How to use the LED resistor calculator
- Enter the supply voltage and choose the LED colour or type the datasheet forward voltage.
- Enter the target current, LEDs per string and the number of strings.
- Use the standard value and wattage shown, and check the actual current.
LED forward voltage and resistor chart (20 mA)
Series resistor R = (supply − forward voltage × LEDs in series) ÷ current. The table uses 20 mA, one LED and a typical forward voltage, rounded up to the next E24 value so the current stays at or below 20 mA. Use the forward voltage from your LED datasheet.
| LED colour | Forward voltage | 3.3 V | 5 V | 12 V | 24 V |
|---|---|---|---|---|---|
| Red | 1.8–2.2 V | 68 Ω | 150 Ω | 510 Ω | 1.1 kΩ |
| Orange | 2.0–2.2 V | 62 Ω | 150 Ω | 510 Ω | 1.1 kΩ |
| Yellow | 2.0–2.2 V | 62 Ω | 150 Ω | 510 Ω | 1.1 kΩ |
| Green (InGaN) | 2.8–3.4 V | 15 Ω | 100 Ω | 470 Ω | 1.1 kΩ |
| Blue | 2.9–3.4 V | 5.1 Ω | 91 Ω | 470 Ω | 1.1 kΩ |
| White | 2.9–3.4 V | 5.1 Ω | 91 Ω | 470 Ω | 1.1 kΩ |
| Infrared 940 nm | 1.2–1.5 V | 100 Ω | 200 Ω | 560 Ω | 1.2 kΩ |
| UV 395 nm | 3.2–3.6 V | — | 82 Ω | 430 Ω | 1.1 kΩ |
Frequently asked questions
Why does an LED need a resistor?
LED current rises steeply with voltage, so connecting one straight to a supply overloads it. A series resistor drops the extra voltage and limits current; use a constant-current driver for power LEDs.
Can parallel LEDs share one resistor?
Avoid it. Forward voltages vary, so current crowds into the LED with the lowest voltage. Give each string its own resistor, or put the LEDs in series with one resistor.
What if the result is not a standard value?
Use the next standard value above; the current will be slightly lower and safer. The calculator rounds up to the chosen series and shows the resulting current.
Are my inputs uploaded?
No. Everything is calculated locally in your browser and nothing is stored; a share link only carries the values you entered.
Results are for design estimates and part selection; for production and safety-related circuits follow component datasheets, applicable standards and measurements.