Confused about Pipe Schedule vs Tube Gauge meanings? Confused as to why 6″ Pipe does not have a 6″ diameter?
Nominal pipe sizes are given by their inner diameter (ID) while nominal round tube sizes are given by their outer diameter (OD). So a 6″ inch pipe might have an OD of 6.5″. “Nominal” means “in name only”. Simple right? Nope. With Schedule 40 Pipe, a 6″ pipe has about a 6″ ID and 6 5/8″ OD. With Schedule 80, a 6″ pipe has a 6 5/8″ OD but a 5.761″ ID.
What’s important to remember, is these are just names. If the dimensions are critical to you, just call it 6 5/8″ OD with a 0.28″ wall, not just 6″ Schedule 40 pipe.
A 2×4 is not 2″ by 4″
Like when you buy 2x4s, you know they aren’t actually 2″ by 4″, but 1.5″ by 3.5″. Yes, with round tube, the nominal size is the same as the outer diameter. But with pipe, as you would think, the concern is with the volume of a fluid that could flow through it, so it is of lesser concern what the OD is.
Wall thickness for tube is given by it’s gauge, while the “schedule” of the pipe is the indication of its thickness. The higher the schedule, the thicker. But unlike the gauge of tube which is the same wall thickness for all tube sizes, the wall thickness of a pipe depends BOTH on it’s nominal size AND schedule. So a pipe the Schedule 40 in a 4″ Pipe will be a different wall thickness than a 6″ Pipe.
Click here for US Standard Pipe Sizes on Wikipedia
Clarification on Bollard Cover Sizes and Thicknesses
When we say a 6″ bollard cover, we mean that it fits a 6″ Pipe. Either 1/4″ or 1/8″ thicknesses in bollard covers will fit over the nominal pipe size as indicated.