NPT Thread Chart: Nominal Sizes, TPI & Identification

Published September 22, 2026 · MfgWorkbench editorial reference

NPT sizes are nominal pipe sizes, not direct measurements across the thread. Use this selected chart for identification, then verify the connection specification and gauge requirements.

What the NPT designation means

NPT uses a 60° thread form with a nominal 1:16 taper on diameter. That means the diameter changes by one unit over sixteen units of axial length; it does not mean a 1:16 change on radius. The calculated taper half-angle is about 1.7899°.

A 1/2 NPT designation therefore does not describe a 0.5-inch outside diameter. The nominal pipe outside diameter in the table is 0.840 in, while the actual thread measurement depends on axial location and allowable dimensions. Gauge-plane requirements matter for acceptance.

Identification workflow

Measure approximate outside diameter, determine TPI with a suitable pitch gauge, inspect taper and identify thread form. For 1/4 NPT, 18 TPI is a useful clue; it differs from the 19 TPI used by the corresponding BSP size. For 1/2 and 3/4 sizes, both families have 14 TPI, so pitch alone is insufficient.

Do not use a matching nut or fitting as a substitute for the required gauge. Damaged or mismatched threads may engage a few turns and still produce an unacceptable connection.

Sealing and limits of this chart

NPT, NPTF and other pipe thread specifications are not interchangeable labels. Sealant choice, assembly procedure, engagement and service suitability come from the fitting manufacturer and applicable specification. This table intentionally omits torque prescriptions and tap-drill recommendations because those require the exact product and process.

For pressure service, confirm every component’s thread, material, temperature and pressure rating. A dimensional match is not a pressure-rating approval.