Uses

• Pipes are utilized for fluids and gases, such as water, oil or propane. That is why the inside diameter is the key measurement -- it indicates how much can flow through the pipe. Tubes, on the other hand, are used for construction or load bearing. This is why the outside diameter is important because it indicates how much it can hold as a stability factor.
  
  

Thickness

• The thickness of the walls of pipes and tubes are an important factor to consider. The thickness of tubing is indicated by a gauge for thinner thicknesses and for thicker tubing it is indicated by fractions of an inch or millimeters. The normal range for tubing is 20 gauge, which measures .035 inch, up to a thickness of 2 inches. The wall thickness of a pipe is referred to as a schedule. The most common schedules are 10, 40 and 80. Schedule 40 is the most common and 80 is extra heavy. Unfortunately there is no set thickness for a schedule; it varies.
  
  

Diameter

• The diameter of a pipe indicates the measurement of the inside width of the piping, not the outside; the inside diameter is sometimes referred to I.D. Tubing is measured by the outside diameter, or O.D. If you see 2-inch tubing, it should measure 2 inches from one outside edge to the other outside edge at the widest part of the circle.
  
  

Material

• Piping is usually made of carbon steel. Tubing is often made of mild steel, steel alloys, aluminum, brass, copper or stainless steel.