Wait! There is a difference between Pipe and Tube?
Knowing the answer to this question and how to properly measure each will win you trivia championships!
Tube and Pipe are not the same. In simple terms, pipe is a vessel used to most often carry fluids and gas. Tube is used for structural projects like car frames and construction. Once you know what you want the next step is figuring out the size you need.
Tube is measured by an exact outside diameter (OD) and wall thickness.
Pipe is measured by a nominal outside diameter (also known as NPS or Nominal Pipe Size) and wall thickness.

         Pipe Schedules & Weights
NPS is a North American set of standard sizes for pipes used for high or low pressures. The schedule number on a pipe relates to the thickness of the wall on the pipe. The higher the number, the thicker the wall. So Schedule 80 pipe is thicker walled material than Schedule 40 pipe. The Schedule number can be the same on two different sized pipes and yet the actual wall thickness will be different. For example a 1” Schedule 40 pipe has a wall thickness of .133”. Whereas a 2” Schedule 40 pipe has a wall thickness of .145”. So, the bottom line on pipe is that the Schedule number will increase the wall thickness, and the wall thickness changes based on the NPS
The dimensions provided for tubing 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.