LED Resistor Calculator

LED Series Resistor Calculator

Circuit:
Vs ──[R]──[LED]── GND
R = (Vs − Vf) / If

LED Series Resistor Calculator — Protect Your LEDs

Every LED needs a current-limiting series resistor to prevent it from drawing too much current and burning out. This LED resistor calculator takes your supply voltage, LED forward voltage, and desired forward current, then computes the exact resistor value needed — along with the nearest standard E12 and E24 values and power dissipation ratings.

The LED Resistor Formula

The series resistor value is calculated using a simple application of Ohm's Law: R = (Vs − Vf) / If, where Vs is the supply voltage, Vf is the LED's forward voltage, and If is the desired forward current in Amperes. The resistor drops the excess voltage (Vs − Vf) while limiting the current to the desired value.

LED Forward Voltage by Color

Different LED colors use different semiconductor materials, resulting in different forward voltage drops:

  • Infrared (IR): ~1.2 V
  • Red: ~2.0 V
  • Yellow / Green: ~2.1 V
  • Blue / White: ~3.2 V

Always check your specific LED's datasheet as values vary between manufacturers and LED families.

Choosing the Resistor Power Rating

The power dissipated in the series resistor is P = (Vs − Vf) × If. For reliability, choose a resistor rated for at least twice this power. For example, if the calculated power is 80 mW, use a 1/4 W (250 mW) resistor rather than a 1/8 W (125 mW) type.

Standard Resistor Series (E12 / E24)

Resistors are manufactured in standardised value series. The E12 series has 12 values per decade (10, 12, 15, 18, 22, 27, 33, 39, 47, 56, 68, 82) while E24 has 24 values. The calculator shows the nearest available standard value in both series along with the actual LED current that value will produce.

Frequently Asked Questions

What happens if I don't use a series resistor?
Without a current-limiting resistor, the LED will draw as much current as the supply can provide. LEDs have a very low forward resistance, so even a small excess voltage causes a large current spike that will destroy the LED almost instantly.
What is a typical LED forward current?
Standard 3mm and 5mm indicator LEDs typically run at 10–20 mA for full brightness. High-brightness LEDs may run at 350 mA or more. Always check the LED's datasheet for the maximum rated current and recommended operating current.
Can I use one resistor for multiple LEDs in series?
Yes, for LEDs in series the forward voltages add up. Use R = (Vs − (Vf1+Vf2+...)) / If. For LEDs in parallel, each needs its own series resistor to ensure even current sharing, as small differences in Vf can cause unequal current distribution.
Why does my LED brightness change with battery level?
As a battery discharges, its voltage drops. Since R = (Vs − Vf) / If, a lower Vs reduces the current through the LED, dimming it. Constant-current LED drivers solve this by maintaining a fixed current regardless of supply voltage.