3 Pipe vs. Tube

Pipe differs from tubing in that pipes are used for transporting materials, while tubing is used for structural applications. Therefore, the inside diameter (ID) is the critical dimension for pipe because the ID is used to calculate capacity. On the other hand, the outside diameter (OD) is the critical dimension for tubing because OD is used to calculate tolerances.

Pipe

Pipes transport fluids under low pressure, such as with cross-country pipelines, and fluids under high pressure, such as in refineries and power plants.
The nominal dimensions for pipe 12” and below are based on ID. Engineers and designers use ID to calculate pipe size based on the amount of fluid to be transported, along with other factors. Therefore, a 6” pipe has an internal diameter of approximately 6 inches.

Tube

Tubing is used for structural applications, boiler tubes, utility poles, bridges, overhead crosswalks. Therefore, the nominal dimensions for tubing are based on outside diameter (OD) because the OD is used to measure outside tolerances: stresses that are on the outside pushing in, instead of on the inside pushing out.