- 1). Measure the distance the pipe needs to cover in feet and inches, from one fitting to the next, using a tape measure. Once you know the length of the open space between the fittings, add about 1 inch to that length, to allow for the threads at both ends to screw into the fittings. At a minimum, allow enough length for the pipe to screw in four to five threads deep at both ends.
- 2). Measure the inside diameter of a fitting in inches, to calculate the nominal size of the pipe that will fit it. Lay the tape measure across the end of the fitting, including the deepest cuts of the threads.
- 3). Subtract 1/4 inch from the measurement if you're using a Schedule 40 pipe, which is the most common size. The result is the estimated actual inside diameter. If you're using Schedule 80, subtract 3/8 inch. If you're using Schedule 120, subtract 11/32 inch. Schedule 80 is also called "extra strong" and Schedule 120 is also called "double extra strong."
- 4). Choose the next smallest nominal size if the actual inside measurement is 2 1/4 inches or less. Round up, if the measurement is over 2 1/4 inches. Nominal sizes generally run in 1/4-inch intervals for pipe from 1/2 inch to 1 1/2 inch, with smaller sizes in 1/8-inch intervals and larger sizes in 1/2-inch intervals.
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