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Ohm's Law Calculator

Enter any two of voltage, current, resistance and power to solve the other two, with unit choices and the formula used for each result.

Change the values and press Calculate to work out your own figures.

Enter any two of voltage, current, resistance and power, each in the unit you like, and the other two are worked out.

Potential difference across the component.
Current through the component.
Resistance of the component.
Power dissipated by the component.
Result
Resistance and power
6 Ω · 24 W
Resistance in milliohms
6,000 mΩ
Resistance in kiloohms
0.006 kΩ
Resistance in megaohms
0.000006 MΩ
Power in milliwatts
24,000 mW
Power in kilowatts
0.024 kW
Power in megawatts
0.000024 MW
Power in horsepower (mechanical, 550 ft·lbf/s)
0.0321845 hp
Power in horsepower (electric)
0.0321716 hp(E)
Power in BTU (IT) per hour
81.8914 BTU/h

With 12 V and 2 A: R = V ÷ I = 6 Ω, and P = V × I = 24 W.

Show the working
  1. In base units: V = 12 V, I = 2 A
  2. R = V ÷ I = 6 Ω
  3. P = V × I = 24 W

How to use the Ohm's law calculator

  1. Enter any two of the four values: voltage, current, resistance or power.
  2. Choose a unit beside each one, for example millivolts, milliamps or kilohms. The unit you pick for an empty box is the unit its answer is shown in.
  3. Press Calculate to get the other two values and the formula used for each.

Leave the other two boxes empty. If more than two are filled in, the values could contradict each other, so the calculator asks you to clear one.

Ohm's law and the power formulas

Ohm's law says the current through a resistor is proportional to the voltage across it: V = I × R. Combined with electrical power, P = V × I, any two of the four quantities give the other two.

Voltage: V = I × R = P ÷ I = √(P × R)
Current: I = V ÷ R = P ÷ V = √(P ÷ R)
Resistance: R = V ÷ I = V² ÷ P = P ÷ I²
Power: P = V × I = I² × R = V² ÷ R

For example, a 12 V supply across a 4 Ω resistor drives 12 ÷ 4 = 3 A of current, and the resistor turns 12 × 3 = 36 W into heat. A 60 W bulb on 120 V draws 0.5 A and has a hot resistance of 240 Ω.

Units

Metric prefixes multiply by powers of 1,000: 1 kΩ = 1,000 Ω, 1 mA = 0.001 A, 1 MW = 1,000,000 W. Power is also shown in horsepower, which has two definitions: mechanical horsepower is 745.69987158227022 W (550 foot-pounds per second) and electrical horsepower, used for motors, is exactly 746 W. It is also shown in BTU per hour, used for heating and cooling.

When Ohm's law applies

The formulas describe resistors and other loads whose resistance stays constant, with direct current or the effective (RMS) values of alternating current in purely resistive loads. Motors, LEDs and other loads with inductance, capacitance or changing resistance also need impedance and power factor, and three-phase power uses different formulas.

Questions

How do I work out the resistor for an LED?

Subtract the LED's forward voltage from the supply voltage, then divide by the current you want. For a 5 V supply, a 2 V LED and 20 mA: (5 − 2) ÷ 0.02 = 150 Ω. You can enter 3 V and 20 mA here to check the resistance and the power the resistor must handle (0.06 W).

What happens if the resistance is zero?

With a voltage across zero resistance, Ohm's law gives an infinite current, a short circuit, so the calculator reports that there is no finite answer.

How many amps does a 1,500 W heater draw on 120 V?

I = P ÷ V = 1,500 ÷ 120 = 12.5 A.

Reviewed by RK Calculator Team. Updated 2026-09-25.

Sources

Limitations

  • Assumes an ideal ohmic (linear) resistor and DC or resistive AC RMS values; reactive AC loads need impedance and power factor.
  • Values are magnitudes; sign conventions for current direction are not modelled.
  • Three-phase circuits are out of scope for this tool.

Formula version 0.1.0Reviewed