LED Resistor for a 5V USB / Arduino Rail
Find the series resistor for one or more LEDs from the supply, the forward drop, the count, and the current.
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Result
About this tool
This page opens the LED resistor calculator with a 5V supply, a 2.2V LED forward voltage, one LED and a 20mA target current already filled in — the resistor value below is computed live on load.
5V is the supply almost everyone hits first: USB power, an Arduino's 5V pin, a phone charger. At that lower voltage the LED's own forward drop matters more, so this preset uses 2.2V — a common figure for many standard-color 5mm and 3mm LEDs — leaving 2.8V across the resistor at the standard 20mA target, for 140Ω and 0.056W of resistor dissipation.
If your LED's datasheet lists a different forward voltage (blue and white LEDs commonly run 3–3.4V, which the 9V preset above handles more comfortably than 5V does), swap in the real number above — a mismatch here is the most common reason a 5V LED circuit ends up dimmer or brighter than expected.
Example. The defaults, 5 V, a 2 V drop, one LED, and 20 mA, return 150 Ω and 60 mW. The nearest E24 values are 150 Ω, 160 Ω, and 130 Ω. 9 V with the same LED and current returns 350 Ω and 140 mW.
Limit. The LEDs are in series. The calculation stops when the combined forward drop is not strictly below the supply. The current unit menu opens on milliamps.