Interestingly, there is nothing about a 1.5 inch PVC pipe that is 1.5 inches.  In this example, the 1.5 inch PVC has an outer diameter of 1-7/8 inches and an inner diameter of 1-11/16 inches.

If you measure a hole close to 1.5 inches, do not assume you need Durso Standpipes made of 1.5 inch PVC - you're wrong.

This begs the question, if no dimension is 1.5 inches, then why is it called 1.5 inch PVC pipe?

Legend has it that back in the 1930's when pipe was made of copper, that pipe did in fact have a nominal diameter inside with 1/16 inch walls, making the outside diameter 1/8 inch larger. Later when metallurgy improved, the walls were made thinner which increased the inner diameter but left the outside diameter unchanged to allow it to work with existing fittings.

When PVC pipe was developed it inherrited its outer diameter from iron pipe.

Nominal Pipe SizePVC ScheduleOuter DiameterInner DiameterWall Thickness
 ¾ inch (.75)  40  ~1-1/16 inch (26mm)  ~ ¾ inch (20mm)  1/8 inch
 1 inch (1.0)  40  1-5/16 inch (33mm)  1 inch (25mm)  1/8 inch
 1¼ inch (1.25)  40  1-5/8 inch (42mm)  1-5/16 inch (34mm)  1/8 inch
 1¼ inch (1.25)  26  1-5/8 inch (42mm)  ~1-7/16 inch (38mm)  1/16 inch
 1½ inch (1.5)  40  1-7/8 inch (48mm)  1-9/16 inch (40mm)  1/8 inch
 1½ inch (1.5)  26  1-7/8 inch (48mm)  1-11/16 inch (44mm)  1/16 inch

 

SOURCE