Electronics CalculatorsOhm's law calculator
Fill in any two of the four boxes. The other two are worked out.
The four formulas
Ohm's law links voltage V (volts), current I (amperes) and resistance R (ohms): V = I × R. Power P (watts) is P = V × I. From these two, every pair of values gives the other two:
| You know | Resistance | Current | Voltage | Power |
|---|---|---|---|---|
| V and I | V / I | given | given | V × I |
| V and R | given | V / R | given | V² / R |
| I and R | given | given | I × R | I² × R |
| P and R | given | √(P / R) | √(P × R) | given |
Typing values
You may use the usual prefixes: k (thousand), M (million), m (milli, a thousandth), u (micro, a millionth), n (nano) and p (pico). 4k7 means 4.7 kilo-ohm, and 20m means 0.02. Units such as V, A or the ohm sign at the end are ignored.
How it was tested
We gave the calculator 1,200 random pairs of values and checked that, in every answer, V equals I × R and P equals V × I, and that the two values you typed were returned unchanged.
Why do I get an error when I fill three boxes?
Two values fix the circuit. A third could disagree with them. Clear one.
Does it work for AC?
For a resistor, with RMS values, yes. Coils and capacitors need impedance, not only resistance.
This is an independent guide. It is not connected with any standards body or company named here. The formulas are the usual textbook ones; the colour code, E-series and AWG values are those printed in common references, because the standards IEC 60062, IEC 60063 and ASTM B258 are paid documents that we did not read. Tables and examples were calculated with Python on 14 November 2026. Check data sheets and local safety rules before you build.