Ohm's law calculator

Divide the voltage by the resistance: I = V/R. With 12 V across 6 Ω the current is 2 A; with 5 V across 1 kΩ it is 0.005 A, or 5 mA. Convert kilohms to ohms (×1,000) before dividing, or pick kΩ and mA in the unit selectors and let the calculator convert.

Divide the power by the voltage: I = P/V. A 2,300 W kettle on a 230 V supply draws 10 A, and a 60 W load on 12 V draws 5 A. For AC this holds for resistive loads such as heaters and kettles; motors and power supplies with a power factor below 1 draw more current than P/V.

P = VI, which becomes P = I²R or P = V²/R once Ohm's law is substituted. A 100 Ω resistor with 5 V across it dissipates 5²/100 = 0.25 W, exactly the limit of a ¼ W part, so a 0.5 W resistor is the safer choice there.

Not with a single fixed resistance. An LED conducts almost nothing below its forward voltage (about 2 V for red, about 3 V for blue and white) and then current rises steeply, so it needs a series resistor. A filament lamp's resistance rises as the tungsten heats, so the V²/P value, 882 Ω for a 60 W, 230 V bulb, applies only when it is lit.

Conductance is the reciprocal of resistance, G = 1/R, measured in siemens (S). A 6 Ω resistor has a conductance of 1/6 ≈ 0.167 S. Conductances in parallel add directly, which is why it is convenient for parallel networks: 1/R_total = G₁ + G₂ + …

Accuracy depends on your inputs and the method's assumptions. Decimal arithmetic uses 50 significant digits, but estimates, numerical methods and source data can be less precise; the displayed rounding does not remove those limits. It is checked against 6 worked examples whose answers come from independent sources; for example, “12 V across 6 Ω” is checked against Python 3.8 decimal: I = 12/6, P = 12²/6.

OpenStax University Physics Volume 2, §9.4 Ohm's law and §9.5 Electrical energy and power; HyperPhysics — Ohm's law, power.