sumivo

LED Series Resistor Calculator

Calculate series resistance and resistor power for a single LED.

01 / INPUTS

V
V
mA

02 / RESULT

Resistance

150 Ω

Resistance
150 Ω
Resistor power
0.06 W

Calculation trace

  1. 20 mA ÷ 1000 = 0.02 A
    0.02 A
    Convert LED current
  2. (5 V − 2 V) ÷ 0.02 A = 150 Ω
    150 Ω
    Series resistance
  3. (5 V − 2 V) × 0.02 A = 0.06 W
    0.06 W
    Resistor power
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How it works

The tool converts LED current from milliamperes to amperes, then computes the resistor voltage drop as Vs − Vf and resistance as R = (Vs − Vf) / I. Resistor dissipation is P = (Vs − Vf)·I.

Assumptions & limits

  • Uses a fixed entered LED forward voltage and an ideal DC supply; actual forward voltage varies with current and temperature.
  • Supply voltage must exceed forward voltage, and all three inputs must be positive finite numbers.
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FAQ

Why must the supply voltage exceed the LED forward voltage?
The series resistor needs a positive voltage drop, Vs − Vf, to set a positive current with R = (Vs − Vf) / I.
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