LED Resistor Calculator
The series resistor an LED needs, with the nearest standard value.
How it works
An LED is a diode, not a resistor: past its forward voltage the current rises almost vertically. Connect one straight to a supply and it destroys itself in a fraction of a second. The series resistor sets the current.
R = (Vsupply - Vforward x n) / I
Forward voltage depends mostly on the colour — a red LED sits near 2 V, blue and white near 3.2 V, because the material's band gap sets the photon energy.
Series LEDs share one current, so one resistor does for all of them, provided the supply exceeds the total forward voltage with headroom to spare.
Common questions
What if the supply is below the forward voltage?
No resistor will help. Blue and white LEDs need over 3.2 V, so they will not light from a single 3 V cell at full brightness. Use a higher supply or a boost driver.
Do LEDs in parallel need one resistor or several?
One each. Forward voltages vary slightly between parts, so a shared resistor lets the lowest-voltage LED hog the current and burn out first.