Last updated: June 2026
Use this electrolysis calculator to calculate mass deposited, charge required, time required or current required using Faraday’s law. It is designed for O-Level, IGCSE, A-Level and Singapore H2 Chemistry students studying quantitative electrolysis and electroplating.
- Select whether you want to calculate mass, charge, time or current.
- Select a common ion preset or enter the molar mass and number of electrons from the half-equation.
- Enter the values requested in the blue guidance box inside the calculator.
- Keep current efficiency at 100% for ideal school calculations unless another value is given.
- Select Calculate to see the unit conversions, formula substitution and final answer.
Faraday’s Law Calculator
Faraday’s law connects the amount of substance formed at an electrode with the electric charge passed through the electrolyte. The calculation usually begins with charge:
Here, Q is charge in coulombs, I is current in amperes and t is time in seconds.
The amount of electrons is then found using the Faraday constant:
If each ion requires n electrons, the amount of substance formed is:
Multiplying by molar mass gives the theoretical mass:
What is electrolysis?
Electrolysis is the decomposition of an ionic substance using electricity. Mobile ions carry charge through the molten compound or electrolyte.
Cations move towards the cathode and gain electrons, so reduction occurs at the cathode. Anions move towards the anode and lose electrons, so oxidation occurs at the anode.
How to find the number of electrons transferred
The value of n comes from the balanced electrode half-equation. It is the number of moles of electrons needed to form one mole of the substance being calculated.
| Half-equation | Value of n |
|---|---|
| Ag+ + e− → Ag | 1 |
| Cu2+ + 2e− → Cu | 2 |
| Al3+ + 3e− → Al | 3 |
| 2Cl− → Cl2 + 2e− | 2 for one mole of Cl2 |
| 4OH− → O2 + 2H2O + 4e− | 4 for one mole of O2 |
Mass Deposited Calculator
To calculate the theoretical mass deposited at an electrode, use:
If the current efficiency is below 100%, only part of the charge produces the desired substance:
Charge, current and time
Current is the rate of flow of charge. One ampere means one coulomb of charge flows every second.
This explains why time must be converted into seconds before using Q = It. Milliamperes must also be converted into amperes.
Current efficiency
In ideal school calculations, current efficiency is normally taken as 100%. In a real electrolytic process, some charge may be used in competing electrode reactions, so the actual mass deposited can be lower than the theoretical mass.
When calculating the charge needed to obtain a specified actual mass, a lower efficiency means that more charge is required.
Worked example: copper deposited
Calculate the mass of copper deposited when a current of 2.50 A is passed through copper(II) sulfate solution for 30.0 minutes. Use M(Cu) = 63.55 g mol−1 and F = 96500 C mol−1.
Copper is formed according to:
Therefore, n = 2.
Common exam traps
Frequently Asked Questions
What does an electrolysis calculator calculate?
It can calculate mass deposited, charge required, time required or current required using Faraday’s law and Q = It.
What is the Faraday constant?
The Faraday constant is the magnitude of charge carried by one mole of electrons. A common examination value is 96500 C mol−1.
How do I know the value of n?
Read it from the balanced half-equation. For Cu2+ + 2e− → Cu, two moles of electrons form one mole of copper, so n = 2.
Why must time be in seconds?
An ampere is one coulomb per second. Therefore, Q = It gives charge in coulombs only when current is in amperes and time is in seconds.
How does current efficiency affect the answer?
A current efficiency below 100% reduces the actual amount of desired product formed by a given charge. More charge is therefore needed to produce a specified actual mass.