Ohm's Law Calculator (Voltage, Current, Resistance and Power)

Ohm's law connects three quantities of an electric circuit: voltage (V), current (I) and resistance (R). With this calculator you enter any two of them, leave the one you need empty, and get the third. It also calculates the power (P) in watts.

Below you will find the formulas, worked examples and answers to common questions, such as what to do if you know the power or your data is in milliamps.

How to use the Ohm's law calculator?

Step 1: Enter two of the three values: voltage in volts (V), current in amperes (A) or resistance in ohms (Ω). Decimals work with a point or a comma.

Step 2: Leave empty the box of the quantity you want to find.

Step 3: Press Calculate or the Enter key.

Step 4: The result appears on the right and the power in watts below it. The Calculation box shows the formulas with your numbers.

Values are entered as numbers of 0 or more. If the current is 0 the resistance cannot be calculated, and if the resistance is 0 the current cannot be calculated, because it would mean dividing by 0. In those cases the calculator shows a message.

Ohm's law formulas

Ohm's law says that the current through a conductor is proportional to the applied voltage and inversely proportional to its resistance. Written another way:

V = I × R

Solving for each quantity gives the three forms of the law:

Electric power, in watts (W), is P = V × I. If you substitute Ohm's law, it can also be found with just two of the three quantities:

The units are the volt (V), the ampere (A), the ohm (Ω) and the watt (W).

Ohm's law examples

Example 1: Finding the resistance

A lamp connected to a 12 V battery draws 2 A. What is its resistance? We divide the voltage by the current:

R = V ÷ I = 12 ÷ 2 = 6 Ω

P = V × I = 12 × 2 = 24 W

The resistance is 6 Ω and the lamp uses 24 W.

Example 2: Finding the current

A 450 Ω resistor is connected to a 9 V battery. What current flows? We divide the voltage by the resistance:

I = V ÷ R = 9 ÷ 450 = 0.02 A

P = V × I = 9 × 0.02 = 0.18 W

The current is 0.02 A, which is 20 mA, and the power is 0.18 W.

Example 3: Finding the voltage

A current of 0.5 A flows through a 240 Ω lamp. What voltage does it need? We multiply the current by the resistance:

V = I × R = 0.5 × 240 = 120 V

P = V × I = 120 × 0.5 = 60 W

It needs 120 V and uses 60 W.

Example 4: A heater

A heater with a resistance of 23 Ω is connected to 230 V. What current flows and what is its power?

I = V ÷ R = 230 ÷ 23 = 10 A

P = V × I = 230 × 10 = 2300 W

The current is 10 A and the power is 2300 W, which is 2.3 kW.

Frequently asked questions

What does Ohm's law say?

That the voltage across a resistor equals the current through it multiplied by its resistance: V = I × R. If the resistance stays the same and you double the voltage, the current doubles too.

How do you calculate power with Ohm's law?

Power is P = V × I. With Ohm's law it is also P = I² × R and P = V² ÷ R. For example, with 12 V and 6 Ω the current is 2 A and the power is 24 W.

What if I know the power and the voltage?

First find the current with I = P ÷ V, then use the calculator. For example, for a 60 W lamp at 120 V: I = 60 ÷ 120 = 0.5 A. With V = 120 and I = 0.5, the resistance is 240 Ω.

How do I use milliamps or kilohms?

Convert them to amperes and ohms before entering them: 1 mA = 0.001 A and 1 kΩ = 1000 Ω. So 20 mA is 0.02 A and 4.7 kΩ is 4700 Ω.

Does it work for series, parallel or AC circuits?

Yes, with one extra step. In series, resistances add up: 100 Ω + 200 Ω = 300 Ω. In parallel you use 1/R = 1/R₁ + 1/R₂: 100 Ω and 200 Ω in parallel give 66.67 Ω. With AC the law holds for resistive loads; with coils or capacitors you use impedance instead of resistance.

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