LEDs are efficient, bright, and long-lasting - but without a series resistor they burn out fast. In this guide I'll show you step by step how to find the right resistor. The quickest way is our LED Series Resistor Calculator, but the math behind it is simpler than you think.
Why LEDs need a series resistor
An LED is not a normal load: it has a fixed forward voltage and would instantly draw too much current and burn out if the voltage is too high. The series resistor limits that current to a safe value. Typical forward voltages are: red LEDs about 1.8-2.2 V, blue and white LEDs about 3.0-3.4 V. A common LED current is 20 mA.
The formula and a worked example
The resistance follows from this formula:
- R = (U_s - n · U_LED) / I_LED
Here U_s is the supply voltage, n the number of LEDs in series, U_LED the forward voltage, and I_LED the LED current. Example: 12 V supply, one LED with 2 V forward voltage, 20 mA current.
- R = (12 - 2) / 0.02 = 500 Ω
- Nearest E12 standard value: 560 Ω
Don't forget power dissipation
The resistor gives off heat. You calculate the power dissipation like this:
- P = I² · R = 0.02² · 500 = 0.2 W
A good rule of thumb: pick double the rating. So a 0.25 W resistor is comfortably enough here.
Series or parallel? A practical tip
If you want to run several LEDs, you have two options. In series, several LEDs share one resistor - the forward voltages add up. In parallel, each LED gets its own series resistor, because otherwise small differences cause uneven brightness. My tip: for even light with mixed LEDs, always use one resistor per LED.
