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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 ¹⁄₁₆ 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 (ID)	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
¹⁄₈	0.405	27	0.180	4½	2½	0.260
¼	0.540	18	0.200	4	3	0.401
³⁄₈	0.675	18	0.240	4½	3	0.408
½	0.840	14	0.320	4½	3	0.534
¾	1.050	14	0.340	4½	3	0.546
1	1.315	11.5	0.400	4½	3¼	0.682
1¼	1.660	11.5	0.420	4½	3¼	0.707
1½	1.900	11.5	0.420	4½	3¼	0.724
2	2.375	11.5	0.436	5	3	0.756
2½	2.875	8	0.682	5½	3	1.136
3	3.500	8	0.766	6	3	1.2000
3½	4.000	8	0.821	6½	3½	1.2500
4	4.500	8	0.844	6¾	3½	1.3000
5	5.563	8	0.937	7½	3¾	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