LED Forward Voltage Chart
Every LED resistor calculation starts with one number people guess wrong: the forward voltage. Guess high and the LED is dim; guess low and it is bright for a minute. Here is the honest range by colour, and the reasons the number moves.
Forward voltage at 20 mA
| Colour | Vf at 20 mA | Notes |
|---|---|---|
| Infrared | 1.1 to 1.5 V | remote controls, IR blasters, night-vision cameras |
| Red (classic) | 1.8 to 2.2 V | the classic GaAsP/GaP; the most stable with heat |
| Orange | 2.0 to 2.2 V | same family as red |
| Yellow | 2.0 to 2.3 V | same family as red |
| Green, low brightness | 2.0 to 2.4 V | old GaP green; a different curve than the bright one |
| Green, high brightness | 2.9 to 3.4 V | InGaN chemistry; this is why green values confuse people |
| Blue | 2.8 to 3.4 V | InGaN; the basis of white LEDs |
| White | 2.9 to 3.4 V | blue die plus phosphor; also the 5 V star and COB parts |
| UV | 3.2 to 3.8 V | treat like blue electrically, treat the light seriously |
| RGB module (each die) | 1.9 to 2.2 red, 3.0 to 3.4 G/B | one package, two resistor values |
These are 5 mm and standard SMD parts at room temperature. Power LEDs at 700 mA or 1 A sit a little higher: a white star is often 3.2 to 3.6 V under load, and an RGB strip runs its red die around 2 V and blue around 3.2 V.
Why the number moves
Three things shift Vf, and each explains a mystery people blame on the resistor.
Current. Vf rises with drive current. A red LED at 1 mA reads around 1.7 V and at 20 mA around 2.0 V. If you size for the current you actually want, the number lands.
Temperature. Vf falls as the junction warms, roughly 2 mV per degree for the classic colours. A part that starts at 3.2 V may sit at 3.0 V when it is warm, which is why a constant-current driver wastes less thought than a resistor does.
Chemistry. This is the big one for green. Older GaP greens run near 2.1 V and modern InGaN greens near 3.2 V, with the same colour and the same 5 mm package. When two people argue about the right resistor for a green LED, this is usually why.
Sizing the resistor: (supply volts minus Vf) divided by amps. The LED resistor calculator does it per colour; a strip or a high-power part is better served by a constant-current driver.
FAQ
FAQ
Should I use the typical or the worst-case Vf?
For one LED either is fine. For a batch or a long life, size the resistor for the low end of the range, so the current stays under the limit when Vf drops with heat.
How do I measure my own?
Feed the LED through a 1 kΩ resistor from a 5 V supply and measure across the LED with a multimeter. That gives Vf at about 3 mA, and it is close enough for most resistor math.
Why does my white LED read 2.7 V?
It is running at a much lower current than 20 mA, or it is warm. Both pull Vf down; neither means the part is broken.
Do I need a resistor at all on a 3.3 V pin with a blue LED?
Yes. 3.3 V is right around the blue forward voltage, so current control collapses: tiny Vf changes swing the current wildly. Add the resistor and check the calculator.
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