This is a chart for the outside of your plumbing pipes so you can figure out what size parts to order. Note: The actual size a part is called is not the actual size of the hole in the part it is really the Inside Diameter of the pipe that would glue into that part which doesn’t make sense, since you cannot see inside the pipe. Therefore I am giving you the measurements of the outside of the pipe so you know what size part to order.  For example a part that ½” plumbing would glue in to would actually measure ¾” inside the hole of the part and on the outside of the pipe that glues into the part.
Pipe Outside Diameter chart:
½” Outside Diameter is ¾”
¾” Outside Diameter is 1”
1” Outside Diameter is 1 ¼”
I have never seen 1 ¼” in Hot Tubs
1 ½” Outside Diameter is 1 ⅞”
2” Outside Diameter is 2 ⅜”
2 ½” Outside Diameter is 2 ⅞”
Threaded Parts Outside Diameter Chart:
1” Pump Thread Outside Diameter is 2”
1” Pump Union Thread Inside Diameter is 1 13/16”
1 1/2” Heater Union or Pump Thread Outside Diameter is 2 7/16”
1 1/2” Heater Nut or Pump Union Inside Diameter is 2 5/16”
2” Heater Union or Pump Thread Outside Diameter is 3 1/16”
2” Heater Nut or Pump Union Inside Diameter is 2 15/16”
2 1/2” Heater Union or Pump Thread Outside Diameter is 3 11/16”
2 1/2” Heater Nut or Pump Union Inside Diameter is 3 9/16”