How to use Ohm's law
- Fill in exactly two of Voltage, Current, Resistance and Power, in volts, amps, ohms and watts.
- Leave the other two boxes empty. If you've filled in three, clear one.
- Read all four values, shown with prefixes such as mA and kΩ. You can type prefixes too: 4.7k for 4.7 kΩ, 20m for 20 mA, 10µ (or 10u) for 10 µA, 2.2M for 2.2 MΩ.
The four formulas
Ohm’s law links voltage, current and resistance, and the power law adds watts. Between them, any two values fix the other two:
| To find | From V and I | From V and R | From I and R | From P and one other |
|---|---|---|---|---|
| Voltage (V) | given | given | I × R | P ÷ I, or √(P × R) |
| Current (I) | given | V ÷ R | given | P ÷ V, or √(P ÷ R) |
| Resistance (R) | V ÷ I | given | given | V² ÷ P, or P ÷ I² |
| Power (P) | V × I | V² ÷ R | I² × R | given |
With the defaults, 12 V and 6 Ω, you get I = 12 ÷ 6 = 2 A and P = 12 × 2 = 24 W. A 100 W signal into a 4 Ω speaker works the other way: I = √(100 ÷ 4) = 5 A and V = √(100 × 4) = 20 V.
Example: choosing an LED resistor
An LED needs a resistor in series to limit its current. Say the supply is 5 V, the LED drops about 2 V and you want 20 mA through it (check your LED’s datasheet for its own figures). The resistor has to take the remaining 3 V:
- Enter Voltage 3 and Current 0.02
- Resistance = 3 ÷ 0.02 = 150 Ω
- Power = 3 × 0.02 = 0.06 W
Resistors come in standard values, so pick the nearest one at or above the result, and make sure its power rating is comfortably above the power shown. A common 0.25 W resistor is fine here.