Free Ohm's Law calculator with clear step-by-step results.
Solves Ohm's law for whichever of voltage, current or resistance you are missing, then derives power from the two known quantities. Division is guarded so a zero resistance or zero current returns zero rather than an infinite result.
Ohm's law
V = I x R, so I = V / R and R = V / I
Power
P = V x I, equivalently I^2 x R or V^2 / R
Mains voltages are lethal. Calculations here are for understanding circuit relationships only, not a substitute for proper electrical design, protection and qualified installation.
Only to ohmic components whose resistance is constant. Diodes, LEDs, incandescent filaments and semiconductors are non-linear, so their resistance changes with the operating point.
It determines the resistor wattage rating you need. A resistor dissipating 0.5 W in a 0.25 W package will overheat and drift or fail.