Ohm's Law Calculator

Ohm's law says voltage equals current times resistance: V = I × R. Enter any two of the four values (volts, amps, ohms, watts) and the calculator solves for the other two, using Ohm's law together with the power formula P = V × I.

How to use the calculator

1. Pick the two values you know, for example the nameplate voltage and wattage of a heater.
2. Enter them with their units (V, A, Ω, W). Use kilo- and milli- prefixes carefully: 4.5 kW is 4,500 W, 500 mA is 0.5 A.
3. Read the two results. For AC loads that are not purely resistive, treat the "ohms" result as impedance and see the power factor note below.

Ohm's law and the power formula

Ohm's law: V = I × R

Power: P = V × I

Combining the two gives the 12 formulas on the Ohm's law wheel, three for each quantity:

These apply exactly to DC circuits and to AC circuits with purely resistive loads (heaters, incandescent lamps). For motors and other inductive loads, resistance becomes impedance, and power also depends on the power factor. To convert a nameplate wattage to current with power factor or three-phase, use the watts to amps calculator.

Field examples

• Heating element check. A 240 V, 4,500 W water heater element should measure about R = V² ÷ P = 57,600 ÷ 4,500 = 12.8 Ω. A reading far from that, or an open circuit, means a failed element. At 240 V that element draws 240 ÷ 12.8 = 18.75 A.
• Baseboard heater resistance from wattage. A 2,000 W, 240 V baseboard: R = 240² ÷ 2,000 = 28.8 Ω, and I = 240 ÷ 28.8 = 8.33 A.
• A 240 V element on 208 V. Resistance stays fixed, so power falls with the square of the voltage: P = 208² ÷ 12.8 = 3,380 W, about 75% of the 4,500 W rating, drawing 208 ÷ 12.8 = 16.25 A. That is why water heaters in 208 V apartment buildings recover slowly.
• Current from a resistive load. A 1,500 W, 120 V heater draws I = P ÷ V = 12.5 A.
• Voltage drop on a conductor. Stranded 12 AWG copper has a DC resistance of about 1.98 Ω per 1,000 ft at 75°C (NEC Chapter 9, Table 8), so 100 feet is about 0.198 Ω. Out and back is 0.396 Ω. At 20 A: V = I × R = 20 × 0.396 = 7.9 V, the same answer the K-factor method gives in the voltage drop calculator. The derivation is on the voltage drop formula page.
• Heat in a bad connection. A loose termination with 0.1 Ω of contact resistance carrying 30 A dissipates P = I² × R = 900 × 0.1 = 90 W in one spot. That is why loose lugs burn.

Series and parallel basics

Building circuits are parallel: every receptacle and fixture sees the full circuit voltage, and branch currents add up at the breaker.

Parallel example. Two 1,500 W, 120 V heaters are each 120² ÷ 1,500 = 9.6 Ω. Plugged into the same circuit they form 1 ÷ (1/9.6 + 1/9.6) = 4.8 Ω, so the circuit carries 120 ÷ 4.8 = 25 A. That trips a 20 A breaker, which is the Ohm's law explanation of the most common overload call.

Series example: the open neutral. If the shared neutral of a multiwire branch circuit opens, a load on one leg ends up in series with a load on the other leg across 240 V. Take a 60 W lamp (120² ÷ 60 = 240 Ω) and a 1,500 W heater (9.6 Ω): total resistance is 249.6 Ω, current is 240 ÷ 249.6 = 0.96 A, and the lamp sees 0.96 × 240 ≈ 231 V while the heater sees about 9.2 V. The small load gets nearly the full 240 V and fails. (Filament resistance changes with temperature, so real values differ, but the result is the same.)

Three-phase power

For balanced three-phase loads: P = √3 × V(line-to-line) × I × PF. A 208 V three-phase heater drawing 27.8 A per phase at unity power factor uses about 10,000 W.

Common mistakes

• Assuming constant wattage at a different voltage. Resistive loads keep constant resistance, so their wattage changes with voltage squared. Electronic power supplies do the opposite and draw more current at lower voltage.
• Measuring resistance on an energized circuit. Isolate and verify zero voltage first; the meter reading is meaningless otherwise, and the meter may be damaged.
• Using DC resistance for AC motor circuits. A motor winding measures a few ohms but draws far less current than V ÷ R suggests while running, and far more at locked rotor. Use nameplate current.
• Dropping the prefix. 500 mA is 0.5 A; at 120 V that is 120 ÷ 0.5 = 240 Ω, not 0.24 Ω.

Frequently asked questions

What is Ohm's law in simple terms?

Current through a conductor equals the voltage across it divided by its resistance. Double the voltage and the current doubles; double the resistance and the current halves.

What are the 12 Ohm's law formulas?

Three each for voltage, current, resistance and power, all derived from V = I × R and P = V × I. They are listed in the table above and shown on the Ohm's law wheel.

How do I find resistance from wattage?

Divide the voltage squared by the wattage: R = V² ÷ P. A 4,500 W element rated at 240 V is 57,600 ÷ 4,500 = 12.8 Ω.

Does Ohm's law work for AC?

Yes for resistive loads. For inductive or capacitive loads, use impedance (Z) instead of resistance and include the power factor when calculating power.

What are the units?

Voltage in volts (V), current in amperes (A), resistance in ohms (Ω) and power in watts (W).

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Last updated: October 6, 2026. Results must be checked against the adopted code and the AHJ; electrical work should be done by qualified people.