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Electrical

Ohm's Law Calculator

Resistance and power from voltage and current.

Your measurementsEvery assumption editable01
Circuit valuesCalculated live02

Result

10Ω

10 ohms — drawing 1440 watts

Voltage
120 V
Current
12 A
ResistanceV ÷ I
10 Ω
PowerV × I
1440 W
Power from resistanceI² × R — the same figure, checked
1440 W

Ohm's law relates three quantities, so any two give the third. The power relationships follow from it, which is why P = VI, P = I²R and P = V²/R all describe the same thing.

These are DC relationships, and they hold for AC only with purely resistive loads — heaters, incandescent lamps, resistive elements. Motors, transformers and electronics introduce reactance and a power factor, which the kVA calculator handles.

Resistance rises with temperature in copper and aluminium. A conductor carrying near its rated current runs warmer and therefore more resistive than the cold figure suggests.

Enter any two of voltage and current and the rest follows. Ohm's law relates three quantities, and the power relationships fall out of it — which is why P = VI, P = I²R and P = V²/R all describe the same thing.

Why use this tool?What it does differently03

Why use this tool?

Power cross-checked

Calculated two ways — V × I and I² × R — so you can see the identities agree.

The whole relationship

Resistance, power and apparent power from two inputs.

AC caveat stated

These hold for DC and for purely resistive AC loads. Motors and electronics need power factor.

Temperature noted

Copper resistance rises as it warms, so a loaded conductor is more resistive than its cold figure.

How this worksThe method04

How this ohm's law calculator works

Ohm's law states that voltage equals current times resistance. Given any two of the three, the third follows — here resistance is voltage divided by current.

Power follows from the same relationship. P = VI is the direct form; substituting Ohm's law gives P = I²R and P = V²/R. All three produce the same answer, which is why the result shows two of them as a check.

These relationships hold exactly for DC, and for AC only where the load is purely resistive — heaters, incandescent lamps, resistive elements. Motors, transformers and electronics introduce reactance and a power factor below one.

How to use itStep by step05

How to use it

  1. Step 1: Enter the voltage

    The supply voltage across the component or circuit.

  2. Step 2: Enter the current

    The current flowing through it, in amps.

  3. Step 3: Read resistance and power

    Both derived from the two inputs.

  4. Step 4: Check the load type

    If it is a motor or electronic load, use the watts-amps-volts calculator with a power factor instead.

Example usageWorked figures06

Example usage

A resistive heater
120 V drawing 12.5 A is 9.6 Ω and 1,500 W — a typical portable heater.
An LED strip
12 V drawing 2 A is 6 Ω and 24 W.
A large appliance
240 V drawing 25 A is 9.6 Ω and 6,000 W — the same resistance as the heater at twice the voltage, so four times the power.
Frequently asked questionsCommon questions07

Frequently asked questions

What is Ohm's law?

Voltage equals current times resistance — V = IR. Given any two of the three quantities, the third follows.

How do I calculate watts?

Multiply volts by amps for a resistive load. Equivalently, current squared times resistance, or voltage squared divided by resistance — all three are the same identity.

Does Ohm's law work for AC?

For purely resistive AC loads, yes. Motors, transformers and electronics introduce reactance, so apparent power in volt-amps exceeds real power in watts by the power factor.

Does resistance change with temperature?

Yes. Copper and aluminium both become more resistive as they warm, so a conductor near its rated current has measurably higher resistance than its cold value.

Can I use this to size a circuit?

Not on its own. Conductor sizing depends on ampacity, installation conditions and the code in force locally — this gives you the physics, not the requirement.

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