Ohm's Law Calculator

Pick the unknown — voltage, current, resistance, or power — and the calculator solves for it from the values you provide. V = IR for the basic three, and P = VI (or P = I²R, or P = V²/R) when power enters the picture.

Use it for sanity-checking circuit designs. If a 5V supply driving a 1kΩ resistor would dissipate more power than the resistor's rating allows, the calculator flags it before the smoke does. Standard 1/4-watt resistors handle 0.25W — a 5V across 100Ω draws 50mA, which is 250mW, right at the limit.

For AC circuits with reactive components (capacitors, inductors), Ohm's law in the simple form doesn't tell the whole story — you need impedance, not pure resistance. The calculator covers DC and resistive AC; for general AC, look at an impedance calculator instead.

The four formulas are algebraically the same: V = IR, I = V/R, R = V/I, and P = VI = I²R = V²/R. Pick whichever has the values you have. The calculator does the substitution automatically once you specify the unknown. Where the math gets used: pick a current-limiting resistor for an LED, size a power resistor in a sense circuit, calculate voltage drop across a long cable run, verify a power supply has enough headroom. All five-second checks once you know the formulas; minutes of reaching for a calculator if you don't.

Worked example: an LED has a 2V forward voltage and a 20mA recommended current. Drive it from a 5V supply. Voltage that needs to drop across the resistor: 5 - 2 = 3V. R = V/I = 3 / 0.020 = 150 Ω. Power dissipated in the resistor: P = VI = 3 × 0.020 = 0.06 W = 60 mW. A 1/4-watt (250 mW) resistor handles this with comfortable headroom. Pick a 150Ω 1/4W resistor from the E12 series and the BOM is done. If the LED's spec sheet listed forward voltage 2.1V at 30mA: R = 2.9 / 0.030 = 96.7Ω, which rounds to a stock 100Ω. Power = 2.9 × 0.030 = 87mW — still fine for 1/4W.

Where it gets you wrong: the law assumes a linear resistor (resistance independent of voltage and current). LEDs aren't linear — their resistance changes with current. Diodes have an exponential current-voltage curve. Filaments heat up and resistance climbs. Inductors and capacitors store energy and respond to AC reactively, not resistively. The calculator works perfectly for the linear-resistor case which covers most everyday electronics calculations. For exact LED biasing, datasheet curves are more reliable than calculated R values; for AC reactive circuits, you need complex impedance Z = R + jX.