Ohm's law calculator
Fill in any two of the four boxes and leave the others empty. The calculator works out the missing pair and shows which formula it used.
How this calculator works
Ohm's law links three things you can measure in a simple circuit: the push (voltage), the flow (current) and the opposition to flow (resistance). Double the voltage across the same resistor and the current doubles. Double the resistance at the same voltage and the current halves.
Power is how fast energy is turned into heat or light, and it follows from the same three quantities. Because any two of the four values fix the other two, there are six possible starting pairs. The calculator detects which pair you gave it and picks the right rearrangement automatically.
The formula behind it
- V = I × R (voltage = current × resistance)
- P = V × I (power = voltage × current)
- P = I² × R and P = V² ÷ R, found by swapping one quantity using Ohm's law
A worked example
A 12 V battery drives 2 A through a lamp. Resistance is 12 ÷ 2 = 6 ohms and power is 12 × 2 = 24 W. Check by the other route: P = I² × R = 4 × 6 = 24 W. Both agree.
Questions people ask
Why can I only fill in two boxes?
Two known values are enough to fix a simple resistive circuit. A third value could contradict the first two, so the calculator asks you to leave it blank and then tells you what it should be.
Does it work for AC circuits?
For purely resistive loads such as heaters and incandescent bulbs, yes, using RMS values. Motors and anything with coils or capacitors also involve reactance and phase, which need a more advanced treatment.
What units should I use?
Volts, amps, ohms and watts. If you have milliamps, divide by 1000 first, so 250 mA becomes 0.25 A.
Why does my resistor need a power rating?
A resistor turns the power it dissipates into heat. If the calculated power is more than its rating, it will overheat, so pick a rating comfortably above the result.