CODDY Tool

Ohm's Law Calculator

Enter any two of voltage, current, resistance and power to find the other two.

Voltage, current, resistance and power are all related by two equations. Enter any two and this solves for the remaining two — no need to remember which rearrangement of V=IR or P=VI applies.

Everything you enter stays in this browser tab. Nothing is sent to our servers, logged or stored.

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About Ohm's Law Calculator

Ohm's law, V=IR, relates voltage, current and resistance. Combined with the power equation, P=VI, every one of the four quantities can be found from any two of the others — six equations in total, and remembering which one to use for which pair is the actual friction, not the arithmetic.

This sidesteps that by taking whichever two values are filled in and solving directly, showing all four once it has an answer. Filling in exactly two is required rather than allowed: with three or four given values there is no single pair to solve from, and if they were not perfectly consistent with each other, picking one pair silently would hide that inconsistency rather than surface it.

How to use the Ohm's law calculator

  1. Enter two values

    Any two of voltage, current, resistance or power.

  2. Leave the other two blank

    Filling in a third turns off the calculation until you clear one.

  3. Read the result

    All four values are shown once exactly two are entered.

Frequently asked questions

Why do I have to enter exactly two values?

With two values there is exactly one way to solve for the rest. With three or four, the extra values could be inconsistent with each other, and silently picking a pair to trust would hide that rather than tell you about it — so the calculator asks for exactly two instead.

What is the relationship between power and the other three?

P=VI is the direct one. Substituting V=IR or I=V/R into it gives the two other common forms: P=I²R and P=V²/R, which is why power can be found from resistance paired with either voltage or current.

Does this work for AC circuits?

It works for resistive AC circuits using RMS voltage and current, the same way it works for DC. It does not account for reactance from capacitors or inductors, which needs impedance (a complex quantity) rather than plain resistance.

Why do my calculated values look slightly off from what I measured?

Real resistors carry a tolerance — a 5% part can genuinely be anywhere within 5% of its marked value — and a multimeter has its own measurement error on top of that. A calculated result a few percent from a measured one is normal, not a sign either is wrong.

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Ohm's Law Calculator is free to use with no account, no watermark and no usage limits. Last updated 3 September 2026.