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Ohm's Law Calculator
Two values in, the third out — plus power. Pick what to calculate, enter the other two, done.
Calculated value
How it works
V = I × R I = V ÷ R R = V ÷ I Power P = V × I (watts)
Worked example: 12 volts driving 2 amps through a circuit means resistance 12 ÷ 2 = 6 Ω and power 12 × 2 = 24 watts. Change the voltage to 24 volts at the same 2 amps and resistance becomes 12 Ω with power 48 watts.
Frequently asked questions
What is Ohm’s law, in plain words?
Voltage pushes, current flows, resistance holds back. Double the voltage and, for the same resistance, double the current flows; double the resistance and half the current flows.
Which value should I calculate?
Whichever you do not know. Enter the two you do know, pick the missing one above, and the calculator fills it in plus the power.
What if the current is zero?
Resistance and power are shown as 0, not an error. With no current flowing there is no power, and resistance cannot be worked out from zero current.
Does this work for mains AC circuits?
Only as an approximation for simple resistive loads like heaters. Real AC circuits with motors or capacitors need impedance, not plain resistance, so treat mains answers as a guide.
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