Pipes and Fittings

Pipe sizes are standardized and are usually expressed in terms of inches ( ” ) or fractions of inches. As a rule, the size of the pipe is given in terms of the outside diameter ( O. D. ) or inside diameter ( I. D. ).  The figure below shows the terminology that applies to a section of a pipe.

The principal dimensions are as follows:

1. Outside Diameter ( O. D. )                    3. Wall thickness
2. Inside Diameter  ( I. D. )                       4. Length

Another consideration  is the weight per foot which varies according to the pipe’s wall thickness and material.

As a rule, pipe sizes are designated by diameter. Commercial sizes, 12 inches and less, are usually known by their nominal or approximate diameter. Above 12 inches, a pipe is ordinarily designated by its outside diameter. The actual O.D. of smaller diameter pipes is greater than the nominal I.D. For example, a 10″ or 12″ pipe is actually 3/4 of an inch greater than the size used. Pipes 14″ and above, however, is very close to size within a few thousanths of an inch. In all cases, whether the O.D. be small or large, the dimensions are closely held to accomodate various fittings.