Tool · Electrical

Ohm's Law Calculator

Pick the two values you know — voltage (V), current (I), resistance (R), or power (P) — and I'll work out the other two.

I know…

and…

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Volts (V)

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Amps (I)

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Ohms (Ω)

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Watts (P)

Important: this covers the basic DC (and simple resistive) relationship. Real AC work — 3-phase, power factor, voltage drop, and code-required wire sizing — involves more. Always follow your local electrical code and work with a licensed electrician for real installations.

The formulas

Ohm's law ties voltage, current, and resistance together. Power adds a fourth piece. A handy water analogy: voltage is the pressure, current is how much water flows, and resistance is how narrow the pipe is.

V = I × R

I = V ÷ R

R = V ÷ I

P = V × I

P = I² × R

P = V² ÷ R

Example: a 120 V circuit drawing 10 A

R = 120 ÷ 10 = 12 Ω

P = 120 × 10 = 1200 W

Related: Voltage drop calculator →