Pipes come in legacy, nominal sizes according to their schedule. The dimensions of pipes are more or less gibberish when cross-referenced with their nominal size (Schedule 40 & 80 Pipe Dimensions ). Schedule 40 pipe is most common, schedule 80 is thicker but has the same OD. Other schedules are much less common.
The dimensions of round tubes, by comparison, make sense. You can just read the OD and wall thickness and those are the actual nominal dimensions of the tube.
The difference is the actual type in real world.
Pipes are the one the you lay in your house. in order to connect them you weld (or screw) a flange to the end. they always goes in straight lines. with pipes you can do a penetration and weld. the route from one point to the other can go through adapters, crosses, reducers... it does not need to be one piece from end to end.
Tubes are those that used to lay a gas line or hydraulic line for example.
tubes are always one whole piece from end to end. they can be flexible or orthogonal but continues in any case.
what @Mike Poge is talking about is the standards used in pipes.
Hope it clarify the differences.
Pipe Vs Tube
What is the difference between Pipe and Tube?
In short: Tube is measured by outside diameter, the pipe is measured by inside diameter.
There is often confusion as to which size die the customer actually needs - Pipe Size or Tubing Size.
Keep in mind that pipe size refers to a nominal - not actual - inside pipe diameter. Schedule refers to the pipe's wall thickness. The actual physical OD is larger than it's nominal OD.
The dimensions provided for tubing, on the other hand, refer to the actual outside diameter. In other words, the actual physical OD of a tube is just the same as it's nominal OD. The size of a tube will keep the same OD no matter what the wall thickness is.
For example: The actual outside diameter of 1¼″ pipe is 1.625″ - while 1¼″ tube has a true 1.25″ outside diameter.
Consequently, both the size of tube and pipe is measured by it's OD and the thickness.
Why the difference between Pipe and Tube?
Pipes are used to transport something, and tubes to construct something; hence, tubes are defined by the outside diameter and wall thickness (for construction stability), and pipes are measured by inside diameter to allow a calculation for transportation viz., speed, volumes, etc. (OD = ID + 2 × WT)
- Inside Diameter
- Outside Diameter
- Wall Thickness
It is just not true that pipe is measured by the inside diameter. Sch 40 and Sch 80 pipe have the same outside diameter. The nominal sizes don't mean anything.