TheCalculatory
Science & Engineering · Electrical Formula verified

Ohm's Law Calculator

Pick which quantity to solve for, enter the other two, and see the result instantly — power is calculated alongside every answer, with the formula and a step-by-step proof underneath.

Solve for
A
0A25A50A
Ω
0Ω500Ω1000Ω
On-screen number pad
Tap to type into the highlighted field
Voltage
12.00 V
I = 2A, R = 6Ω
Power
24.00 W

Step-by-step proofCheck by hand

How voltage, current and resistance relate

Ohm's Law says voltage equals current times resistance — V = IR. Rearranging that one equation gives the other two: current equals voltage divided by resistance, and resistance equals voltage divided by current. Power, from the separate but related power law (P = VI), is shown alongside every result regardless of which two quantities you entered.

The formulas

V = IR  ·  I = V ÷ R  ·  R = V ÷ I  ·  P = VI

Worked example

A 2A current through a 6Ω resistor: voltage = 2 × 6 = 12.00 V, and power = 12 × 2 = 24.00 W.

Frequently asked questions

What is Ohm's Law?
Ohm's Law states that the voltage across a conductor is directly proportional to the current flowing through it, with resistance as the constant of proportionality: V = IR. It's named after Georg Ohm, who published the relationship in 1827, and it's one of the most fundamental equations in electrical engineering.
How is power calculated if I only enter two of V, I, R?
Power (P = VI) technically needs voltage and current, but this calculator always has enough information to find it regardless of which two quantities you entered — if you gave current and resistance, it first works out voltage from Ohm's Law, then multiplies. The page shows power alongside the main answer either way.
What units does this calculator use?
Volts (V) for voltage, amps (A) for current, ohms (Ω) for resistance, and watts (W) for power — the standard SI units for each quantity, and the ones Ohm's Law and the power law are defined in.
Sources and method
  • Ohm's Law (V = IR) and the electrical power law (P = VI) — standard published physics and electrical engineering.
Last reviewed: 20 Sep 2026 Sources last verified: 20 Sep 2026 Results use the assumptions explained on this page. Report an error How we check calculations

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