Ohm's Law Calculator

What It Does

This page answers one relation family — V = I × R together with P = V × I — by asking which two of the four quantities you already know. Choose the pair from the selector, enter its two values, and the page returns the other two together with the relation each was computed from. Under the fields sits a fixed table of the six relations: voltage from current and resistance, current from voltage and resistance, resistance from voltage and current, power from voltage and current, power from current and resistance, and power from voltage and resistance.

Real-World Usage

Use it to size a resistor, to work out what a supply will deliver into a load, or to check a figure an exam question gave you. It is the page to reach for when you know the voltage and the current and need the resistance, when you know a device draws a certain current at a certain voltage and you want to know how much heat it must shed, or when a power rating and a resistance are given and you need the voltage that drives them. Electrical work, electronics, physics homework and appliance specifications all ask these four quantities of each other.

Local Context

Ohm’s law is a property of the circuit and not of the country, so nothing here changes with your location and the page serves one set of answers everywhere — the relation is consolidated to a single page per language. Two conventions are worth knowing. In the input fields the decimal separator is the point, as in 0.5 and 2.5, whatever separator your language uses in ordinary prose; the result is formatted in your language’s own convention, so the same answer reads 3,333333333 in German and 3.333333333 in English. And an exact answer is printed in full, while a rounded one is marked as rounded beside its ten-significant-digit value.

Which two values do I need?

Any two of the four. Voltage and resistance give the current directly, voltage and current give the resistance, and current and resistance give the voltage; any pair that includes power gives the rest through P = V × I. Choose the pair you have from the selector and the page renders exactly those two fields.

Why is 12 V across 4 Ω exactly 3 A and 36 W?

Because the arithmetic is exact: 12 ÷ 4 is 3 and 12 × 3 is 36, and the page checks that the inverse operation returns the numbers it started from bit for bit before it calls a figure exact. Many calculators divide in floating point and print a stray digit at the end; here an inexact answer is rounded to ten significant digits and marked as rounded, so the difference between the two is visible on the page.

Why does the page refuse some values instead of printing a number?

Because some pairs have no finite answer. Zero resistance with a voltage across it is an unbounded current, and zero current with a voltage across it is an unbounded resistance — neither is a number, and printing Infinity for them would be a worse answer than saying so. A pair where both values are zero in a way that fixes neither unknown is refused as undefined for the same reason.

What do V, I, R and P stand for?

Voltage in volts, current in amperes, resistance in ohms and power in watts. Voltage is the potential difference across the component, current is the charge flowing through it each second, resistance is how much it opposes that flow, and power is the rate at which it turns electrical energy into heat or work.