Water well casings with 7″ INSIDE diameter are very unusual, but they do exist. Here’s what you need to know to make sure the diameter is accurate.
The water well industry standard is to specify diameter using INSIDE DIAMETER (ID). So, in your case, if you have a standard 7″ pipe, the inner diameter (ID, measured from the inside edges of the pipe, viewed in cross section) would be 7″. The outer diameter (OD, measured from outside edges of the pipe, in cross section) would be 7 5/8 (7.625) inches.
If you meant 7″ OUTSIDE diameter, that is 6.25″ INSIDE diameter.
We have well caps for either size.