LED's...How to calculate the dropping resistor

[Geezer]

This is a handy picture showing the formula for calculating the required voltage dropping resistor for a known value/rating LED.
If I don't know the LED's rating, I usually just hook up a 5k-25k pot in-line as a variable resistor, adjust it to an acceptable brightness, then measure the pot & replace it with the next closest higher value resistor.

When you buy a LED, it will have a voltage and current rating. Typical values you might see at Radio Shack would be 2 volts at 20ma. This pic will show you how to calculate for any LED. The calculated resistance will give you the maximum brightness for a particular LED. I would not go below that value, but it's fine to go above. The higher the resistance, the dimmer the LED. And of course, you'll reach a point where the LED stops emitting light.

So, like TubeGeek said, it's not too critical.