Ohm's law calculator

Reviewed by the Calcwise engineering team ·

Fill in any two of the four fields and leave the other two empty. The calculator applies Ohm's law and the power formula.

V
A
Ω
W
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How it is calculated

V = I · R
P = V · I = I² · R = V² / R

Any pair of values gives the other two. For example, with voltage and power: I = P / V and R = V² / P.

These formulas apply to DC and to resistive AC loads (power factor 1). For motors or equipment with a lower power factor, use the watts to amps calculator.

Frequently asked questions

How many amps is 1500 W at 120 V?

I = 1500 / 120 = 12.5 A for a resistive load such as a space heater.

What is the resistance of a 1500 W heater at 120 V?

R = 120² / 1500 = 9.6 Ω when hot.

Does Ohm's law work for AC?

Yes for resistive loads. For motors, fluorescent lights or switch-mode power supplies, include the power factor.

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