Ohm's Law Calculator
Calculate voltage, current, resistance, power
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Ohm's Law Calculator is built for calculating voltage, current, resistance, power — fast, free, and private. You supply Voltage (V) and Current (A) to Ohm's Law; the answer follows at once. In Ohm's Law the working sits beside the answer. Anyone comparing options in Science can use Ohm's Law to compare scenarios. Ohm's Law runs in your browser. Science data stays in the tab. Ohm's Law sits in the Science section next to free ohms law calculator and ohms law calculator voltage current resistance. Keep Ohm's Law bookmarked for next time. Ohm's Law is free, instant and local.
What the defaults give
12 V across a circuit carrying 2 A means 6.00 Ω of resistance and 24.00 W of power.
Resistance is V ÷ I; power is V × I.
The formulas
V = I × R (Ohm law)
P = V × I (power)
P = I² × R = V² ÷ R (the two substitutions)
Knowing any two of voltage, current and resistance gives the third; adding power requires only the voltage and current you already have.
Worked examples
12 V, 2 A → 6.00 Ω, 24.00 W
5 V, 0.5 A → 10.00 Ω, 2.50 W
120 V, 10 A → 12.00 Ω, 1,200.00 W
3.3 V, 0.02 A → 165.00 Ω, 0.07 W
9 V, 1 A → 9.00 Ω, 9.00 W
230 V, 0.5 A → 460.00 Ω, 115.00 W
The power relationships are not linear
Power is voltage times current, and both change when resistance changes at a fixed supply. Halve the resistance and current doubles, so power quadruples: a 12 V supply into 6 Ω dissipates 24 W, but into 3 Ω it pushes 4 A and dissipates 48 W. This is why a short circuit is dangerous — resistance near zero means current and power head toward the limit of the supply, and the heat lands in the wiring.
The same arithmetic explains why a 2,000 W kettle at 230 V draws about 8.7 A while a 60 W bulb draws 0.26 A. The circuit breaker is sized for the first, and the thin flex on a cheap extension lead is sized for neither if several appliances share it.
What the formula assumes
Ohm law holds for resistive loads — heaters, lamps, wire — where resistance is roughly constant. It does not hold directly for diodes, LEDs, motors or anything with a non-linear characteristic. An LED does not have a fixed resistance; it has a forward voltage, and the current must be limited by a resistor or driver. A motor draws a large inrush current at start-up when its effective resistance is low, which is why motor circuits need different protection.
Temperature matters too. A filament bulb has about a tenth of its running resistance when cold, so the switch-on surge is around ten times the running current — the reason bulbs fail at the moment you flip the switch. And for AC, the impedance includes reactance from capacitance and inductance, so the simple V = IR becomes V = IZ with a phase angle between them.
Frequently Asked Questions
What is the resistance at 12 V and 2 A?
6 ohms, from R = V ÷ I. The power dissipated is 24 W, from P = V × I.
How much power does 12 V at 2 A use?
24 watts. Multiply voltage by current, or use I²R which gives the same 24 W.
Why does halving resistance quadruple power?
Because current doubles at a fixed voltage, and power is voltage times current. Both factors rise, so the product rises fourfold.
Does Ohm law work for LEDs?
Not directly. An LED has a forward voltage rather than a fixed resistance, so current must be limited by a resistor or a constant-current driver.
What changes with AC?
Reactance from capacitors and inductors adds to resistance, giving impedance Z. The relationship becomes V = IZ, with a phase angle between voltage and current.
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