LED Vorwiderstand berechnen - Dominik Ewers Solutions

Calculate LED series resistor

24. May 2025
Electroblog

If you want to use an LED in your circuit, there is one question you will hardly be able to avoid:
Which series resistor do I need to ensure that my LED lights up safely and reliably?
In this article, I'll explain it to you very simply:

why a series resistor is necessary at all,
how you can calculate it yourself (incl. formula),
which are typical values (table),
and how you can get started right away with my interactive tool.

Why does an LED need a series resistor at all?

An LED (light-emitting diode) is not a normal load. It allows current to flow above a certain voltage - but without a built-in current limiter.
This means that:

If you connect an LED directly to a voltage source, too much current will flow and it will be destroyed.
The solution: A series resistor that limits the current.

Formula for calculating the series resistance

You can simply calculate the required resistance with :

Formula:
R = (U_Supply – U_LED) / I_LED

Explanation of the values:

$_{U Supply}$ : e.g. 5 V or 12 V
$_{U LED}$ : typical forward voltage of the LED (e.g. 2 V)
$_{I LED}$ : desired LED current, e.g. 20 mA = 0.02 A

Sample calculation:

You want to operate a red LED (approx. 2.2 V) at 5 V, with 20 mA current.

Example:
You want a red LED with 2.2 V forward voltage powered by 5 V – with 20 mA current

R = (5 V − 2,2 V) / 0,02 A = 2,8 V / 0,02 A = 140 Ω

Da 140 Ω is not a standard value in the E12 series, take the next larger one: 150 Ω.
This keeps the LED slightly below its maximum load - good for service life and power consumption.

Table: Typical LED voltages and current consumption

LED color	Forward voltage (V)	Typical current (mA)
Red	1,8 – 2,2	10 – 20
Yellow	1,9 – 2,2	10 – 20
Green	2,0 – 3,2	10 – 20
Blue	2,8 – 3,4	10 – 20
White	3,0 – 3,4	10 – 20

Calculate series resistance automatically - with my tool

Don't want to do the math by hand every time? Then simply use my interactive tool:
Enter the supply voltage, LED voltage and desired current - and you get it immediately:

the calculated value
a suitable standard value from the E12 series

🔧 Click here to go to the tool

Conclusion: series resistors are mandatory - but easily calculated

Even if it sounds trivial: the right series resistor protects your LEDs from death by overcurrent - and ensures that they work with the desired brightness.

With the formula and tool from this article, you can now correctly dimension every LED - without any guesswork.