Calculating resistance – Ohm’s Law

The resistance of an electrical component can be found by measuring the electric current flowing through it and the potential difference across it.

This equation, called Ohm’s Law, shows the relationship between potential difference, current and resistance:

voltage = current × resistance

V = I × R


V is the potential difference in volts, V

I is the current in amperes (amps), A

R is the resistance in ohms, Ω

The equation can be rearranged to find the resistance:

R = V ÷ I


3 A flows through a 240 V lamp. What is the resistance of the lamp?

Resistance = 240 ÷ 3 = 80 Ω

To calculate the resistance of an electrical component, an ammeter is used to measure the current and a voltmeter to measure the potential difference. The resistance can then be calculated using Ohm’s Law.