Ohm's Law Calculator

About this tool

Pick which two of voltage, current, and resistance you know, and this solves Ohm's law for the third and shows the power dissipation alongside it. Values update as you type.

The two laws. Ohm's law is V = I × R — voltage across a component equals the current through it times its resistance. The power law is P = V × I. Rearranging each gives you every combination: I = V ÷ R, R = V ÷ I, P = I² × R, and P = V² ÷ R. This tool computes power from the pair you entered, which is algebraically the same as any of those forms.

Units. Everything is in volts, amps, and ohms. The single most common mistake here is mixing milliamps with amps — if a datasheet gives a current of 20 mA, enter 0.02, not 20. Likewise a 4.7 kΩ resistor is 4700 Ω.

Worked example — an LED resistor. To run a red LED (about 2.0 V forward drop, 20 mA target) from a 5 V supply, the resistor must drop the remaining 3.0 V at 0.02 A: R = 3.0 ÷ 0.02 = 150 Ω. Its power is P = 0.02² × 150 = 0.06 W, so a standard ⅛ W or ¼ W resistor is fine. The LED series resistor calculator does this specific job end to end.

Worked example — heating in a wire. Push 10 A through 0.05 Ω of wire and it dissipates P = 10² × 0.05 = 5 W as heat. The current is squared in that formula, so doubling the current quadruples the heating — which is why wire gauge and length matter so much for high-current runs.

Where the simple law stops. Diodes, LEDs, and transistors are non-ohmic — their resistance isn't constant, which is why the LED example used a fixed forward voltage from the datasheet rather than a resistance. Under AC, capacitors and inductors add frequency-dependent reactance and the arithmetic moves into complex numbers. Resistor tolerance also matters: a 5% 150 Ω part is anywhere from 142.5 to 157.5 Ω.

For a two-resistor divider, use the voltage divider calculator; to check a resistor's wattage on its own, use the resistor power calculator. The guide to Ohm's law and the power triangle works through more examples.

Frequently asked questions

What units does this use?

Volts, amps, and ohms throughout. Convert first if your figures are in other prefixes — 20 mA is 0.02 A, 4.7 kΩ is 4700 Ω.

How is power calculated?

Always P = V × I, using the value you entered directly together with the one just solved for. That's equivalent to P = I²R or P = V²/R depending on which pair you supplied.

Why does it show a hint instead of a number for zero resistance?

Zero ohms with a non-zero current implies an undefined voltage relationship in some directions, so those combinations show a hint rather than a misleading result.

Does Ohm's law work for an LED?

Not directly. An LED is non-ohmic — its current rises steeply once it conducts, so you design around its datasheet forward voltage and pick a series resistor for the leftover voltage, as in the example above.

Does this apply to AC circuits?

For purely resistive AC loads, yes, using RMS values. Once capacitance or inductance is involved, resistance becomes impedance, which depends on frequency and shifts the phase between voltage and current.

How do I size a resistor's wattage?

Work out the power it dissipates (shown here as "Power") and choose a resistor rated comfortably above it — commonly at least double, to allow for tolerance and temperature.