Ohm's Law Calculator
Enter any two of voltage, current, resistance and power — in any unit prefix — and get the other two with the formulas and working shown.
Ohm's Law Calculator
Fill in exactly two boxes; leave the other two empty.
Example: 12 V across 6 Ω. Type any two values of your own.
Working
- I = V ÷ R = 12 V ÷ 6 Ω = 2 A
- P = V² ÷ R = 12² ÷ 6 = 144 ÷ 6 = 24 W
- Check: P = V × I = 12 V × 2 A = 24 W ✓
Safety: household mains (120 V or 230 V) can kill, and a correct calculation does not make a circuit safe. Use this calculator to learn and to design low-voltage circuits; leave work on fixed wiring and mains appliances to a qualified electrician.
How to use the Ohm’s law calculator
Type values into any two of the four boxes and pick a unit prefix for each — millivolts or kilovolts, microamps or milliamps, milliohms to megohms, milliwatts to kilowatts. The calculator converts everything to volts, amperes, ohms and watts, solves for the two missing quantities and shows each formula with your numbers substituted, followed by an independent check using a formula it did not use. If you set a unit on an empty box, the answer for that quantity is also shown in that unit. The page address updates as you type, so you can bookmark or share a calculation.
The formulas
Everything follows from two relationships: Ohm’s law, V = I × R, and the power law, P = V × I.
Substituting one into the other gives P = I² × R and P = V² ÷ R, and rearranging all four gives
the full set of 12:
| To find | From V and I | From V and R | From V and P | From I and R | From I and P | From R and P |
|---|---|---|---|---|---|---|
| Voltage V | — | — | — | I × R | P ÷ I | √(P × R) |
| Current I | — | V ÷ R | P ÷ V | — | — | √(P ÷ R) |
| Resistance R | V ÷ I | — | V² ÷ P | — | P ÷ I² | — |
| Power P | V × I | V² ÷ R | — | I² × R | — | — |
A dash means that quantity is one of the two you entered.
Worked example: an LED series resistor
A red LED on a 5 V supply should run at 20 mA, and its datasheet gives a forward voltage of about 2 V. The resistor has to drop the remaining 5 − 2 = 3 V. Enter 3 V and 20 mA:
- Convert to base units: 20 mA = 0.02 A.
- R = V ÷ I = 3 V ÷ 0.02 A = 150 Ω.
- P = V × I = 3 V × 0.02 A = 60 mW.
- Check: P = I² × R = 0.02² × 150 = 60 mW ✓
150 Ω is a standard resistor value, and a common ¼ W (250 mW) resistor has plenty of margin over 60 mW. The LED itself is not a resistor — its voltage stays roughly constant as current changes — which is exactly why it needs a series resistor to set the current.
A mains example: what a 2 kW heater draws
Enter 230 V and 2 kW. The calculator gives I = P ÷ V = 2,000 W ÷ 230 V = 8.69565 A and R = V² ÷ P = 230² ÷ 2,000 = 26.45 Ω. The resistance is the element’s value when hot; measured cold with a multimeter it will read lower, because the resistance of most metals rises with temperature. For the running cost of the same heater, use the electricity cost calculator.
Common mistakes
- Mixing prefixes. 12 V ÷ 4.7 kΩ is 2.55 mA, not 2.55 A. Choosing the prefix in the dropdown avoids the slip.
- Forgetting power ratings. A resistor that is the right value but dissipates more than its rating will overheat. Always check P.
- Using nameplate figures for motors. Motors and power supplies are not simple resistors; their current depends on load and power factor.
- Treating wire as zero resistance. Long runs of thin wire drop real voltage — look up the resistance per meter in the AWG wire gauge chart and include it as R.
Is it safe?
The formulas are the same at 5 V and at 230 V, but the consequences are not. A current far smaller than one ampere through the body can stop the heart, and mains circuits can deliver much more than that. Switch off and isolate before touching any circuit, never work live on mains wiring, and follow your local wiring rules. Unit conversions for related quantities are in the millivolts to volts and watts to kilowatts converters, and more tools are in the electricity section.
Frequently asked questions
What is Ohm’s law?
Ohm’s law says that the current through a resistor is proportional to the voltage across it: V = I × R, where V is in volts, I in amperes and R in ohms. Rearranged, I = V ÷ R and R = V ÷ I. Combined with the power law P = V × I, it gives the 12 formulas in the wheel.
How do I calculate watts from volts and amps?
Multiply them: P = V × I. A device drawing 0.5 A from a 12 V supply uses 12 × 0.5 = 6 W. If you know the resistance instead of the current, use P = V² ÷ R.
How many amps does a 2,000 W appliance draw?
Divide the power by the voltage: on a 230 V supply, 2,000 ÷ 230 ≈ 8.7 A; on a 120 V supply, 2,000 ÷ 120 ≈ 16.7 A. That is why the same kettle needs a much heavier circuit in North America than in Europe or the UAE.
Does Ohm’s law work for AC circuits?
For purely resistive loads such as heaters, kettles and incandescent lamps, yes — use RMS voltage and current. Motors, transformers, LED drivers and other electronics have reactance, so their current depends on impedance rather than resistance and real power is P = V × I × power factor. The calculator’s results then describe apparent values only.
Why can’t I enter three values?
Two values fix the other two completely, so a third value can only agree or contradict. If you have three measurements, enter the two you trust most and compare the calculated third value with your reading — a large difference usually points to a measurement error or a non-resistive load.