How does one figure the diameter of a pipe based on the measurement of it's circumfrance.?

I'm doing a small bathroom remodel and need to create a hole in some beadboard wainscotting for the feed pipe to the toilet. The circumfrance is 2 1/8 inches. My guess is I'll need to buy a circular drill bit so I can create a smooth hole as close to the size of the pipe as possible but I'm not sure what size I will need.

Answers:
Divide the circumferance by pi...
2.125 divided by 3.14 = 0.676 inches...
You will probably need to drill about a 5/8" hole
and then hog it out a little if it is too tight..
You might just go with a 3/4" hole for a toilet feed as
it isn't usually necessary to make that tight a fit...

Other answers:
circ= pi*diameter => diameter=circ/pi where pi=3,14....
circ=2*pi*radius where 2*radius=diameter
circ= pi*diameter => diameter=circ/pi where pi=3,14....
circ=2*pi*radius where 2*radius=diameter
divide it by pi
circumference is equal to pi radius squared. Divide both sides by pi, then you will have the radius that you will need to find the sqaure root of. Double that and you should have you hole size.
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