Ohm's Law Calculator
Enter any two of volts, amps, ohms and watts โ get the rest.
Using the calculator
Fill in any two of the four values โ volts, amps, ohms, watts โ and the other two are solved instantly from Ohm's law (V = I ร R) and the power law (P = V ร I). Type a third and the calculator re-solves from your two most recent entries, so playing with scenarios is frictionless. The formula wheel below shows all twelve rearrangements for reference.
A worked example: an LED strip drawing 2 A from a 12 V supply dissipates 24 W and presents 6 ฮฉ. A 60 W bulb on 230 V mains draws 0.26 A through an effective 883 ฮฉ.
Where the simple law stops
Ohm's law as used here assumes DC circuits and resistive loads โ resistors, heaters, incandescent filaments. Real AC devices complicate it: motors and electronics have power factor and impedance, so their VA and watts differ, and a filament's resistance changes as it heats (that 883 ฮฉ bulb measures ~40 ฮฉ cold). The arithmetic here is the right starting point for electronics hobby work, LED supplies, automotive 12 V circuits and sizing resistors.
Reading the resistor you just calculated for is the colour code calculator's job. And the standing caution: mains voltage work belongs with a qualified electrician โ this page does arithmetic, not safety.
Frequently asked questions
What is Ohm's law?
V = I ร R: voltage equals current times resistance. With the power law P = V ร I, any two of the four quantities determine the other two.
How many watts is 12 volts at 2 amps?
24 watts โ power is volts times amps. The same load presents 6 ohms of resistance.
How do I find current from watts and volts?
Divide: I = P รท V. A 60 W device on 12 V draws 5 A โ the reason high-power 12 V circuits need thick cables and proper fuses.
Does Ohm's law work for AC mains devices?
For resistive loads like heaters, yes. Motors and electronics involve power factor and impedance, so real current can exceed the naive calculation โ and mains work needs a qualified electrician.