LED Formulas: Resistor Value and Power
LED formulas for the series resistor from supply voltage, forward voltage and current, resistor power, LEDs in series, with worked examples and a table.
The formulas
| To find | Formula |
|---|---|
| Series resistor | R = (Vs − Vf) / I |
| Resistor with n LEDs in series | R = (Vs − n × Vf) / I |
| Power in the resistor | PR = I² × R |
| Power in the LED | PLED = Vf × I |
| Current with a chosen resistor | I = (Vs − Vf) / R |
Vs is the supply voltage, Vf the forward voltage of the LED (from the data sheet, often about 2 V for red and 3 V or more for white and blue) and I the current you want, in amperes. Vs must be larger than n × Vf. LEDs in parallel should each have their own resistor.
Worked example: one LED on 12 V
The LED has a forward voltage of 3.2 V and a target current of 20 mA.
- R = (12 − 3.2) / 0.02 = 440 Ω
- The next standard value (E12 series) is 470 Ω, which gives I = 8.8 / 470 = 18.72 mA
- PR = 18.72² mA × 470 Ω = 164.8 mW, and PLED = 3.2 × 0.01872 = 59.91 mW
Worked example: three LEDs in series on 12 V
- R = (12 − 3 × 3.2) / 0.02 = 120 Ω
Resistor for one 3.2 V LED at 20 mA
| Supply | Resistor |
|---|---|
| 5 V | 90 Ω |
| 9 V | 290 Ω |
| 12 V | 440 Ω |
| 24 V | 1,040 Ω |
Frequently asked questions
Why round up to the next standard value?
A larger resistor gives a slightly lower current, which is safer for the LED. A smaller value would push the current above the target.
What power rating should the resistor have?
Use at least twice the calculated dissipation. A quarter-watt resistor is ample for a single indicator LED at 20 mA.
Can I connect LEDs in parallel with one resistor?
It is risky. The forward voltages differ slightly, so one LED can take most of the current. Give each LED its own resistor.
Does the forward voltage change?
Yes. It varies with current and temperature, and between batches. A constant-current driver is better for high-power LEDs.
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