LED current-limiting resistor calculator
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KEY POINTS
An LED's current-limiting resistor is R = (supply voltage − Vf × number in series) / current. Driving a white LED (Vf about 3.1 V) at 20 mA from 5 V needs about 95 Ω, so 100 Ω is chosen from the E24 series. Vf is set mainly by the band gap of the emitting layer: about 2 V for red (AlGaInP) and about 3 V for blue and white (InGaN).
Inputs
Vf varies with color, part, current and temperature (it falls by about 2 mV/°C as temperature rises). Check the datasheet of the LED you use. For power LEDs (hundreds of mA or more), resistor losses are large and a constant-current DC-DC driver is the usual choice.
Vf and resistor value by color (5 V supply, 20 mA, one LED)
| Color | Emitting layer material | Wavelength (nm) | Typical Vf | Resistor (E24) |
|---|---|---|---|---|
| Infrared (940 nm) | GaAs / AlGaAs | 850 to 940 | 1.3 V | 200 Ω |
| Red | AlGaInP | 620 to 630 | 2 V | 150 Ω |
| Orange | AlGaInP | 600 to 610 | 2.05 V | 150 Ω |
| Yellow | AlGaInP | 585 to 595 | 2.1 V | 150 Ω |
| Green (high brightness) | InGaN | 520 to 530 | 3 V | 100 Ω |
| Blue | InGaN | 455 to 470 | 3.1 V | 100 Ω |
| White (blue + phosphor) | InGaN + YAG phosphor | — (broadband) | 3.1 V | 100 Ω |
| Ultraviolet (365 nm) | InGaN / AlGaN | 365 to 405 | 3.6 V | 75.0 Ω |
LED color and forward voltage
An LED is a semiconductor that passes forward current through a pn junction and emits the energy released when electrons and holes recombine as light. The photon energy, and so the color, is set mainly by the band gap of the emitting layer: λ [nm] ≈ 1240 / Eg [eV]. Blue (about 2.7 eV) has a larger band gap than red (about 1.9 eV), so its forward voltage is higher.
LED current rises exponentially with voltage, so connecting one directly to a voltage source lets a tiny voltage difference send the current out of control. The current-limiting resistor tames this steep characteristic with its linear one and keeps the current stable. Most white LEDs combine a blue InGaN LED with a yellow phosphor; practical blue LEDs are what made white LED lighting possible.
FAQ
Why pick the next higher E-series value?
A resistor at or above the calculated value keeps current at or below the design target, which protects the LED.
Why do blue LEDs need a higher voltage than red?
Photon energy follows the band gap of the emitting layer. Blue InGaN emits at about 2.7 eV and red AlGaInP at about 1.9 eV, so the forward voltage differs accordingly.
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Last updated: October 23, 2026