Calculating led resistor values manually?

[thewise1]

I know there are tons online, but not always will I have access to internet. (like when I have extra time at school, that's where I design a lot of stuff)

So what's the formula?

Or series of steps?

1000th post!

[Kyo]

R = (Vs - Vl) / Al
Vs = supply voltage
Vl = LED voltage
Al = LED draw

for example
3.4V led with 20 mA draw, Vs = 5V

R = (5 - 3.4)/0.02 (remember to convert mA -> A!)
R = 80 Ohm

[thewise1]

Thanks a lot kyo!