⚡ Ohm's Law Calculator
Enter any two of voltage, current, resistance and power, and the calculator fills in the other two using Ohm's law.
Enter any two values — leave the other two blank.
Voltage
12 V
Current
2 A
Resistance
6 Ω
Power
24 W
Ohm's law ties voltage, current and resistance together (V = I × R), and power adds a fourth (P = V × I = I²R = V²/R). Give any two and the other two are fixed. Use consistent units — volts, amps, ohms and watts. 🔒 In your browser.
How the ohm's law calculator works
Ohm's law states that voltage equals current times resistance (V = I × R), and electrical power extends it: P = V × I, which also equals I²R and V²/R. Those relationships form the "power wheel" — with any two of the four quantities known, the remaining two are fixed. The tool detects which two you entered, picks the right formula, and shows voltage (V), current (A), resistance (Ω) and power (W) together. Use consistent base units.
Keep everything in base units: volts, amps, ohms and watts. If you're working in milliamps or kilohms, convert first (1 mA = 0.001 A, 1 kΩ = 1000 Ω) or the power and resistance will be off by factors of a thousand.
Frequently asked questions
What is Ohm's law?
Ohm's law says the current through a conductor is proportional to the voltage across it and inversely proportional to its resistance: V = I × R. Rearranged, I = V / R and R = V / I.
How do I calculate power from voltage and current?
Power is voltage times current: P = V × I. A 12 V supply delivering 2 A provides 24 W. You can also use P = I²R or P = V²/R when you know the resistance instead.
Can I find resistance from power and voltage?
Yes — R = V² / P. For 12 V dissipating 24 W, R = 144 / 24 = 6 Ω. Enter just the voltage and power and the calculator returns the current and resistance too.
What is the power wheel?
A chart of the twelve rearrangements of V = IR and P = VI, letting you get any quantity from any two others. This calculator does the same thing: give it two values and it computes the rest.
What units should I use?
Volts, amps, ohms and watts — the SI base units. Convert milliamps to amps (÷1000) and kilohms/megohms to ohms (×1000 / ×1,000,000) before entering, so the results come out in watts and ohms correctly.
Why do I need exactly two values?
Ohm's law has four linked quantities and two independent equations, so any two fix the other two. With only one value the system is underdetermined; with three, the inputs could conflict — so enter exactly two and leave the rest blank.