Ohm's law calculator

Any two give the other two

Ohm's law ties four quantities together: voltage in volts, current in amps, resistance in ohms and power in watts. Give it any two and the rest follow:

• V = I x R and I = V / R and R = V / I
• P = V x I, or P = I squared x R, or P = V squared / R

The tool shows which two formulas it used, so the answer can be checked by hand rather than trusted.

Units are converted before the arithmetic

This is where these calculators usually go wrong. Enter 5 volts and 10 milliamps and the answer is 500 ohms, not 0.5 — but only if the milliamps are converted to amps first. Everything here is normalised to base units, calculated, then converted back to whatever reads most naturally, so a milliamp is never quietly treated as an amp.

Division by zero is refused, not printed

Zero resistance with a voltage across it is a short circuit: the equation gives infinite current, which is not an answer. Zero current with a voltage present means an open circuit, where resistance is undefined rather than infinite. Both are explained rather than shown as "Infinity".

Example

12 V across 4 ohms draws 3 A and dissipates 36 W. A 9 V supply through a 2.2 kilohm resistor draws about 4.1 mA and dissipates about 37 mW — comfortably inside a quarter-watt resistor.

What this does not model

This is direct current and purely resistive. Alternating current adds reactance, impedance and power factor; a real component's resistance changes with temperature; and a power supply has its own internal resistance. For sizing a resistor or a fuse on a DC circuit it is exactly right, and for designing a filter it is not the tool.

Related tools

See the number base converter and the percentage calculator.