The word “taper” points to the big difference between pipe threads and those on bolts and screws. Pipe threads must make not only a mechanical joint but also a leak-proof 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 3/4 inch in a foot.

The standards below specify this distance and the effective thread. It also specifies another distance, the engagement, the distance the fixture can be screwed in by hand without much effort. For mounting and or installation it is 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. A tolerance of plus or minus one turn is allowed, and in practice threads are often routinely cut shorter than the standard.

American Standard Taper Pipe Threads
Nominal size	Actual OD	Threads per inch	Length of engagement (tightened by hand)	Hand tight turns	Wrench makeup turns	Length of effective thread
3⁄8	0.675	18	0.240	4½	3	0.408
½	0.840	14	0.320	4½	3	0.534