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ASME B1.20.1

NPT Thread — American Tapered Pipe

Mechanical interference seal — 1:16 taper (¾ in/ft)

NPT (National Pipe Taper) is the American standard for sealing pipe connections. The 1:16 taper (¾ inch per foot) generates mechanical interference when tightened, eliminating the need for a gasket when sealant is applied (Teflon tape or compound). Flank angle is 60°.

Dimension table

Nominal sizeTPIRef. OD (in)Pitch (in)Pitch (mm)Taper
1/16 NPT270.31250.037040.9401:16
1/8 NPT270.40500.037040.9401:16
1/4 NPT180.54000.055561.4111:16
3/8 NPT180.67500.055561.4111:16
1/2 NPT140.84000.071431.8141:16
3/4 NPT141.05000.071431.8141:16
1 NPT11.51.31500.086962.2091:16
1-1/4 NPT11.51.66000.086962.2091:16
1-1/2 NPT11.51.90000.086962.2091:16
2 NPT11.52.37500.086962.2091:16
2-1/2 NPT82.87500.125003.1751:16
3 NPT83.50000.125003.1751:16
3-1/2 NPT84.00000.125003.1751:16
4 NPT84.50000.125003.1751:16
5 NPT85.56300.125003.1751:16
6 NPT86.62500.125003.1751:16

Source: ASME B1.20.1-2013, Table 1. Ref. OD = nominal pipe outside diameter. Actual thread diameter at gauge plane varies with engagement (L1). Taper 1:16 equals 0.75 in/ft (6.25%).

Key technical characteristics

Flank angle
60°
Taper
1:16 (¾ in per foot)
Standard
ASME B1.20.1
Sealing mechanism
Mechanical interference + sealant
Designation
Inch fraction (e.g. ½ NPT)
NPTF variant
Dryseal — seals without sealant (ASME B1.20.3)

Typical applications

  • Hydraulic and pneumatic connections in North American manufactured equipment.
  • Lubrication and coolant systems in CNC machines.
  • Process instrumentation: pressure sensors, flow transmitters.
  • Pipe fittings in boilers, compressors, and steam systems.

Compatibility and interoperability

  • ✗NOT interchangeable with BSPT (R): both are tapered pipe threads but flank angles differ — NPT 60° vs BSPT 55°. Forcing assembly damages threads.
  • ✗NOT compatible with BSPP (G) — BSPP is parallel, NPT is tapered. Geometries are incompatible even if TPI appears close in some sizes.
  • ✓NPTF (Dryseal, ASME B1.20.3) is dimensionally compatible with NPT — NPTF/NPT assembly works, though without sealant-free guarantee in that case.

Selection guide

  • 01Always use sealant (PTFE tape or thread compound) — interference alone is not sufficient for high pressures.
  • 02For sealant-free sealing, specify NPTF (Dryseal) instead of NPT.
  • 03When replacing BSP fittings with NPT (or vice versa): always verify angle with a thread gauge before assembly.
  • 04The NPT size number (½, ¾, 1 in) does NOT correspond to actual diameter — it is a nominal pipe number.
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