Ohm's law calculator
V, I, R, and power — enter two, get the rest, with a wattage warning when parts start cooking.
How it works
One relationship, three arrangements: V = I · R. Voltage pushes, resistance resists, current is what results. Power is the part everyone forgets: P = V · I — and it's the number that decides whether your resistor survives.
Common questions
They're all the same triangle: V = I·R, I = V/R, R = V/I. Pick the one whose unknown you need. This calculator does the picking — choose what to solve for and feed it the other two.
Because parts fail by wattage, not by being 'wrong'. A 1k resistor across 12 V is a perfectly legal resistance — dissipating 144 mW, fine for a ¼ W part, fatal for an 0402. Every resistance decision is secretly also a power decision.
This calculator takes milliamps because that's the realistic range for signal-level electronics (LEDs ~10 mA, GPIO ~20 mA max, logic ~µA-mA). For motors and heaters, think in amps — and stop using ¼ W resistors.