WorldCalcs

Ohm's Law Calculator

Enter any two of voltage, current, resistance, or power and get the other two using V = I × R and P = V × I.

Reviewed by the WorldCalcs team · Methodology · Last reviewed: July 2026

Voltage

12 V

Current

2 A

Resistance

6 Ω

Power

24 W

V = I × R and P = V × I

What is Ohm's law?

Ohm's law is the cornerstone of electrical circuits. It says the voltage across a resistor equals the current through it multiplied by its resistance: V = I × R. Voltage (V) is the electrical "push," current (I) is the flow of charge, and resistance (R) is how much the material opposes that flow. Pair Ohm's law with the power equation P = V × I and you can move between all four quantities — volts, amps, ohms and watts — knowing just two of them.

The formulas

From V = I × R and P = V × I, every other relationship follows by algebra:

  • V = I × R and V = P / I and V = √(P × R)
  • I = V / R and I = P / V and I = √(P / R)
  • R = V / I and R = V² / P and R = P / I²
  • P = V × I and P = I² × R and P = V² / R

That's why entering any two values is enough — the calculator picks the right pair of formulas and fills in the rest.

Worked example

A 12-volt supply pushes 2 amps through a resistor. Resistance is voltage divided by current: 12 ÷ 2 = 6 ohms. Power is voltage times current: 12 × 2 = 24 watts. So the resistor draws 24 W and has a resistance of 6 Ω. If instead you knew the resistor was 10 Ω and dissipated 90 W, you'd get V = √(90 × 10) = 30 V and I = √(90 ÷ 10) = 3 A.

The Ohm's law wheel

Electricians often picture the four quantities in a "wheel" or "pie" with V, I, R and P in the middle and the twelve formulas around the edge. You don't need to memorise all twelve — just V = I × R and P = V × I. Every other version is those two rearranged, which is exactly what this calculator does for you.

Related: pair with the Force Calculator and Density Calculator in the same Science cluster, or explore more tools on Math Calculators.

All calculations happen in your browser. Nothing is sent, stored, or tracked.

Results are estimates and may contain errors — for general information only, not professional advice. Always verify before relying on them. Disclaimer

How to use

Fill in exactly two of the four boxes — voltage (V), current (I), resistance (R) or power (P) — and leave the others blank. The calculator picks the right rearrangement of V = I × R and P = V × I to solve for the two missing values.

If a combination would require dividing by zero (for example, finding resistance from a zero current), the calculator flags it instead of returning an invalid result.

Frequently asked questions

What is Ohm's law?+

Ohm's law states that voltage equals current times resistance: V = I × R. It links the three basic electrical quantities so that knowing any two gives you the third.

How do I calculate power from voltage and current?+

Multiply them: power = voltage × current, or P = V × I. A device on 12 V drawing 2 A uses 12 × 2 = 24 watts.

How do I find resistance from voltage and current?+

Divide voltage by current: R = V / I. For 12 V and 2 A, resistance is 12 ÷ 2 = 6 ohms.

How do I find current from power and resistance?+

Take the square root of power divided by resistance: I = √(P / R). This comes from P = I² × R rearranged.

What are the units in Ohm's law?+

Voltage is measured in volts (V), current in amps (A), resistance in ohms (Ω), and power in watts (W). One watt equals one volt-amp.

What is the Ohm's law wheel?+

It's a chart that arranges V, I, R and P with all twelve rearranged formulas around them. It's a handy reference, but everything on it derives from V = I × R and P = V × I.

Does Ohm's law work for AC circuits?+

For purely resistive loads, yes. In AC circuits with capacitance or inductance you also have to account for reactance and impedance, so the simple V = I × R applies to the resistive part.

How do I calculate voltage from power and resistance?+

Use V = √(P × R). For a 10 Ω resistor dissipating 90 W, voltage is √(90 × 10) = √900 = 30 V.

Related calculators