from a single answer into a practical decision range.
Review Checklist
Confirm every input uses the unit and time period requested by the calculator.
Run a low, expected, and high scenario so the answer has a useful range.
Check whether rounding or a missing decimal place changes the decision.
Update the calculation for every new mixture, batch, reaction, or homework data set.
How to Validate the Result
Use Oxidation Number Calculator as a repeatable checkpoint rather than a one-time answer. The safest workflow is to record the original inputs, save the output, and write down which assumption you are testing. Then rerun the calculator with one changed value. If the result changes sharply, that input deserves more attention before you act on the number.
For this topic, the main validation lens is units, concentration, limiting assumptions, temperature, precision, and significant figures. That means a result can be mathematically correct and still be misleading if the inputs come from the wrong time period, use inconsistent units, or mix expected values with best-case values. Keep baseline, conservative, and optimistic runs separate so the final decision is easier to explain later.
When you share the result with someone else, include the assumptions and the date of the calculation. Many calculator outputs become stale after prices, schedules, measurements, or constraints change. A short note about the source of each input makes the calculation auditable and prevents later confusion about why the answer moved.
Label the source for each input before comparing scenarios.
Use the same rounding method across every run.
Flag any input that is estimated rather than measured.
Recalculate for every new mixture, batch, reaction, or homework data set.