Ohm's Law Calculator

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Solve voltage, current, resistance and power instantly. Enter any two values and the calculator works out the other two using Ohm's law and the power equations.

Fill in any two values — the other two are calculated automatically.

V
A
Ω
W
V Voltage I Current R Resistance
Voltage –
VVoltage –
ICurrent –
RResistance –
PPower –
Uses V = I × R and P = V × I (with P = I²R and P = V²/R). Values assume a purely resistive DC circuit at a steady operating temperature.

About Ohm's Law Calculator

The Ohm’s law calculator solves the four fundamental electrical quantities — voltage, current, resistance and power — from any two you know. Type two values and the remaining pair is computed automatically, with the tool telling you which relationship it used.

The math combines Ohm’s law, V = I × R, with the power equations P = V × I, P = I²R and P = V²/R. Because every pair of knowns maps to a unique solution, you can start from a supply voltage and load resistance just as easily as from a measured current and power rating.

Results assume a purely resistive DC circuit at a steady operating temperature — the textbook case that covers resistors, heating elements and LED series calculations. It is a free, instant companion for electronics students, hobbyists sizing components and technicians double-checking bench measurements.

How to use Ohm's Law Calculator

  1. Enter any two of the four values: voltage, current, resistance or power.
  2. The other two are calculated automatically as you type.
  3. Check the “Solved from” note to see which values were treated as given.
  4. Press Clear to start over with a different pair.

Frequently asked questions

Ohm’s law states that voltage equals current times resistance: V = I × R. Combined with the power relation P = V × I, it lets any two of the four quantities determine the other two.

Use P = I²R: the current squared times the resistance. A 2 A current through a 10 Ω resistor dissipates 40 W. Enter the two knowns and the calculator applies the right identity for you.

Two independent quantities fully determine a resistive circuit’s operating point. With only one value the system is underdetermined, and a third input could contradict the first two — so the calculator solves from a pair.

Only for purely resistive AC loads. Circuits with inductance or capacitance involve impedance and phase angle, which plain Ohm’s law does not capture — use an impedance calculation for those.

Resistance changes with temperature — a lamp filament, for instance, has far higher resistance hot than cold. The calculator assumes a steady operating temperature, so use values measured under real operating conditions.
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