Ohm's Law Calculator

What this calculates

Choose which two of voltage, current, resistance and power you know, enter them, and this works out the other two, with the formula for each shown step by step. All four appear in a grid that marks which values you entered and which were calculated.

It works in base units — volts, amps, ohms and watts — and refuses combinations that have no real answer, such as a voltage across zero resistance, rather than showing infinity.

The formula

Two relationships tie the four quantities together.

Rearranged, they give a pair of formulas for every combination of two known values.

Whichever two you start from, the same circuit comes out: every pair describes one set of four values.

Worked example

Start from any other pair of those four and you get the same circuit back. Given 12 V and 3 A, the resistance is 12 ÷ 3 = 4 Ω and the power 12 × 3 = 36 W. Given 3 A and 36 W, the voltage is 36 ÷ 3 = 12 V.

The square-root forms work the same way. For 240 Ω dissipating 60 W, the current is √(60 ÷ 240) = 0.5 A and the voltage √(60 × 240) = 120 V.

Not every answer comes out round. 9 V across 470 Ω gives 9 ÷ 470 = 0.01914894 A and 9² ÷ 470 = 0.1723404 W, each shown to seven significant figures.

When to use it, and the mistakes to avoid

Use it when choosing a resistor for a known voltage and current, when checking how much power a resistor has to handle, when working out the current a load draws from its voltage and power, and when checking a measurement against what the values say it should be.

The mistakes that cost the most:

• Entering milliamps or kilohms as if they were amps or ohms. This calculator takes base units. 20 mA is 0.02 A: through 220 Ω that is 4.4 V and 0.088 W. Entered as 20 A, the same resistor shows 4,400 V and 88,000 W — a thousand times the voltage and a million times the power. Likewise 470 kΩ is 470,000 Ω: 9 V across it drives 0.00001914894 A, while 470 Ω gives 0.01914894 A, a thousand times more.
• Using the wrong power formula for what you know. With voltage and resistance, the power is V² ÷ R — for 12 V and 4 Ω, 36 W. Dividing 12 by 4 gives 3, which is the current in amps, not the power. With current and resistance, the power is I² × R, still 36 W; multiplying 3 by 4 gives 12, which is the voltage.
• Entering a negative value. The calculator works with the size of each quantity and refuses a minus sign. A negative voltage or current describes a direction, which depends on how the circuit is drawn, and a resistor's resistance and power are never negative.
• Expecting an answer from zero resistance or zero current. Zero resistance with a voltage across it is a short circuit, and a voltage with no current is an open circuit; neither has a finite answer, and the calculator says which one it has found.
• Assuming every component obeys Ohm's law. V = I × R describes a resistance that stays the same as the voltage and current change. Many real conductors change resistance as they heat up, and some components do not behave like a fixed resistance at all, so the figures here describe the ideal case.

FAQ