LED Series Resistor Calculator

The formula and the forward voltages

R = (Vs − Vf) ÷ If. The only part you need to look up is Vf, and it tracks the color: red and yellow sit around 1.8 to 2.2 V, green and blue around 3 to 3.4 V, white around 3.2 V. Most indicator LEDs are happy anywhere from 5 to 20 mA; they're plenty visible at the low end.

Round up, not down

The calculator suggests the next standard E12 value above the exact answer. Rounding up means slightly less current, which an LED never minds. Rounding down means more current, which it eventually does. Check the dissipation figure too: past a quarter watt you want a bigger resistor body, not just the right ohms.

Powering several LEDs

Two LEDs in series share one resistor and one current, which is the tidy way to do it: add their forward voltages together and subtract the total from the supply. Two LEDs in parallel behind a single resistor is the arrangement to avoid, because the one with the slightly lower forward voltage takes most of the current and burns brighter until it fails. Parallel strings each want their own resistor, one per branch.

When the supply is close to the forward voltage

A blue LED at 3.2 V on a 3.3 V supply leaves 0.1 V for the resistor, which makes the current wildly sensitive to every tolerance in the circuit. Either raise the supply or drive the LED with a constant-current source rather than a resistor. The same trap appears with white LEDs on a single lithium cell as the cell discharges: what looked correct at 4.2 V misbehaves at 3.4 V.