Pipe Thread Chart: BSP and NPT

Identify a pipe thread from two measurements, then look up its sizes. Covers British Standard Pipe threads (BSP: parallel G to ISO 228 and taper R to ISO 7) and American National Pipe Taper threads (NPT to ASME B1.20.1).

Identify a thread

Measure across the crests near the start of the thread with a vernier. Count the threads over 25.4 mm, or measure over 10 threads and work out the pitch: 14 threads per inch is a pitch of 1.81 mm. For a taper, compare the diameter at the first and the fourth full thread.

Results

Most likely:

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BSP threads (ISO 228 and ISO 7)

55° thread with rounded crests and roots. The same sizes are used for parallel G threads, which seal on a washer or O-ring, and for taper R (male) and Rc (female) threads, which seal on the thread with a 1 in 16 taper on diameter. Diameters are at the gauge plane.

SizeThreads per inchPitch, mmMajor diameter, mmMinor diameter, mm

NPT threads (ASME B1.20.1)

60° thread with flat crests and roots and a 1 in 16 taper on diameter. NPT threads seal with tape or compound; the dryseal NPTF form seals metal to metal. A thread of the same size but with no taper is NPS.

SizeThreads per inchPitch, mmPipe outside diameter, mmPitch diameter at the pipe end, mmEffective thread length, mm

Tap drill sizes for pipe threads depend on whether the hole is taper reamed, so take them from your tap maker's chart. The minor diameter of a parallel G thread is the smallest the bore can be.