Here's another handy shortcut involving pi: pi = 3.1416... which is very
close to 3 1/7, which is not too far from 3 1/8. An old sheet metal man
showed me this. If you multiply the diameter of a sheet metal pipe by 3
1/8, you get pretty close to the circumference, and it comes out in
fractions that you can read on a ruler. (The little bit extra helps make
the seam. :-)