Ohm's Law Calculator — Solve for Any Unknown
Enter any two values. Leave the unknown field blank or zero.
The Three Forms of Ohm's Law
The Ohm's Law triangle is a memory aid: write E at the top, I and R at the bottom. Cover the unknown to reveal the formula — cover E to get I × R; cover I to get E / R; cover R to get E / I.
Worked Examples — Ohm's Law in Electrical Circuits
Example 1: Find current drawn by a 1500W electric heater on 120V
First, find current from watts and volts: I = P / E = 1500W / 120V = 12.5 amps.
Then check resistance: R = E / I = 120V / 12.5A = 9.6 Ω
Application: A 15-amp breaker protecting this circuit has only 2.5A of headroom. Adding another load would trip it. Connect this heater to its own 15A or 20A circuit.
Example 2: Voltage drop on a wire run (applying Ohm's Law)
A #12 AWG copper wire has a resistance of approximately 1.98 Ω per 1000 ft (NEC Table 9). For a 50-foot one-way run (100 ft total round-trip), the wire resistance = 1.98 × (100/1000) = 0.198 Ω.
For a 20A load: Voltage drop = I × R = 20A × 0.198 Ω = 3.96V on a 120V circuit = 3.3% — just over the NEC 3% recommendation.
Application: Upsize to #10 AWG (1.24 Ω/1000 ft) to bring drop to 2.5A × 1.24 × 0.1 = 2.48V = 2.1%. Use the Voltage Drop Calculator for any run length.
Example 3: Find breaker size from load watts and voltage
A 240V, 5000W electric water heater: I = P / E = 5000W / 240V = 20.8A. Round up to the next NEC 240.6 standard size — a 25A breaker for non-continuous, or 30A for continuous (125% × 20.8A = 26A → 30A).
Power Formula (Watt's Law) — Related to Ohm's Law
These power formulas combine Ohm's Law with the definition of power. They are used to find watts when you know current and resistance (useful for heat loss calculations in conductors) or to find voltage when you know watts and resistance.
Ohm's Law in AC vs. DC Circuits
Ohm's Law (E = I × R) applies directly to DC circuits and resistive AC loads (electric heaters, incandescent lights). For AC circuits with inductive loads (motors, transformers) or capacitive loads, the opposition to current is called impedance (Z), measured in ohms, and the relationship becomes E = I × Z.
For practical residential wiring calculations involving voltage drop and conductor sizing, treating the circuit as resistive (using wire resistance from NEC Table 9) is standard practice and produces accurate results for NEC compliance.
Frequently Asked Questions
Ohm's Law states E = I × R: voltage equals current times resistance. It describes how voltage, current, and resistance are related in an electrical circuit. Knowing any two lets you calculate the third.
Voltage (E or V) in Volts, Current (I) in Amperes (amps), Resistance (R) in Ohms (Ω), Power (P) in Watts (W).
Directly for resistive loads. For inductive or capacitive AC loads, impedance (Z) replaces resistance — E = I × Z. For wire resistance and voltage drop calculations in residential wiring, the resistive approximation is standard and accurate.
Wire has a small but real resistance per foot (from NEC Table 9). Ohm's Law lets you calculate the resulting voltage drop: V_drop = I × R_wire_total. The NEC recommends keeping this to ≤ 3% of circuit voltage. Use the Wire Size Calculator to find the minimum AWG automatically.
P = E × I (Watts = Volts × Amps), also written P = I² × R and P = E² / R. These "Watt's Law" combinations let you solve for power from any two of the four electrical quantities.