LED Resistor Calculator
Find the right resistor to safely power an LED.
Calculate
Formulas
R = (Vs - Vf) / IVs = Supply voltage, Vf = LED forward voltage, I = LED current
LED Forward Voltages
Red: 1.8-2.2V. Yellow: 2.0-2.2V. Green: 2.0-3.5V. Blue: 3.0-3.5V. White: 3.0-3.5V. Standard current: 20mA.
How to Choose an LED Resistor
An LED is a current-driven device: connect it directly to a supply and it will draw uncontrolled current and burn out. A series resistor limits that current to a safe value. The calculator uses Ohm's law applied to the voltage left over after the LED's forward voltage:
R = (Vs - Vf) / IVs is the supply voltage, Vf is the LED forward voltage, and I is the desired LED current (typically 20 mA for standard 5 mm LEDs).
The resistor drops the difference between supply and forward voltage. The larger that difference, the larger the resistor needed for a given current.
Worked Examples
A red LED (Vf ≈ 2.0 V) at 20 mA from a 5 V supply:
R = (5 - 2.0) / 0.02 = 150 Ω
The nearest standard value is 150 Ω (or 220 Ω for a dimmer, safer result).
A blue LED (Vf ≈ 3.2 V) at 20 mA from 12 V:
R = (12 - 3.2) / 0.02 = 440 Ω → use 470 Ω standard.
Always round up to the next standard value so you never exceed the rated current.
Don't Forget Resistor Power Rating
The resistor must dissipate the power it drops without overheating. Power equals voltage across the resistor times current:
P = (Vs - Vf) × IIn Example 2, P = 8.8 × 0.02 = 0.18 W. A standard 1/4 W (0.25 W) resistor is fine. For higher currents or larger voltage differences, check this figure and choose a 1/2 W or 1 W resistor as needed.
Typical LED Forward Voltages
| LED colour | Forward voltage (Vf) | Typical current |
|---|---|---|
| Red | 1.8 – 2.2 V | 20 mA |
| Yellow / Amber | 2.0 – 2.2 V | 20 mA |
| Green | 2.0 – 3.5 V | 20 mA |
| Blue | 3.0 – 3.5 V | 20 mA |
| White | 3.0 – 3.5 V | 20 mA |
Forward voltage varies between manufacturers; check the datasheet when precision matters.
Frequently Asked Questions
What happens without a resistor?
The LED draws excessive current and overheats, usually failing within seconds. A current-limiting resistor (or a constant-current driver) is essential for any LED on a fixed-voltage supply.
What if the calculated value isn't a standard resistor?
Round up to the next standard E12 value (such as 150, 180, 220, 330 Ω). A slightly larger resistor means slightly less current, which is safe.
Can one resistor drive multiple LEDs?
For LEDs in series, subtract the sum of their forward voltages from the supply. For LEDs in parallel, give each its own resistor — sharing one causes uneven current and premature failure.
Why round current to 20 mA?
20 mA is the standard rating for common 3 mm and 5 mm indicator LEDs. Many run fine and last longer at 10–15 mA, which is often bright enough.