American pipe threads

Pipe thread sizes are described much as bolt sizes are, although the shapes are different. For example, “½–14 NPT” identifies a pipe thread with a nominal inside diameter of ½ inch and 14 threads to the inch, made according to the NPT standard. If “LH” is added, the pipe has a left hand thread. In the United States, the pipe thread standards are:

NPT American Standard Pipe Taper Thread
NPSC American Standard Straight Coupling Pipe Thread
NPTR American Standard Taper Railing Pipe Thread
NPSM American Standard Straight Mechanical Pipe Thread
NPSL American Standard Straight Locknut Pipe Thread

The word “taper” in several of these names points to the big difference between many pipe threads and those on bolts and screws. Many pipe threads must make not only a mechanical joint but also a leakproof one. To accomplish this, the threads become shallower the farther they are from the end of the pipe or fitting. The bottoms of the threads aren't on a cylinder, but a cone; they taper. The taper is 116 inch in an inch, which is the same as ¾ inch in a foot.

drawing of tapered pipe thread

Because of the taper, a pipe can only screw into a fitting a certain distance before it jams, unlike threading a nut on a bolt. The standard specifies this distance, the effective thread. It also specifies another distance, the engagement, the distance the pipe can be screwed in by hand, without much effort. For workers, instead of these distances, it is more convenient to know how many turns to make by hand and how many with a wrench.

The table shows the distances and number of turns called for in the standard. A tolerance of plus or minus one turn is allowed, and in practice threads are often routinely cut shorter than the standard specifies.

All dimensions are in inches.

American Standard Taper Pipe Threads (NPT)
Nominal size
(inside
diameter)
Actual OD Threads per inch Length of engagement
(tightened by hand)
Hand tight turns Wrench makeup turns Length of effective thread
1/16 0.3125 27 0.160     0.2611
18 0.405 27 0.180 0.260
¼ 0.540 18 0.200 4 3 0.401
38 0.675 18 0.240 3 0.408
½ 0.840 14 0.320 3 0.534
¾ 1.050 14 0.340 3 0.546
1 1.315 11.5 0.400 0.682
1.660 11.5 0.420 0.707
1.900 11.5 0.420 0.724
2 2.375 11.5 0.436 5 3 0.756
2.875 8 0.682 3 1.136
3 3.500 8 0.766 6 3 1.2000
4.000 8 0.821 1.2500
4 4.500 8 0.844 1.3000
5 5.563 8 0.937 1.4063

The standard also defines larger pipes: every whole inch to 12 inches. At that point pipes begin to be sized by outside diameter instead of inside diameter: every inch from 14 to 20 inches OD, then every other inch to 30 inches OD.

ASA B2.1-1960

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