TITLE: Calculate length and circumference based on bead size
QUESTION [1 upvotes]: I'm working on a tool, it will have a simple size reference. I'm trying to create a formula with 3 variables.
x = bead size (2, 4, 6, 8, 10, 12 mm)
y = length on string
z = circumference of chain

Suppose I need a 20 inch chain in length (y), using a beadsize 8mm (x), how would i calculate the circumference?
The end result would be for me to just add these numbers in excel and I'd get any possible option.
I do of course know the circumference = π × diameter.
Probably an easy question for the wizzards here, but I'm struggling;-)
Edit: I added an image that I made to clarify, hope that helps. Thank you.
length or circumference needed, based on variable beadsize

REPLY [0 votes]: Eventually, at a moment of clarity, I figured it was actually very easy.
All I had to do was subtract half the beadsize from the radius and use the known formula. In retrospect I cannot believe it had me confused for this long;-)
So, length became: 
r = C/2π
L = 2π × (r + 0.5*beadsize)

Or vice versa when the length is known, circumference is:
C = 2π × (r - 0.5*beadsize)
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