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For indicator LEDs. High-power LEDs need a constant-current driver. Everything is calculated in your browser; nothing you type is uploaded.

R = (Vsupply − n × Vf) ÷ I, rounded up to the E12, E24 or E96 series; the rating is at least twice the dissipated power.

A red LED (2 V, 20 mA) on 5 V needs 150 Ω, which dissipates 60 mW: a 1/8 W resistor is enough.