Inputs
Check the LED datasheet for the exact value
A standard 5mm LED is usually rated for 20mA. 10mA is a practical choice
Result
Recommended resistor (E24 series)
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- Calculated value
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- Actual current
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E24 resistor values (tap to expand)
How to use
- Select the supply voltage3.3V for Pico / ESP32, 5V for Arduino
- Select the LED colorA typical Vf (forward voltage) is filled in for each color
- Select the currentIf unsure, choose 10mA (balanced)
- Tap CalculateThe recommended E24 resistor is shown
Typical Vf by LED color
| Color | Vf range | Typical |
|---|---|---|
| Red | 1.8–2.2V | 2.0V |
| Green (AlGaP) | 2.0–2.6V | 2.2V |
| High-brightness green (InGaN) | 2.8–3.4V | 3.0V |
| Blue | 3.0–3.6V | 3.2V |
| White | 3.0–3.6V | 3.2V |
Check the LED datasheet for the exact value. Vf varies by manufacturer and part number, even for the same color.
FAQ
What happens if I connect an LED without a current-limiting resistor?
Without a current-limiting resistor, too much current flows through the LED and it fails almost instantly. An LED cannot limit its own current, so always connect a resistor in series.
Can I drive blue or white LEDs from a Raspberry Pi Pico (3.3V)?
Blue and white LEDs have a Vf (forward voltage) of about 3.2V, which is very close to the 3.3V supply of a Pico or ESP32, so the current is unstable. Use the VBUS pin (5V over USB) with a transistor driver, or choose a low-Vf LED rated for 3.3V.
What is the E24 series, and why is the chosen resistor different from the calculated value?
The E24 series is the set of standard resistor values sold commercially (24 values per decade). A resistor with exactly the calculated value is usually not available, so the tool picks the nearest E24 value at or above the calculated value to keep the LED from overcurrent.
What should I do if the power dissipation exceeds 250mW?
A typical 1/4W (250mW) resistor may overheat or burn out. Use a resistor rated 1/2W (500mW) or higher. Lowering the current (for example, from 20mA to 10mA) also reduces the power dissipation.
What is the formula for an LED current-limiting resistor?
The formula is R = (Vcc − Vf) ÷ IF, where Vcc is the supply voltage, Vf is the LED forward voltage, and IF is the target current in amperes. Example: a 5V supply, a red LED (Vf = 2.0V), and 10mA give R = (5 − 2.0) ÷ 0.01 = 300Ω, so you choose 300Ω from the E24 series.