Ohm Law Calculator (V/I/R/P)

Ohm's law with power (V, I, R, P)

Ohm's law ties voltage, current and resistance together as V = R · I. Bring power into it and you get three equivalent ways to write the same thing: P = V · I = R · I² = V² / R. Give the calculator any two of the four quantities (V, I, R, P) and it works out the rest. You'll often see this drawn as the Ohm-Watt wheel or the VIRP triangle. Example: hook a 12 V battery to a 4 Ω resistor and you get I = V/R = 3 A, with P = V·I = 36 W turning into heat. Want to double-check it? P = V²/R = 144/4 = 36 W lands on the same number.

Applications

This is where pretty much all electronics work begins. You lean on it to size wire gauge (NEC / IEC 60364 / NBR 5410 in Brazil) so the I²R losses don't cook the cable, to choose circuit breakers and fuses, and to pick a resistor wattage that won't go up in smoke. It also tells you battery runtime from capacity (Wh) and load (W), and how big a heat sink a power transistor needs. Every voltage divider, LED driver, motor controller and PSU you size traces back to these few equations.

FAQ

AC or DC? The equations hold at every instant for either one. With AC, plug in RMS values for V and I, and swap R for the impedance Z when there's reactance in the mix.

Does Ohm's law apply to everything? No. It only holds for ohmic parts like resistors and wires. Diodes, LEDs, transistors and lamps are non-linear, though even there the law still describes things well enough if you stay near a fixed operating point.

Why does my resistor get hot? Because the P = R·I² it dissipates leaves as heat. Choose a part rated comfortably above the P you calculated, usually with about a 2x cushion.