Free online calculator
LED series resistor calculator
Nobody wires just one LED. Each string is its own series chain of LEDs behind its own resistor — stringing more LEDs raises the headroom the resistor has to eat; adding more strings in parallel just repeats the same string, each with its own resistor, never a shared one.
Example R = 150 Ω per string — dissipation: 60 mW per string — total supply current: 20 mA.
Formula
With the default values
The resistor drops whatever the LED does not, and turns that headroom into heat:
Example Nearest E24: 150 Ω → I = 20 mA (0.0%)
What you enter
- Source voltage (V)
- LED Vf (V)
- Desired current
- Not sure of Vf? Pick a color (fills in a typical value):
- LEDs in series per string
- Parallel strings
- Nearest buyable value
