Electrical calculator
LED Resistor Calculator
Calculate the resistor needed for one LED or a series string, then check the current and resistor wattage.
Formula
Resistance = supply voltage minus LED voltage drop, divided by current in amps
Resistor power = current squared × resistance
Worked example
Three 2.1 volt LEDs in series drop 6.3 volts. On a 12 volt supply, the resistor must drop the remaining 5.7 volts. At 20 milliamps, the exact resistor is 285 ohms. The calculator chooses the next standard value so the current does not exceed the target.
Common mistakes
- Entering milliamps as amps.
- Using one resistor for several parallel LED branches.
- Assuming every LED color has the same forward voltage.
- Choosing a resistor wattage equal to the exact calculated dissipation.
Check the actual LED specifications
Forward voltage changes with the part, current and temperature. Each parallel branch should normally have its own resistor. This calculator does not replace the component data sheet.