Ohm's Law Calculator

Ohm's Law Formulas

V = I × R  |  I = V ÷ R  |  R = V ÷ I  |  P = V × I = I² × R = V² ÷ R

Units

Voltage (V) is measured in volts, current (I) in amperes (amps), resistance (R) in ohms (Ω), and power (P) in watts (W). Household circuits in North America run at 120 V or 240 V.

Circuit Safety

A 15A circuit breaker protects up to 1,800 W (120 V × 15 A), but NEC guidelines recommend loading no more than 80%, or 1,440 W. A 20A circuit handles 2,400 W, with an 80% limit of 1,920 W.