Ohm's Law Calculator
Enter any two of voltage, current, resistance and power, and the other two are solved for you — with the formula used shown alongside.
What You Know
Pick any two of the four quantities and the other two are worked out for you.
Using: R = V / I and P = V × I
Results
Resistance
The Core Relationships
Ohm's law: voltage equals current times resistance.
The power law: power equals voltage times current.
Power dissipated when you know current and resistance.
Power dissipated when you know voltage and resistance.
Every other form is these four rearranged. Units are volts (V), amps (A), ohms (Ω) and watts (W).
Ohm's law applies to DC circuits and to resistive loads. Reactive AC circuits, semiconductors and filament lamps do not hold a constant resistance, so treat the result as the ideal resistive case.
Frequently Asked Questions
- What is Ohm's law?
- Ohm's law states that the current through a conductor between two points is directly proportional to the voltage across those points, written as V = I x R. Voltage is measured in volts, current in amps and resistance in ohms. It was published by Georg Ohm in 1827 and is the single most used relationship in electronics.
- How do I find power if I only know voltage and resistance?
- Use P = V squared divided by R. For example, a 120 volt supply across a 60 ohm heating element dissipates 14400 divided by 60, which is 240 watts. You do not need to work out the current first, though it would be 2 amps if you did.
- Why do I need to know two values before the others can be found?
- Voltage, current, resistance and power are linked by two independent equations, V = I x R and P = V x I. Two equations can pin down two unknowns, so supplying any two of the four quantities fixes the remaining two. A single value leaves infinitely many possible circuits.
- What happens if I enter a resistance of zero?
- Zero resistance is a short circuit, and the maths breaks down because current would be infinite for any voltage. The calculator returns no result instead of showing infinity. In a real circuit a short across a supply draws whatever current the source and the wiring resistance allow, which is why fuses and breakers exist.
- Does Ohm's law work for AC circuits?
- Only for purely resistive loads. Once capacitance or inductance is present, the opposition to current is impedance rather than plain resistance, and voltage and current fall out of phase. Power then has to account for the power factor, so real power is less than volts times amps. Use this calculator for DC and for resistive AC loads such as heaters and incandescent lamps.
- Which components do not obey Ohm's law?
- Any component whose resistance changes with the applied voltage or current is non-ohmic. Diodes, transistors, thermistors and filament lamps are all examples. A tungsten filament can be ten times more resistive when hot than when cold, which is why lamps draw a large inrush current at switch-on.