Ohm's law calculator
Enter any two of voltage, current, resistance and power, and find the other two.
About this calculator
Fill in any two boxes and leave the other two empty. The calculator solves Ohm's law and the power formulas for the rest.
Values are in volts, amps, ohms and watts, with milliamps and kilowatts shown too.
Measurements and terms explained
- Voltage (V)
- The electrical push that drives current round a circuit, in volts.
- Current (I)
- The flow of charge, in amps (A). 1 A = 1,000 mA.
- Resistance (R)
- How much a part resists current, in ohms (Ω).
- Power (P)
- The rate energy is used, in watts (W).
Common questions
What is Ohm's law?
Voltage equals current times resistance, V = I × R.
How do I find power?
Power in watts equals voltage times current, P = V × I.
More free calculators
Resistor colour code calculator
Read a resistor from its colour bands.
Voltage drop calculator
Voltage lost along a cable run.
Electricity cost calculator
What an appliance costs to run.
Salary calculator
UK take-home pay with tax code, student loan, pension and more.
Income tax calculator
UK income tax and National Insurance for 2026/27.
Mortgage calculator
Monthly payment, total interest and loan to value.