This calculates the maximum number of LEDs that you can put in series while keeping the current within the limits you specify, while using a simple resistor to control the current.
The supply voltage variation, LED voltage variation and resistor tolerance are all accounted for.
The voltage drop across the resistor is often larger than the voltage across one LED.
For better control or efficiency, you can use more complicated control systems than a simple resistor. Diodes Inc have a good range of ICs for driving LEDs
If you want to pretend that power supplies are perfect, LED voltages are exact, and all resistors below 1 Ω have the same characteristics, go to https://ledcalculator.net/ Someone even registered a domain for that rubbish.
This doesn't take any account of LED resistance.
Just because it is on the internet does not mean it is magic. It only checks a couple of basic bits of maths in the numbers
It's probably got some bugs in it.
It's come from someone who, for all you know, is less trustworthy than a bloke you just met in a pub, who won't show his face, and whose real name you don't know
If you design anything from these numbers, and blow anything up, it's you who will be held responsible for it.
Any constructive comments welcome. Contact the author though the mTrak website