Ohm's Law Calculator

Solve V = I × R from any two values — voltage, current, or resistance — plus power in watts, with the full Ohm's-law wheel, worked electronics examples, and honest AC caveats.

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Ohm's law ties the three fundamental circuit quantities together: V = I × R. Enter any two (leave the unknown at 0) and the third is solved, along with power P = V × I in watts. The law describes ohmic components — resistors, heating elements, wires — whose resistance stays constant with voltage. LEDs, motors, and AC circuits with capacitors or inductors don't follow plain resistance math; AC reactive loads need impedance, which is Ohm's law's grown-up sibling.

About Ohm's Law

V = I × R is the first equation of electronics and still the one used most: it sizes resistors for LEDs, tells you what current a heater pulls, whether a wire gauge can carry a load, and why a 12 V accessory dies on a 5 V supply. Every multimeter measurement lands in one of its three slots.

Enter the two values you know — from a label, a spec sheet, or a meter — and set the unknown to 0. You get the missing value, the power dissipation in watts, and the exact formula used. Watts matter as much as the missing value: components have power ratings, and exceeding them is how resistors become smoke detectors' first customers.

The percent-style proportional reasoning behind circuit ratios lives in the Percentage Calculator.

The Formula Wheel

Three base rearrangements plus the power family — the classic 12-formula wheel condenses to these:

V = I × R I = V ÷ R R = V ÷ I P = V × I P = I² × R P = V² ÷ R

Worked example: a 12 V supply pushing 2 A implies R = 12 ÷ 2 = 6 Ω and dissipates P = 12 × 2 = 24 W. Double the voltage on the same resistance and current doubles too — but power quadruples (P = V²/R), which is why overvoltage kills components so quickly.

Everyday Circuit Examples

Real numbers through the same formula — each row computed by this calculator:

Scenario	V	I	R	P
USB phone charger	5 V	2 A	2.5 Ω	10 W
LED + resistor (from 12 V)	12 V	0.02 A	600 Ω	0.24 W
Car headlight	12 V	4.6 A	2.6 Ω	55 W
US space heater	120 V	12.5 A	9.6 Ω	1,500 W
EU kettle	230 V	8.7 A	26.4 Ω	2,000 W

Note the heater row: 12.5 A is most of a US 15 A circuit — Ohm's law is also why running two heaters on one outlet trips the breaker.

Where Ohm's Law Bends

Ohm's law assumes resistance is constant, and real components disagree in interesting ways: an incandescent filament's resistance rises tenfold as it heats (cold inrush current is huge), LEDs are diodes with a forward-voltage cliff rather than a resistance (that's why they need a series resistor, sized with this calculator), and semiconductors generally do their own thing.

AC adds a second dimension: capacitors and inductors resist changing current — reactance — and the combination of resistance and reactance is impedance (Z), with V = I × Z as the generalized law. For resistive AC loads (heaters, kettles, incandescent bulbs) the plain math here works fine with RMS values, which is exactly what the examples table does.

Frequently Asked Questions

How do I calculate resistance from voltage and current?

R = V ÷ I. A 12 V circuit carrying 2 A has 6 Ω of resistance. Measure voltage across the component and current through it, divide, done — that's also exactly how a multimeter's ohm mode thinks.

What resistor do I need for an LED?

R = (supply voltage − LED forward voltage) ÷ desired current. A typical 2 V LED at 20 mA from 12 V: (12 − 2) ÷ 0.02 = 500 Ω — use the next standard size up (560 Ω). The resistor exists because LEDs don't self-limit current.

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

I = P ÷ V = 1500 ÷ 120 = 12.5 A on US mains — most of a standard 15 A branch circuit, which is why heaters get their own outlet. On 230 V systems the same heater draws only 6.5 A.

What's the difference between volts, amps, ohms, and watts?

Volts push (electrical pressure), amps flow (charge per second), ohms resist the flow, and watts measure the work rate (V × A). The plumbing analogy holds up: pressure, flow rate, pipe narrowness, and power delivered.

Does Ohm's law work for AC circuits?

For purely resistive loads (heaters, kettles), yes — use RMS values, which is what mains figures like 120 V already are. Once capacitors, inductors, or motors join in, resistance generalizes to impedance and phase enters the math; plain V = I × R becomes an approximation.

Why did my resistor burn out if the resistance was right?

Power rating. A correct 500 Ω resistor passing 20 mA from 12 V dissipates only 0.2 W — fine for a ¼ W part. The same resistance across mains would try to dissipate 29 W and fail instantly. Always check P = V × I against the component's wattage.

Methodology. This calculator uses standard, peer-reviewed mathematical formulas. It is reviewed and maintained by the Vast Calculators editorial team.

Last updated · July 10, 2026

Results are estimates for general use; verify critical figures independently.

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