Ohm's law calculator
Enter any two values — voltage, current or resistance — and find the rest, including power.
Fill in the fields and press Calculate to see your result here.
How this calculation works
Ohm’s law relates the three quantities as V = I × R (voltage = current × resistance). Power follows from P = V × I. Whichever two values you provide, the calculator rearranges these formulas to solve for the other two.
Worked example
A 12V battery connected to a 4Ω resistor draws I = V ÷ R = 12 ÷ 4 = 3A, and the resistor dissipates P = V × I = 36W.
Tips
Watch your units
Enter volts, amps and ohms directly — for milliamps or kilohms, convert to base units first (e.g. 150 mA = 0.15 A) to avoid a result that's off by a factor of 1,000.
Check component power ratings
A resistor's wattage rating should be well above the calculated power, or it can overheat.
One component at a time
This works for a single resistance. For several resistors, combine them first: series values add, and parallel values combine through their reciprocals.
Mains voltage is dangerous
Only work on household wiring if you are qualified. This tool is for learning and low-voltage planning.
Frequently asked questions
What if I know power instead of one of these three?
Power (P = VI) combined with any one of V, I or R is enough to find the other two, but this version expects two of V, I, R directly — use P = VI and Ohm's law by hand for that combination.