Ohm's Law Calculator
Enter any two of voltage, current, resistance and power to get the other two with Ohm's law (V = I × R) and P = V × I, with mA and kΩ support.
Ohm's law links voltage, current and resistance: V = I × R. Together with the power equation P = V × I, knowing any two of the four quantities fixes the other two. Enter the two you know — a supply voltage and a resistor value, say — and the calculator returns the current, the power and the rest, with current in A, mA or µA and resistance in Ω, kΩ or MΩ.
How to use this tool
- Enter exactly two of voltage, current, resistance and power.
- Pick the current and resistance units that match your numbers; mA and kΩ are common in electronics.
- Leave the other two boxes empty.
- Read the missing values and the formula that produced each one.
Formula
V = I × R · P = V × I = I² × R = V² ÷ R
V is voltage in volts, I current in amperes, R resistance in ohms and P power in watts. Rearranged: I = V ÷ R, R = V ÷ I, I = √(P ÷ R) and V = √(P × R). Inputs are strictly positive magnitudes; zero-input cases can be underdetermined and are outside this four-variable solver.
Example
A 12 V supply across a 6 Ω resistor drives I = 12 ÷ 6 = 2 A, and the resistor turns P = 12 × 2 = 24 W into heat, so it needs a power rating well above 24 W.
Frequently asked questions
Does Ohm's law apply to every component?
Only to ohmic parts such as resistors and wires at a steady temperature, whose resistance does not change with current. LEDs, diodes and filament bulbs are non-linear: V ÷ I still gives the resistance at that one operating point, but it is not a constant.
How do I size a resistor for an LED?
Subtract the LED forward voltage from the supply for the series resistor. With 5 V supply, 2 V forward drop and 20 mA, enter 3 V and 20 mA: 150 Ω dissipates 0.06 W. Check component tolerances and the resistor derating curve for its operating conditions.
Why exactly two values?
Four quantities are tied together by two equations, so two must be known to pin down the others. Three or four entries could contradict each other, so the calculator asks for exactly two.
Does it work for AC?
Yes for purely resistive loads such as heaters, using RMS voltage and current. Motors and other inductive or capacitive loads also have reactance and a power factor, which this simple form ignores.
Last updated: 2026-10-30