Characteristics of NPT:

angle between taper and center axis of pipe is 1° 47’ 24” (1.7800°)
truncation of roots and crests are flat
60° thread angle
pitch is measured in threads per inch (TPI)

Sizing
The NPT threads sizing chart includes data for external and internal pipe threads. The taper rate for all NPT threads is 1/16—3/4 in. per foot (62.5 mm per m)—which is measured by the change of diameter over distance. The outside diameter (OD) of a pipe or fitting must also be measured; both the TPI and OD are required for accurate identification of thread size because more than one size can have the same TPI.

Commonly used sizes include: 1/8, ¼, 3/8, ½, ¾, 1, 1 ¼, 1 ½, and 2 in. These can be found on pipes and fittings used by most U.S. suppliers. Sizes less than 1/8 in. are sometimes used for compressed air, while sizes more than 2 in. are less common, mainly because other joining methods are more often employed with these larger sizes.

Measuring for a leak-free seal

In the imperial system, TPI is measured by how many crests (threads) there are per inch; the metric system measures the distance between two crests.

Using the trial-by-error method, try different thread gauges until you determine the tightest fit; engage as many threads as possible—the more threads engaged, the more accurate the reading. Hold the fitting and thread gauge up to the light, looking for gaps between the gauge and thread. This is easier to see on a male connection than a female one.

Next, measure the thread diameter with an ID/OD caliper. Male thread diameter is measured on the OD; female thread diameter is measured on the ID. The Hose Safety Institute Handbook recommends holding the caliper at a slight angle for a more accurate male reading, and holding it perpendicular to the thread for a more accurate female reading.

If the pipe or fitting connection seals on a flared surface or inverted angle seat, determine that angle of seal with a seat angle gauge on the male connection. Place the gauge on the sealing surface; if the centerlines of connection and gauge are parallel, the correct angle has been determined. For a female connection, insert the gauge into the connection and place it on the sealing surface. As with the male connection, if the centerlines are parallel, the correct angle has been determined.