How about a mandrel that has a diameter of 2.5 inches? No math...
Richard has a great point here. One of the things I learned doing
blacksmithing was that often, the inches or millimeters is not the
important part, it’s that sizes match, or that the volume of the raw
billet matches the volume of the end product, no matter what forging
steps were taken, etc… so measuring with a ruler is sometimes not
the best way.
For example… to measure a bangle blank… draw a circle 2.5 inches
in diameter, using a compass or a circle template or even a properly
sized can or bottle to trace around. Then take a piece of string and
wrap it around, mark the overlap point and then measure it. Done!
Or, for forging… Sometimes it’s really hard to estimate how much
raw material you need in order to form a forged part - you change
every single dimension, length, width, thickness, while forging, how
much of “this” was in “that” ?
This was one of the best tricks for that problem that I ever
learned. Take very firm modeling clay, and model the piece you want
to make. Then, smash the model you made, and roll it into the size
stock you have available. So if I wa= nt to make a a big, long forged
earring out of 6 mm silver rod, I first model the earring in clay,
quite carefully - then I smash it and roll up the clay so it’s a
round rod 6 mm in diameter.
The length of that clay rod tells me exactly how much silver rod I
need to start with to forge the earring I want!
Hope that helps, and good luck with your bracelet project.