How is the inches on bicycle wells measured?

Answer 1

I'm assuming that you're asking about wheels, not wells.

Basically they're measured straight across the centre, from one edge to the other. What makes things rather confusing is that usually the measurements refer to from one edge of the tire to the other.

But this means that if you use a tire that is thinner than what was originally intended when the size was defined, then your measurement might not make much sense. It's safer to use the I.S.O. method, where you measure to the point of the rim where the tire attaches and gives a more well-defined value in mm. As long as the BSD (bead seat diameter) matches, each rim can take a couple of different tire widths w/o trouble.