Ohm's Law Calculator
Fill in any two of voltage, current and resistance, leave the third blank, and this calculator solves for it with Ohm's law.
Ohm's law: V = I × R. Leave any one field blank and the calculator solves for it from the other two.
How to use Ohm's law by hand
- Write down the two values you know, and which one (V, I or R) you're solving for.
- Pick the matching rearrangement: V = I × R, I = V ÷ R, or R = V ÷ I.
- Substitute the numbers in and compute. Keep everything in volts, amps and ohms — convert first if a value is in mA or kΩ.
Worked example
A 9 V battery is connected across a 3 Ω resistor. Current = voltage ÷ resistance = 9 ÷ 3 = 3 A. If the same circuit instead carries 1.5 A, the resistance would be 9 ÷ 1.5 = 6 Ω.
Wire up real resistors, batteries and bulbs and watch the current change in the Circuit Builder, or check what each symbol in a circuit diagram means on the Circuit Symbols chart.
Frequently asked questions
What is Ohm's law?
Ohm's law states that voltage equals current times resistance: V = I × R. If you know any two of voltage, current and resistance, you can find the third.
How do you calculate resistance from voltage and current?
Rearrange Ohm's law to R = V ÷ I. For example, 12 V across a component carrying 2 A gives a resistance of 12 ÷ 2 = 6 Ω.
How do you calculate current from voltage and resistance?
Rearrange Ohm's law to I = V ÷ R. For example, 12 V across a 6 Ω resistor gives a current of 12 ÷ 6 = 2 A.
What units does Ohm's law use?
Voltage (V) in volts, current (I) in amps, and resistance (R) in ohms (Ω). Keep all three in these base units — don't mix in milliamps or kilohms without converting first.
