Ohm's Law Calculator (V, I, R, P)
Pick the two values you know, enter them, and the calculator returns voltage, current, resistance and power together. Fields you are not using are ignored.
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Formula
V = I × RP = V × I = I² × R = V² ÷ R- From power and resistance:
V = √(P × R)andI = √(P ÷ R) - From voltage and power:
I = P ÷ VandR = V² ÷ P - From current and power:
V = P ÷ IandR = P ÷ I²
Units are volts (V), amperes (A), ohms (Ω) and watts (W). Zero resistance or zero current cannot be divided by, so those cases report an error.
Worked example
A 12 V supply drives 2 A. Resistance R = 12 ÷ 2 = 6 Ω and power P = 12 × 2 = 24 W. Check with the other mode: 12 V across 6 Ω gives I = 12 ÷ 6 = 2 A and P = 144 ÷ 6 = 24 W.
Frequently asked questions
What is Ohm's law?
Ohm's law says voltage equals current times resistance, V = I × R. A current of 2 A through 6 Ω needs 12 V across it.
How do I calculate power from voltage and current?
Multiply them: P = V × I. At 12 V and 2 A the power is 24 W. Combined with Ohm's law this also gives P = I² × R and P = V² ÷ R.
How do I find resistance from voltage and power?
Use R = V² ÷ P. A 230 V heater rated 2300 W has R = 52900 ÷ 2300 = 23 Ω and draws 10 A.
What units does the calculator use?
Volts, amperes, ohms and watts. Convert milliamps first: 20 mA is 0.02 A, and 4.7 kΩ is 4700 Ω.
Can I use this to check a real circuit?
It is for learning and design checks only, not safety certification. Real parts have tolerances, heat up and change resistance, and mains work needs a qualified person.