Ohm's Law Calculator

Ohm's Law and power

V = I × R

P = V × I = I² × R = V² / R

Ohm's Law links voltage (V, volts), current (I, amps), and resistance (R, ohms); the power law adds power (P, watts). These four quantities are tied together so tightly that knowing any two of them is enough to find the other two. Enter the two you know and the calculator fills in the rest.

How it works

Voltage is the electrical "push," current is the resulting flow of charge, and resistance is how much the component holds that flow back. The law says the current through a resistor is directly proportional to the voltage across it and inversely proportional to its resistance — double the voltage and the current doubles; double the resistance and the current halves. The three power forms all give the same watts; you just pick whichever uses the two quantities you already have, so you never need to solve for a third value first.

Two worked examples

Finding current and power. With 5 V across a 1 kΩ resistor, the current is 5 V ÷ 1000 Ω = 5 mA, and the power is 5 V × 5 mA = 25 mW — so an ordinary ⅛ W resistor is comfortable here.

Working back from a power rating. Suppose a resistor is rated ¼ W and sits across 12 V. The most it can safely dissipate gives a minimum resistance of R = V² / P = 144 / 0.25 = 576 Ω, so you would choose the next standard value above that (680 Ω) to stay within the rating.

Common mistakes

The usual slip is mixing unit prefixes — entering 5 mA as "5" instead of "5m", or 1 kΩ as "1" instead of "1k". Amps and milliamps differ by a factor of 1000, so a prefix error throws the answer off by three orders of magnitude. The other trap is reaching for the wrong power form: if you only know current and resistance, use P = I²·R, not P = V²/R, so you don't have to compute the voltage first and risk rounding it twice.

Where it's used

Ohm's Law is the starting point for almost every DC calculation on this site — sizing anLED series resistor, setting avoltage divider, or checking the drop along a wire run.

It applies to ohmic components only

Ohm's law describes components whose resistance is constant, and plenty are not. A filament lamp's resistance rises sharply as it heats, so its cold-start current is many times its running current. Diodes and LEDs are exponential rather than linear — which is why they need aseries resistor instead of a voltage. Even ordinary resistors drift with temperature, at a rate given by their temperature coefficient. For the everyday case of a resistor at a steady temperature the law is exact; outside that, it is a starting point.