Ohm's Law Calculator
Enter any two values (voltage, current, resistance or power) and get the other two instantly, with the formula and working shown.
Enter any two values. The other two are calculated instantly.
Working
- Enter two values to see the calculation.
How to use the Ohm's Law Calculator
- Type any two values, for example voltage and resistance. Pick the right unit (mA, kΩ, kW…) next to each box.
- Read the answer: the other two boxes fill in straight away, and the results panel shows every value with a sensible unit.
- Check the working under the calculator. The formulas used light up on the Ohm's law wheel.
- Change any value to recalculate. The calculator always uses the two values you typed most recently.
Ohm's law formulas
Ohm's law links voltage (V), current (I) and resistance (R): V = I × R. Combined with the power law P = V × I, you get 12 formulas, three for each quantity. They are the same formulas shown on the wheel above.
| To find | Formula 1 | Formula 2 | Formula 3 |
|---|---|---|---|
| Voltage (V), volts | V = I × R | V = P / I | V = √(P × R) |
| Current (I), amps | I = V / R | I = P / V | I = √(P / R) |
| Resistance (R), ohms | R = V / I | R = V² / P | R = P / I² |
| Power (P), watts | P = V × I | P = I² × R | P = V² / R |
Worked examples
Current through a resistor: a 9 V battery connected to a 470 Ω resistor gives I = 9 / 470 = 0.0191 A, which is about 19.1 mA. The resistor dissipates P = 9² / 470 = 0.172 W, so a standard ¼ W resistor is fine.
Current drawn by an appliance: a 2000 W heater on a 230 V supply draws I = 2000 / 230 = 8.7 A. On a 120 V supply the same heater would draw 16.7 A.
LED series resistor: to run an LED at 20 mA from 5 V when the LED drops 2 V, the resistor sees 3 V. R = 3 / 0.02 = 150 Ω, and it dissipates 3 × 0.02 = 0.06 W.
Units and prefixes
Pick a prefix next to each box instead of converting by hand: 1 kΩ = 1,000 Ω, 1 MΩ = 1,000,000 Ω, 1 mA = 0.001 A, 1 µA = 0.000001 A and 1 kW = 1,000 W. You can also type a prefix straight into a box, for example 4.7k or 20m.
Frequently asked questions
What is Ohm's law?
Ohm's law says the current through a conductor is proportional to the voltage across it: V = I × R, where V is voltage in volts, I is current in amps and R is resistance in ohms.
How do I calculate current from voltage and resistance?
Divide the voltage by the resistance: I = V / R. For example, 12 V across a 4 Ω resistor gives 12 / 4 = 3 A.
How do I calculate power?
Power in watts is voltage times current (P = V × I). You can also use P = I² × R or P = V² / R if you know resistance instead.
Why do I only need two values?
Voltage, current, resistance and power are linked by two equations (V = I × R and P = V × I), so any two known values fix the other two.
Does this work for AC circuits?
Yes for purely resistive AC loads such as heaters and kettles, using RMS voltage and current. Motors and other loads with reactance also need the power factor, which this calculator does not include.
What is the Ohm's law wheel?
It is a circle that shows all 12 formulas that link voltage, current, resistance and power. Each quarter shows the three ways to calculate one quantity.