Ohm's Law Calculator

Enter any two of voltage, current, resistance or power and the calculator solves the remaining two live, using Ohm's Law and the power wheel.

This free ohms law calculator turns any two known electrical values into all four. Pick the pair you know, voltage and resistance, current and power, or any other combination, and it works the rest out as you type. It doubles as a voltage current resistance calculator and a v=ir calculator, covering the full power wheel: V=IR, P=VI, P=I²R and P=V²/R. Everything runs in your browser, nothing uploaded.

Ohm's Law

I know
Enter any two values and the other two are solved live.

Current

3 A

Power: 36 W

Voltage (V)
12 V
Current (I)
3 A
Resistance (R)
4 Ω
Power (P)
36 W

The two values you entered stay put; the solved pair is highlighted. Relations used: V = I × R, P = V × I, P = I²R and P = V²/R.

These are ideal DC relations. Real circuits shift with temperature, AC power factor and component tolerance, so size wire and fuses with a safety margin, not the exact figure.

How it works

  1. 1

    Pick the pair you know

    Choose which two of voltage, current, resistance and power you already have from the selector.

  2. 2

    Type the two values

    Enter your two numbers in volts, amps, ohms or watts. Results update on every keystroke.

  3. 3

    Read the other two

    The solved voltage, current, resistance and power appear together, with the pair you entered kept in view.

Instant & 100% private — nothing is uploaded

Every calculation runs locally in your browser. The income, balances and goals you enter stay on your own device and are never sent to a server — nothing is stored, logged or shared.

Frequently asked questions

What is Ohm's Law?
Ohm's Law says voltage equals current times resistance, written V=IR. Rearranged, current is V divided by R and resistance is V divided by I. This calculator adds the power relations so any two of the four values give you the other two.
How do I calculate current from voltage and resistance?
Divide the voltage by the resistance: I = V / R. For 12 volts across a 4 ohm load that is 3 amps, and the power comes out to 36 watts. Enter the voltage and resistance and the calculator shows both.
How is power worked out?
Power uses whichever pair you have: P = V×I, P = I²R or P = V²/R. All three agree for the same circuit, so the tool picks the relation that fits your two known values.
Do I need to press a button to solve?
No. Pick the two quantities you know, type their values, and the other two update as you type. There is no submit step, so you can nudge a number and watch the result move.
Does it work for AC circuits?
The formulas are the DC form of Ohm's Law. They hold for AC resistive loads too, but reactive loads such as motors and transformers need a power factor, so read the watts there as apparent power (volt-amps) rather than true power.
What units should I enter?
Volts for voltage, amps for current, ohms for resistance and watts for power, the standard electrical units. Enter milliamps or kilo-ohms as their decimal value, so 500 mA is 0.5 and 2.2 kΩ is 2200.

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