Ohm's law and the power formulas
Ohm's law links the voltage across a resistor, the current through it and its resistance: V = I × R. Combine it with the power equation P = V × I and you can work out all four quantities as soon as you know any two of them. That is what this calculator does.
The four quantities
- Voltage (V), in volts: the electrical pressure pushing charge around the circuit.
- Current (I), in amperes: how much charge flows per second.
- Resistance (R), in ohms (Ω): how strongly a component opposes the current.
- Power (P), in watts: the rate at which energy is turned into heat (or light, or motion).
All twelve formulas
| To find | From V and I | From V and R | From V and P | From I and R | From I and P | From R and P |
|---|---|---|---|---|---|---|
| V | — | — | — | I × R | P ÷ I | √(P × R) |
| I | — | V ÷ R | P ÷ V | — | — | √(P ÷ R) |
| R | V ÷ I | — | V² ÷ P | — | P ÷ I² | — |
| P | V × I | V² ÷ R | — | I² × R | — | — |
Entering values
You can type SI prefixes instead of counting zeros: 4.7k is 4,700 Ω, 20m is 0.02 A (20 mA), 250m is 0.25 W and 1M is one megaohm. The resistor-style shorthand 4k7 works too. To change which values are inputs, just start typing in a calculated box; it becomes an input and the oldest input becomes calculated.
A worked example
A 470 Ω resistor across a 12 V supply carries 12 ÷ 470 = 25.5 mA and dissipates 12² ÷ 470 = 0.306 W. A standard 0.25 W resistor would run too hot here, so you would pick a 0.5 W part. As a rule of thumb, choose a resistor rated for at least twice the power it will dissipate.
Where Ohm's law applies
The relationship holds for ordinary resistors and wires at a steady temperature. It does not describe diodes, LEDs, transistors or lamps, whose resistance changes with the voltage across them. For AC circuits with capacitors or inductors you need impedance rather than plain resistance, although the same formulas still work for purely resistive loads using RMS values.
