Ohm's Law Calculator guide
Ohm's Law Calculator: Voltage, Current, Resistance and Power
Use our Ohm's law calculator to find voltage, current, resistance or power from any two of them, using V = IR and P = VI.
By the AllCalculator editorial teamUpdated 16 October 20261 min read
Ohm's law is the foundation of electronics. Our Ohm's Law Calculator finds any two of voltage, current, resistance and power from the other two.
How to use the calculator (Ohm's Law Calculator)
Enter any two of:
- Voltage (V), in volts
- Current (I), in amps
- Resistance (R), in ohms
- Power (P), in watts
Leave the other two blank.
The formulas
V = I × R (Ohm's law) and P = V × I
| To find | Formulas |
|---|---|
| Voltage | V = I × R, V = P ÷ I, V = √(P × R) |
| Current | I = V ÷ R, I = P ÷ V, I = √(P ÷ R) |
| Resistance | R = V ÷ I, R = V² ÷ P, R = P ÷ I² |
| Power | P = V × I, P = I² × R, P = V² ÷ R |
A worked example
With the calculator's default settings, 12V across a 6Ω resistor gives a current of 12 ÷ 6 = 2A, and the resistor uses 12 × 2 = 24W of power.
Frequently asked questions
What size fuse do I need?
Work out the current, then choose the next fuse size up. A 2kW heater on 230V draws about 8.7A, so it needs a 13A fuse.
Does Ohm's law always apply?
It works for resistors and wires, but not for components like LEDs and diodes, whose resistance changes with the voltage.
Is mains electricity dangerous?
Yes. UK mains is 230V and can kill. Leave wiring work to a qualified electrician.
To read a resistor's value, use our Resistor Calculator.
Sources and further reading
Links open official and independent websites in a new tab.
This guide is general information to help you understand your options. It isn't financial, tax, legal or building advice and doesn't take your personal circumstances into account. Figures were checked when the guide was last updated and can change, so confirm anything important with GOV.UK, HMRC, a regulated adviser or a qualified tradesperson.