Ohm’s Law Calculator
Enter any two of voltage, current, resistance and power to calculate the other two using Ohm’s law and the power formula.
Ohm’s law and power formulas
V = I × R I = V ÷ R R = V ÷ I P = V × I = I² × R = V² ÷ R
Voltage is measured in volts (V), current in amperes (A), resistance in ohms (Ω) and power in watts (W).
Worked example
A 12-volt supply across a 4-ohm resistor drives a current of 12 ÷ 4 = 3 A, and the resistor dissipates 12 × 3 = 36 W.
For learning and estimating only. Mains electricity is dangerous; follow local electrical codes and use a qualified electrician for wiring work.
Frequently asked questions
What is Ohm’s law?
The current through a conductor equals the voltage across it divided by its resistance: I = V ÷ R.
How do I calculate amps from watts?
Divide watts by volts. A 60 W bulb on 120 V draws 0.5 A.
Does Ohm’s law apply to everything?
It applies to ohmic components like resistors. Diodes, bulbs heating up and motors do not follow it exactly.
What is the power triangle?
A memory aid for P = V × I, showing how power, voltage and current relate.
Related calculators
- kWh CalculatorEnergy use and running cost of an appliance.
- Horsepower CalculatorHorsepower from torque and RPM, plus kW.
- Speed Distance Time CalculatorSolve for speed, distance or time.
- Significant Figures CalculatorCount sig figs and round to significant figures.
- Factor CalculatorList all factors, factor pairs and the prime factorization.
- Area CalculatorArea of rectangles, circles, triangles and 5 more shapes.