LED resistor

R = (Vsupply − Vf) / I + nearest E24 value and dissipated power

Parameters

Supply voltage, V

LED

Current, mA

LEDs in series

Resistor

Resistor power: 400 мВт

1 000 Ом

exact: 500 Ом → E24: 1 000 Ом

Resistor power: 400 мВт (¼ Вт+)
Actual current: 10.0 мА

Typical 5-mm LED current is 20 mA (5–10 mA is enough for indication and lasts longer). The resistor must handle the power — take ×2 margin. Never power an LED without a resistor, even for a second — current rises exponentially.