Enter any two values below — voltage, current, resistance, or power — and the other two are calculated instantly. The most fundamental relationship in electronics.
Ohm's Law states that the current through a conductor between two points is directly proportional to the voltage across the two points. The constant of proportionality is resistance: V = I · R.
Power dissipated by a resistive element is the product of voltage and current: P = V · I. Combining the two gives P = I²·R = V²/R.
Tip: when sizing a resistor, always check its power rating against the calculated P. A 1/4 W resistor will fail in seconds at 500 mW.