Ohm’s Law Calculator
Enter any two of voltage (V), current (I), resistance (R) and power (P), and the calculator works out the other two.
How to use the Ohm’s Law Calculator
- Fill in exactly two of the four boxes.
- Leave the other two empty.
- Read all four values.
How it works
Ohm’s law relates voltage, current and resistance: V = I × R. Combined with the power formula P = V × I, any two values determine the rest:
- I = V ÷ R, R = V ÷ I
- P = I² × R = V² ÷ R
- I = √(P ÷ R), V = √(P × R)
Ohm’s law holds for resistive components such as resistors and heating elements; diodes, LEDs and motors are non-linear.
Ohm’s law formula wheel
| To find | Formulas |
|---|---|
| Voltage (V) | I × R, P ÷ I, √(P × R) |
| Current (I) | V ÷ R, P ÷ V, √(P ÷ R) |
| Resistance (R) | V ÷ I, V² ÷ P, P ÷ I² |
| Power (P) | V × I, I² × R, V² ÷ R |
Frequently asked questions
What is Ohm’s law?
Voltage equals current times resistance: V = I × R.
What resistor do I need for an LED?
R = (supply voltage − LED voltage) ÷ LED current. For a 2 V, 20 mA LED on 5 V: (5 − 2) ÷ 0.02 = 150 Ω.
How much current flows through a 100 Ω resistor at 12 V?
0.12 A (120 mA), dissipating 1.44 W.
What are the units?
Volts (V), amps (A), ohms (Ω) and watts (W).