Solve a circuit with Ohm's law
Know any two of V, I, R, P and the rest follow — the four equations are really one.
Tools used in this guide
The job
An LED needs 20 mA from a 5 V rail: what resistor? R = V/I = 5/0.02 = 250 Ω. Or the reverse — a 9 V supply into 100 Ω draws 90 mA and dissipates 0.81 W.
Which pair to start from
The solver needs exactly two of the four quantities. V and R give current and power directly; V and I give resistance and power; P and R work too — the tool tries every pair.
Units are base SI: volts, amps, ohms, watts. Convert first — 20 mA is 0.02 A, and a 4.7 kΩ resistor is 4700 Ω.
Where the math stops
Ohm's law assumes ideal resistive DC behavior. AC circuits add impedance and phase, LEDs are nonlinear, and real resistors drift with temperature — the calculator gives the ideal answer, the circuit decides if it holds.