Ohm's Law Calculator
Choose which two values you know and type them in. You get voltage, current, resistance and power, all four.
Your numbers
How we calculate it
Ohm's law says voltage equals current times resistance, V = I × R. The power law says power equals voltage times current, P = V × I. Putting them together gives the other forms: P = I² × R and P = V² ÷ R. Any two of the four values fix the other two.
For example, 12 V and 2 A give a resistance of 12 ÷ 2 = 6 Ω and a power of 12 × 2 = 24 W. Given 6 Ω and 24 W the calculator works out V = √(P × R) = 12 V and I = √(P ÷ R) = 2 A.
Assumptions and limits
- All units are the base units: volts, amps, ohms and watts. Convert first, so 20 mA is 0.02.
- Both values must be above zero, and they must be two different quantities.
- Ohm's law holds for simple resistors and DC circuits. Lamps, diodes, motors and AC circuits with reactance do not follow it so simply. Real parts need a safety margin on power, and mains work should be done by a qualified person.
Frequently asked questions
What is the formula for Ohm's law?
V = I × R. Voltage in volts equals current in amps times resistance in ohms. Rearranged, I = V ÷ R and R = V ÷ I.
How do I find the resistor for an LED?
Subtract the LED voltage from the supply, then divide by the current you want. For a 5 V supply, a 2 V LED and 20 mA, the resistor drops 3 V, so R = 3 ÷ 0.02 = 150 Ω. Enter 3 V and 0.02 A here.
How do I calculate power from voltage and resistance?
P = V² ÷ R. At 12 V across 6 Ω the power is 144 ÷ 6 = 24 W. Choose voltage and resistance as the two known values.
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Worked example (a fixed example using the starting numbers, it does not change as you type)
Inputs: First known value = Voltage (volts, V); First value = 12; Second known value = Current (amps, A); Second value = 2.
- Voltage 12 V
- Current 2 A
- Resistance 6 Ω
- Power 24 W
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