First, you need to know the resistance needed. For this, you need Ohm's Law.
Vs = Source Voltage (either 3.3v, 5v, or 12v)
Vf = Forward Voltage (almost always stated on the package)
If = Forward Current (usually 20-30mA, stated as 0.02 to 0.03 in the formula)
R = Resistance in Ohms
For example, a 2.1v, 20mA LED using a 5v power source would figure like this:
Use this number as a rough guide and purchase the resistor(s) that is the next highest value of R (in this case, I believe it'd be 170 ohms).
You also need to know what Watt rated resistor you need.
P=If*(Vs-Vf) <--- Power Law
P = Power in Watts
If = Forward Current from the previous formula
Vs-Vf = what you used in the previous formula also
Continuing with the example:
If P <= 0.25, then use 1/4 Watt rated
If P > 0.25 but <= 0.5 use 1/2 Watt rated
If P > 0.5 then use 1 Watt rated
Using these numbers will help you purchase the resistor(s) you will need for a project working with LEDs.
Here's an update to an old post (for what it's worth)...calculating for parallel and series-wired LEDs.
Parallel-wired LEDs maintain the same Vf, but the If increases for each LED in parallel (e.g., two 20mA LEDs in parallel would use .04 for If), making it more or less IfT=If1+If2+etc.
Series-wired LEDs maintain the same If, but the Vf increases for each LED in series (e.g., two 2.1VDC LEDs in series would use 4.2 for Vf), making it more like VfT=Vf1+Vf2+etc.
Edited by EvilWays, 23 October 2004 - 11:03 PM.