GlenCalc

LED resistor calculator

An LED needs a resistor to limit its current. Find the forward voltage and current on its datasheet (typical red 2.0 V, blue or white 3.0–3.3 V, 20 mA).

Your answer

Use a standard resistor of150 ΩExact value 150 Ω
Resistor power0.06 W
Actual current with that resistor20 mA
Choose a rating of at least1/4 W

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How we worked it out

Updated as you change the numbers.

  1. R = (supply − LEDs × forward voltage) ÷ current. Round up to the next standard (E24) value. Power = voltage across R × current.

Method and assumptions

Rounding up keeps the current at or below the target. Choose a resistor rated for at least twice the power it dissipates.

Questions

What resistor for a red LED on 5 V at 20 mA?

150 Ω (exactly 150), dissipating 0.06 W.

Sources

Method and figures checked on . Every worked answer on this page is covered by an automated test. Found a mistake? Tell us.

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