⚡ Ohm's Law Calculator
Enter exactly two of the four values — voltage (V), current (I), resistance (R), or power (P) — and the other two are solved using V = IR and P = VI.
Two knowns fix the whole circuit
Ohm's law ties voltage, current and resistance together and the power law adds the fourth, so any two give the other two:
V = I × R P = V × I
I = V / R P = I²R = V²/R
I = √(P / R) R = V / IThe page opens with 12 V across 6 Ω. That draws 12 / 6 = 2 A, and the power is 12 × 2 = 24 W. Give it any other pair - say 24 W and 6 Ω - and it comes back to the same 2 A, because √(24 / 6) = 2.
Units, and a resistor worth sizing properly
- Everything is in base units: volts, amps, ohms and watts. A 20 mA LED current goes in as 0.02, not 20.
- Exactly two boxes, no more. Fill a third and the tool says how many it found rather than guessing which to trust.
- Sizing an LED resistor: a 5 V supply feeding an LED that drops 2 V leaves 3 V. Enter 3 V and 0.02 A for 150 Ω at 0.06 W.
- Watch the wattage. The 12 V / 6 Ω example burns 24 W and would destroy a quarter-watt hobby resistor in seconds.
Frequently asked questions
How many amps does 12 volts through 6 ohms draw?
2 amps, dissipating 24 watts. Current is voltage divided by resistance, so 12 / 6 = 2.
What resistor do I need for an LED on 5 volts?
Subtract the LED's forward voltage from the supply first. A red LED dropping 2 V at 20 mA leaves 3 V, and 3 / 0.02 is 150 ohms.
How do I get watts from volts and ohms?
Square the voltage and divide by the resistance: 144 / 6 = 24 W. The tool gets there by finding the current first.