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Ohm's law calculator
Calculate voltage, current, resistance and power with Ohm's law. Enter any two of V, I, R and P and this free calculator gives the other two, in volts, amperes, ohms and watts. It runs in your browser.
How to use Ohm's law
- 1Enter any two of voltage, current, resistance, power
- 2Leave the other two fields empty
- 3Read all four values in the result
- Voltage, current, resistance and power, all four filled in.
What you get
- All four values.V, I, R and P come out together, from any two you know.
- Six input pairs.Every pair of the four works, for example V and R, or I and P.
- Nothing is uploaded.The numbers are worked out in your browser and not sent anywhere.
What you can do
- Find the current from voltage and resistance: I = V / R.
- Find the resistance from voltage and current: R = V / I, plus the power P = V·I.
- Work out a resistor's power rating from current and resistance (P = I²·R).
- Starts with 12 V and 6 Ω, which gives 2 A and 24 W.
Good to know
- Enter exactly two of the four fields. Any other number shows "Enter exactly two of the four."
- The tool starts with voltage and resistance filled in. Clear one and fill in a different value to switch pairs.
- Use volts, amperes, ohms and watts. Convert mA or kΩ first (1 mA is 0.001 A).
- Ohm's law is for resistive loads and direct current. It does not cover coils or capacitors at alternating current.
Questions
What is Ohm's law?
V = I·R: the voltage across a resistor equals the current through it times its resistance.
How do I calculate current?
Divide voltage by resistance. 12 V over 6 Ω gives 2 A.
How do I calculate the power of a resistor?
Multiply voltage by current, P = V·I. The tool also works it out from I and R, or V and R.
Why does it want exactly two values?
Two known values fix the circuit. With one value there is too little to solve, with three or four it is already over-defined.