LED Resistor Calculator: Series Resistor for Any LED

Enter the supply, the LED's forward voltage and the current you want, and get the exact resistor, the next E12 value up and the power it dissipates.

Calculator

Exact resistance
310 Ω
Next E12 value up
330 Ω
Current with that resistor
9.394 mA
Power in the resistor
29.12 mW

Type numbers like 4.7, 0.002 or 2e-3. Results are rounded to 4 significant figures.

The formulas

R=VS−VFIFR = \dfrac{V_S - V_F}{I_F}

The resistor drops whatever the LED doesn't. VF comes from the LED's datasheet at the chosen current.

PR=IF 2 RP_R = I_F^{\,2}\,R

Pick a resistor rated for at least twice this.

  • Typical forward voltages: red about 2.0 V, yellow and green about 2.1 V, blue and white about 3.0 to 3.3 V. Check the datasheet.
  • Rounding the resistor up keeps the current at or below what you asked for.
  • Small indicator LEDs are typically rated for 20 to 30 mA continuous; 5 to 10 mA is often bright enough.
  • Several LEDs in series share one resistor: subtract the sum of their forward voltages.

Worked example: a red LED from 5 V at 10 mA

  1. The Kingbright WP7113ID red LED lists VF = 1.9 V typical at 10 mA.
  2. R = (5 V − 1.9 V) / 10 mA = 3.1 V / 0.010 A = 310 Ω. The next E12 value up is 330 Ω.
  3. With 330 Ω the current is 3.1 V / 330 Ω = 9.394 mA, just under the 10 mA asked for.
  4. Power in the resistor: (9.394 mA)² × 330 Ω = 29.12 mW, so a 1/4 W (250 mW) resistor is plenty.
  5. From a 9 V battery at 15 mA, taking VF as 1.9 V again: R = 7.1 V / 0.015 A = 473.3 Ω. That is just above 470 Ω, so the next E12 value up is 560 Ω, giving 12.68 mA.

Learn the topic

Sources