Ohm’s Law Calculator

Pick which two values you know, enter them, and the calculator solves the other two with V = I × R and P = V × I. It applies to DC circuits and to resistive AC loads such as heaters, incandescent lamps and resistors. The other two inputs are ignored for the mode you choose.

Important: This calculation provides a preliminary estimate. Final conductor, conduit and overcurrent protection sizing must follow the NEC as adopted locally, including ampacity, temperature, derating, terminal ratings, equipment listings and local amendments. Electrical work should be done or checked by a qualified electrician.

Tapeline — Ohm’s Law Calculator

Printed October 9, 2026

Values you knowVoltage and resistance
Voltage12 V
Current2 A
Resistance6 Ω
Power24 W

Results update as you type.

Only the two values named here are used; the other two are calculated.

V
A
Ω

For a heating element, resistance = volts² ÷ rated watts (a 1,500 W heater at 120 V is 9.6 Ω).

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Watch the step-by-step video

Watch it explained

A step-by-step video using your numbers: what to enter, what the result means, and a “what if” example. On-screen instructions follow each step as it happens.

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What the animation shows

The animation and the step-by-step video above use your numbers. Here is the same walkthrough written out for the default example.

Finished frame of the ohm’s law calculator animation for the default example
The last frame of the animation for the default example.

What you enter

  • Values you know: Voltage and resistance. Only the two values named here are used; the other two are calculated.
  • Voltage: 12 V.
  • Resistance: 6 Ω. For a heating element, resistance = volts² ÷ rated watts (a 1,500 W heater at 120 V is 9.6 Ω).

What happens, step by step

  1. Close the switch and current flows out of the plus terminal.
  2. I = V ÷ R: the current is 2 A.
  3. More current means the dots move faster around the loop.
  4. P = V × I: the power is 24 W.
  5. The filament turns 24 W into heat and light.

The answer: current is 2 A.

How it’s calculated

  • Ohm’s law: voltage V = current I × resistance R
  • Power: P = V × I
  • Combining the two: P = I² × R = V² ÷ R
  • From any two known values, rearrange: I = V ÷ R, R = V ÷ I, I = P ÷ V, V = √(P × R), I = √(P ÷ R)

Worked example

A 12 V battery across a 6 Ω resistor.

Inputs

Values you know
Voltage and resistance
Voltage
12 V
Current
2 A
Resistance
6 Ω
Power
24 W

Results

Current
2 A
Voltage
12 V
Resistance
6 Ω
Power
24 W

Load this example into the calculator

Assumptions and limits

  • DC circuit, or an AC circuit with a purely resistive load (power factor 1). Motors, LED drivers and transformers need power factor and impedance, not plain resistance.
  • Resistance is taken as constant. Real heating elements and lamp filaments have a lower resistance when cold than at operating temperature.

Verify the result against the applicable local code, adopted code edition, manufacturer instructions and project requirements. See our methodology.

Questions

How many ohms is a 1500 watt heater?

At 120 V, R = V² ÷ P = 14,400 ÷ 1,500 = 9.6 Ω, and it draws 1,500 ÷ 120 = 12.5 A. At 240 V a 1,500 W element is 38.4 Ω.

How do you calculate amps from volts and ohms?

Divide volts by ohms: I = V ÷ R. A 12 V source across 6 Ω pushes 2 A, and the resistor dissipates 12 × 2 = 24 W.

What size resistor do I need for an LED?

R = (supply volts − LED forward volts) ÷ LED current. For a 12 V supply, a 2 V red LED at 20 mA: (12 − 2) ÷ 0.02 = 500 Ω (use the next standard value, 510 Ω), dissipating 0.2 W, so a 1/2 W resistor.

Does Ohm’s law work for AC?

Yes for resistive loads like heaters and incandescent bulbs, using RMS volts and amps. For motors and electronics, current also depends on reactance and power factor.