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Ohm's law calculator

Ohm's law links voltage, current and resistance as V = I × R. Enter any two values in the calculator to find the third, plus the power in watts, with the formulas and worked examples.

By Synced Updated 24 July 2026 3 min read

Voltage

The supply voltage: 12 V for many LED rails, 230 V for mains.

Resistance

The resistance, in ohms (Ω).

Current

3 A

12 V ÷ 4 Ω = 3 A36 W power

VoltageV = I × RCurrentI = V ÷ RResistanceR = V ÷ IPowerP = V × I

By SyncedSmart-home editorial team

Updated 24 July 2026

What Ohm's law tells you

Ohm's law is the rule that ties together the three basic quantities in any circuit: voltage, current and resistance. Put simply, the current flowing through a resistance is the voltage across it divided by that resistance. Know any two of the three and you can always work out the third, which is what the calculator above does.

The formulas

Voltage

Volts equal amps times ohms. Use it when you know the current and the resistance.

3 A × 4 Ω = 12 V
Current

Amps equal volts divided by ohms. This is the sum most DIY jobs need.

12 V ÷ 4 Ω = 3 A
Resistance

Ohms equal volts divided by amps. Handy when sizing a resistor for an LED.

12 V ÷ 3 A = 4 Ω

For example, a 12 V supply across a 4 ohm load draws 3 A (12 / 4). It is the same maths whether you are sizing a resistor for an LED or checking that a device's current draw sits within the rating of the plug or switch feeding it.

Working out power (Watt's law)

Power tells you how much energy the circuit uses, in watts. The base formula is P = V x I, so 12 V drawing 3 A is 36 W. Because the three quantities are all linked, you can also find power from current and resistance (P = I squared x R) or voltage and resistance (P = V squared / R). The calculator gives you the power alongside whichever value you are solving for.

For everyday AC appliance sums, our watts, amps and volts calculator handles the power factor for you.

Frequently asked questions

How do you calculate Ohm's law?

Take any two of the three values and use the matching formula. For current, divide voltage by resistance (I = V ÷ R). For voltage, multiply current by resistance (V = I × R). For resistance, divide voltage by current (R = V ÷ I). The calculator does the rearranging for you.

How do you work out volts from ohms?

You cannot get volts from resistance alone, you also need the current. Multiply the two: V = I × R. So 2 A through 6 Ω is 12 V.

What is Ohm's law in simple terms?

Voltage is the push, current is the flow, and resistance is what holds the flow back. More push gives more flow; more resistance gives less. It is often compared to water in a pipe: pressure is voltage, flow rate is current, and a narrow section is resistance.

Does Ohm's law work for AC circuits?

For simple resistive AC loads, yes. Once a circuit has motors, transformers or electronics, the current and voltage fall out of step and you need power factor and impedance rather than plain resistance.