Ohm's Law Calculator
Any two of volts, amps, ohms and watts in, all four out.
The two laws in play
U = R × I P = U × I
Ohm's law ties voltage, current and resistance together; the power law adds watts. Combine them and any two quantities determine the other two: P = I²R, P = U²/R, I = √(P/R), and so on. 12 V across 24 Ω drives 0.5 A and dissipates 6 W.
Where it applies
Ohm's law holds for resistive loads: resistors, heaters, incandescent bulbs, wire. LEDs, motors and anything with a switching power supply are not simple resistances, so measured current will differ from the naive calculation. For AC mains loads with poor power factor, real power is U × I × cosφ, less than the volt-amp product.