How this calculator works
The resistor absorbs the supply voltage left after the LED forward drops. A series string carries the same current through every LED. Parallel LEDs need individual resistors because differences in forward voltage can cause unequal currents. The resistor dissipates I²R; choose a rated part with thermal allowance. LED colour alone does not specify its forward voltage, so enter the datasheet value at the target current.
Formula
Series: R = (Vs - N Vf)/I. Parallel branches: Rbranch = (Vs - Vf)/Ibranch; Isupply = N Ibranch. Presistor = Ibranch² Rbranch.
Worked example
Two series LEDs at 2 V each and 20 mA from 12 V need 400 ohms and dissipate 0.16 W in the resistor. Three separate 2 V LEDs on 5 V, each with 150 ohms, draw 60 mA in total.
Advertisement
Frequently asked questions
Why not one common resistor for parallel LEDs?
Small forward-voltage differences can cause one LED to take most of the current. A resistor per branch improves sharing.
Which resistor power rating should I use?
The output is calculated dissipation, not a recommended rating. Allow for supply tolerance, temperature and the manufacturer's derating curve.
Can I enter LED colour?
Use a measured or datasheet forward voltage instead. LEDs of the same colour can have different forward drops and current ratings.