Half Reaction Calculator
A redox reaction is one in which electrons move from one species to another, and it is split into two half reactions: an oxidation, where a species loses electrons and its oxidation state rises, and a reduction, where a species gains electrons and its oxidation state falls. Balancing the electrons in each half reaction is the central step in balancing redox equations. This calculator finds the number of electrons transferred in a single half reaction from two inputs: the oxidation state of the element before and after the reaction, and the number of atoms of that element involved. The electrons transferred per atom is the size of the change in oxidation state, and multiplying by the number of atoms gives the total electrons in the half reaction. The calculator also reports whether the change is an oxidation or a reduction by the direction of the oxidation-state change. You enter the starting and ending oxidation states and the atom count, and the calculator returns the electrons transferred and the type of half reaction. Every figure is computed deterministically from the values you enter, never guessed, so the same inputs always return the same result. The method and a worked example that reconciles to the calculator default are shown in full below.
Electrons transferred equal the change in oxidation state times the number of atoms: e = |final - initial| x atoms. For Fe2+ to Fe3+ (one atom, +2 to +3), one electron is lost, so it is an oxidation releasing 1 electron.
Formula
electrons = |final - initial| x atoms
If final > initial: oxidation (electrons lost)
If final < initial: reduction (electrons gained)
The change in oxidation state per atom is the difference between the final and initial states. Its sign shows oxidation or reduction, and its magnitude times the atom count gives the total electrons transferred.
Worked example
Balance the half reaction Fe2+ to Fe3+, with one atom going from oxidation state +2 to +3.
- Change = 3 - 2 = +1 per atom, so it is an oxidation
- Electrons per atom = |+1| = 1
- Electrons transferred = 1 x 1 = 1
- Half reaction: Fe2+ to Fe3+ + 1 electron
These are the calculator's default inputs, so the result of 1 electron lost matches the widget exactly.
Half Reaction Calculator: frequently asked questions
What is a half reaction?
A half reaction is one half of a redox reaction, showing either the oxidation (loss of electrons) or the reduction (gain of electrons) on its own, with the electrons written explicitly.
How do I count electrons transferred?
Take the difference between the final and initial oxidation states of the element, which is the electrons per atom, then multiply by the number of atoms involved.
How do I tell oxidation from reduction?
If the oxidation state increases the species is oxidised and loses electrons; if it decreases the species is reduced and gains electrons. The calculator reports which one occurs.
What if the oxidation state is negative?
Negative oxidation states are fine. Enter the signed values and the calculator uses the difference, so a change from -1 to +1 transfers two electrons.
Does this balance the full equation?
It balances the electrons in one half reaction. To balance a full redox equation you make the electrons of both half reactions equal and add them.
Official sources
- Chemistry and oxidation state reference: US National Institute of Standards and Technology (NIST). As at 25 June 2026.
Reviewed by the CalculatorHub team, edited by James Graham, 25 June 2026. See our methodology. This is general information, not financial, tax, legal or investment advice.