Ohm's law and power formulas
V = I × R I = V ÷ R R = V ÷ I
P = V × I P = I² × R P = V² ÷ R
V is voltage in volts, I is current in amps, R is resistance in ohms and P is power in watts.
Worked examples
- A 12 V, 2 A light: R = 12 ÷ 2 = 6 Ω, and P = 12 × 2 = 24 W.
- A cable with 0.01 Ω resistance carrying 50 A loses 50² × 0.01 = 25 W as heat and drops 0.5 V.
- A 1 kΩ resistor across 12 V passes 12 mA and dissipates 0.144 W.
Where Ohm's law applies
These formulas apply exactly to resistive loads and DC circuits. For AC motors and electronics, power also depends on power factor; see the watts to amps calculator.
Frequently asked questions
What is Ohm's law?
Voltage equals current multiplied by resistance (V = I × R). It describes how voltage, current and resistance relate in a circuit.
How do I calculate watts from volts and amps?
Multiply them: watts = volts × amps. 12 V at 5 A is 60 W.