The primary difference between pipe and tubing is how the size is designated. Pipe is designated by a "Nominal Pipe Size" based upon the ID (inside diameter) of the most common wall thickness. Tubing is designated by the measured OD (outside diameter). For Example: A 3/4 inch iron pipe has an OD of 1.050 inches, while a 3/4 inch steel tube has an OD of 0.75 inches.