Ohm's Law Calculator

Solve for V, I, R or P — enter any two values and get all four results instantly.

Formula Reference

V = I × R I = V ÷ R R = V ÷ I P = V × I

Enter any two values — leave the other two blank. The calculator will solve for all four.

Voltage (V) — volts

Current (I) — amps

Resistance (R) — ohms

Power (P) — watts

Voltage (V)

—

volts

Current (I)

—

amps

Resistance (R)

—

ohms

Power (P)

—

watts

How Ohm's Law Works

Ohm's Law describes the relationship between voltage (V), current (I) and resistance (R) in an electrical circuit: V = I × R. From these three values, a fourth — power (P = V × I) — can also be derived. This calculator accepts any two of the four quantities and solves for the remaining two. For example, if you know a 120V circuit draws 15 amps, it has 8 ohms of resistance and consumes 1,800 watts. These formulas are fundamental to circuit design, breaker sizing, wire selection and troubleshooting. The NEC uses Ohm's Law principles throughout its load calculations, making this tool essential for electricians planning residential, commercial or industrial installations.

Frequently Asked Questions

What is Ohm's Law?

Ohm's Law states that voltage (V) equals current (I) multiplied by resistance (R): V = IR. It's the foundation of basic circuit analysis.

How do I calculate power from voltage and resistance?

P = V² ÷ R. For a 120V circuit with 10 ohms of resistance, power = 14,400 ÷ 10 = 1,440 watts.

What is the unit of resistance?

Resistance is measured in ohms (Ω), named after Georg Ohm. One ohm equals one volt per ampere.

How does Ohm's Law relate to circuit breaker sizing?

A 20-amp breaker on a 120V circuit can safely supply 20A × 120V = 2,400 watts. Always stay at 80% continuous load (1,920W) per NEC 210.20.

Ohm's Law Calculator

How Ohm's Law Works

Frequently Asked Questions

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