V = I × R
Ohm's law links voltage, current and resistance. Choose the value to solve for and enter the other two in convenient units — millivolts, milliamps, kilohms and so on. The calculator shows the answer and the power dissipated (P = V × I), which tells you what wattage resistor you need.
Great for choosing a resistor for an LED, checking a circuit or learning electronics.
How to use Ohm's law
- 1
Choose what to solve for
Voltage, current or resistance.
- 2
Enter the other two
With their units.
- 3
Check the power
Pick a resistor rated above it.
Key features
Solve any variable
V, I or R.
Power output
In W and mW.
Convenient units
mV, mA, µA, kΩ, MΩ.
Instant
Updates as you type.
Common uses
- Electronics hobbyists: LED resistor values.
- Students: Circuit homework.
- Technicians: Quick checks on the bench.
Tips and good to know
- For an LED, use the supply voltage minus the LED's forward voltage as V.
- Choose a resistor rated at least twice the calculated power for safety.
Frequently asked questions
What is Ohm's law?
V = I × R: voltage equals current times resistance.
How do I calculate current?
Choose Current and enter voltage and resistance: I = V ÷ R. 12 V across 470 Ω draws about 25.5 mA.
How much power does a resistor dissipate?
P = V × I = I²R. The calculator shows it for every result.