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Ohm's Law Calculator

Ohm's law is the arithmetic every student, hobbyist and electrician reaches for first: in a resistive DC circuit, voltage equals current times resistance, and power is voltage times current. Four quantities, one triangle — know any three and the fourth falls out. This calculator solves for whichever you are missing, shows the formula behind each answer, and ships defaults that already agree with each other.

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The other three values are used to solve for this one

Only used when finding another quantity

Results update as you type.

Results

Voltage (V)
12
Formula
V = I × R

How is the Ohm's Law calculated?

Choose what to solve for — voltage, current, resistance or power. Enter the other three quantities in volts, amps and ohms; the power field is only read when one of the others is being solved. The result comes back in the solved quantity's own unit, rounded to three decimals, with the formula printed beneath it. The shipped defaults agree with each other: 12 V = 2 A × 6 Ω, and 24 W = 12 V × 2 A, so switching modes never produces a nonsense figure.

Formula

V = I × R · I = V ÷ R · R = V ÷ I · P = V × I

where:
V = voltage in volts
I = current in amps
R = resistance in ohms
P = power in watts

P = V² ÷ R and P = I² × R are the same relationships rearranged — useful when only voltage, or only current, is known.

Example

Voltage: 12 V · Current: 2 A · Resistance: 6 Ω
Check: V = 2 × 6 = 12 V — the three agree. Power: P = 12 × 2 = 24 W
Car-battery logic: a 12 V bulb drawing 2 A needs a 6 Ω filament. At 230 V the same 24 W bulb draws only about 0.104 A, so its resistance must be near 2,204 Ω — the same power, a very different circuit.

Things to remember

The law describes an ideal resistor: wires heat, batteries sag, and LED drivers are not plain resistance — treat this as a design's starting point, not a lab measurement. For AC, use RMS values and remember motors add a power factor. Anything at mains voltage is worth leaving to a licensed electrician.

Frequently Asked Questions

What is Ohm's law in simple terms?

Voltage is the push, current is the flow, resistance is the squeeze: V = I × R. Double the push and the flow doubles through the same resistance; double the resistance and the flow halves for the same push. Power follows as P = V × I — the push times the flow.

What is the power triangle in Ohm's law?

P = V × I, which with V = I × R also gives P = I² × R and P = V² ÷ R. The last form explains high-voltage power transmission: halving the current by doubling the voltage quarters the I² × R heating lost in the lines.

Does Ohm's law work for AC circuits?

For resistive loads, yes — use RMS voltage and current, so 230 V mains is simply 230 in the formula. With motors, transformers and switching supplies, voltage and current fall out of phase, and real power needs a power factor on top of the plain V × I.

Why does my answer differ slightly from the multimeter?

Component tolerance — a 5% resistor is allowed to be 5% off — wire heating, meter accuracy and battery sag all shift real measurements a few percent from the ideal calculation. Ohm's law predicts the design; the meter reports the circuit as actually built.

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