Ohm's Law Calculator

Ohm's Law (V = I × R) combined with the power equation (P = V × I) lets you solve any electrical circuit value from any two others. This calculator handles all six combinations: V+I, V+R, V+P, I+R, I+P, R+P. Pick the pair you know, enter values with sensible unit prefixes (mV, A, kΩ, mW, etc), and the other two appear with the formula used. Useful for resistor sizing, LED current limiting, power supply calculations, and basic electronics homework.

The law describes an ohmic component, which is a resistor and not much else. An LED is a diode: its voltage barely moves while its current climbs steeply, so there is no resistance value to plug in and connecting one directly across a supply destroys it. The standard approach is a series resistor sized from the voltage left over, (supply − forward voltage) / desired current, and it is that subtraction rather than the raw supply voltage that goes into the calculation. Motors, batteries and semiconductor junctions are all non-ohmic in the same way.

The power figure is the one that decides whether a part survives. A quarter-watt resistor dissipating 0.4 W does not fail immediately; it runs hot, drifts out of tolerance and dies later, which makes it a fault that appears weeks after the build. Derating to about half the rated dissipation is the usual habit.

Resistance is also not a constant across conditions. A resistor's value shifts with temperature by its stated coefficient, and a filament or a fuse changes by a large factor between cold and hot, which is why an incandescent lamp draws a surge at switch-on that the cold-resistance calculation predicts and the hot one does not.

Everything here is direct current. With alternating current the resistance term becomes impedance, phase separates real power from apparent power, and a power factor enters the arithmetic, none of which this models.