Ohm's law calculator

Fill in any two of voltage, current, resistance and power. You get the other two, the route that produced them, and the component you could actually buy. Nothing you type here leaves your browser — it is worked out on your own device, so there is nothing to send and nothing to log.

Two of four, and there are twelve formulas

Everything on the wheel chart is a rearrangement of two statements: V = IR and P = VI. Give any two of the four quantities and the other two follow, by one of twelve routes. This page names the route it took, so you can check it rather than trust it.

The answer is a number, not a component

Ohm's law will happily tell you that you need 1,847 Ω. Nobody makes one. Resistors come in the E series — a geometric ladder of preferred values spaced so that consecutive values just overlap within their tolerance — and the nearest E24 value to 1,847 is 1.8 kΩ, 2.5% away.

So every answer here comes with the value you could actually buy, and the tolerance band that value really covers. A 5% resistor marked 1 kΩ is anywhere between 950 and 1,050 Ω, which makes a result quoted to four decimal places a fiction.

The E series does not match its own formula

The preferred values are supposed to be 10^k/n rounded to two significant figures. Eight of the twenty-four E24 values are not. The formula gives 2.61, 2.87, 3.16, 3.48, 3.83, 4.22, 4.64 and 8.25; the standard prints 2.7, 3.0, 3.3, 3.6, 3.9, 4.3, 4.7 and 8.2. IEC kept the historical numbers in the 1950s because too much already existed with them printed on it, and the standard has disagreed with its own construction ever since.

This is direct current and a resistive load

V = IR holds for alternating current too, but only if you replace resistance with impedance, which has a phase angle attached. Anything with capacitance or inductance in it — a motor, a transformer, a mains filter — draws current out of step with the voltage, and then power is no longer simply V × I. These pages do not carry any of that.

What it will not do

It will not do alternating current, impedance or power factor. See above.
It will not know how hot the resistor gets. It gives the power dissipated and the wattage you should buy; the rest is airflow.
It will not tell you whether the circuit is safe. Mains voltage is a separate subject and it is not this one.