Ohm's Law & Power Calculator

Ohm’s law states that current through a conductor is proportional to the voltage across it, giving the three arrangements V = I × R, I = V ÷ R and R = V ÷ I. Add the power relationship P = V × I and its substitutions P = I²R and P = V² ÷ R, and any two known quantities out of voltage, current, resistance and power determine the other two. That is all the power wheel is: twelve formulas covering every pairing. The calculator shows which one it used for each result, so it stays useful as a working tool rather than a black box.

A worked example

Suppose a 12 V fan draws 250 mA. Its effective resistance is 12 ÷ 0.25 = 48 Ω, and it dissipates 12 × 0.25 = 3 W. Drop that fan onto a 5 V rail instead and, treating the resistance as fixed, current falls to about 104 mA and power to roughly 0.52 W — a sixth of the original. Halving the voltage quartering the power is the single most useful intuition the power relationships give you, and it explains why supply-rail mistakes produce such dramatic thermal results in both directions.

Where the answers stop applying

Ohm’s law describes linear resistances. Semiconductors are not linear: an LED does not have a resistance you can divide into, which is why it needs a series resistor sized from the voltage across that resistor rather than from the supply voltage alone. Filament lamps and motors change resistance dramatically as they warm up or load up, so a cold measurement understates their running behaviour. Anything reactive — a capacitor or an inductor on AC — needs impedance rather than plain resistance.

Two mistakes worth avoiding

The first is unit slippage: entering milliamps where the formula wants amps is a factor of a thousand and produces answers that look plausible on a rail sized in volts. The second is treating a calculated power figure as a component rating. A resistor dissipating 0.4 W in a 0.5 W package will run hot enough to discolour a board and drift in value over time; give it a comfortable margin, and more still if it sits in still air or beside other warm parts.

Sizing a resistor for an LED specifically? The LED series resistor calculator applies this arithmetic and rounds to the nearest E24 value for you.