NPT Pipe Thread Size Chart
American National Standard Taper Pipe Thread (NPT) dimensions per ANSI/ASME B1.20.1. Includes Table 1a and 1b dimensions for all 13 standard sizes. All NPT threads taper at 3/4" per foot (1°47’).
| Nominal Size | OD, D (in) | TPI, n | Pitch, p (in) | E0 (in) | L1 (in) | E1 (in) | L2 (in) | E2 (in) |
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Need to thread mill an NPT port? Use our NPT Thread Milling PDO Calculator for tapered bore dimensions and Fusion 360 PDO values.
How to Use This NPT Chart
Find your nominal pipe size in the table. The default Table 1a view shows the most-used dimensions: outside diameter, TPI, pitch, pitch diameters (E0, E1, E2), and engagement lengths (L1, L2). Click “+ Table 1b” to expand additional columns including vanish thread length, overall thread length (L4), thread height (h), and minor diameter (K0).
Key dimension: E0 is the pitch diameter at the beginning of the external thread — this is the primary reference dimension for NPT per ASME B1.20.1. K0 (in Table 1b) is the basic minor diameter at the small end of the pipe, used as the bore diameter reference for internal threads.
If you need to thread mill an NPT port, use our NPT thread milling PDO calculator to get bore dimensions and Fusion 360 PDO values. For straight (non-tapered) threads, see our tap drill size chart.
Frequently Asked Questions
What is the taper angle for NPT threads?
NPT threads have a taper of 3/4 inch per foot on diameter, which equals 1 degree 47 minutes (1.7833 degrees) measured from the centerline. This taper creates a wedging action that forms a pressure-tight seal when assembled with thread sealant (Teflon tape or pipe dope). The taper rate is 0.0625 inches per inch on diameter (0.03125 per side).
What is the difference between NPT and NPS threads?
NPT (National Pipe Taper) threads are tapered at 3/4 inch per foot and create a seal by wedging together. NPS (National Pipe Straight) threads are parallel (no taper) and do not seal by themselves — they require an O-ring or gasket. NPT is used for pressure-tight plumbing and hydraulic connections. NPS (specifically NPSM for free-fitting or NPSC for couplings) is used for mechanical joints where an external seal provides the pressure barrier.
Why doesn’t nominal pipe size match the actual outside diameter?
NPT nominal pipe sizes are historical designations that do not equal the actual outside diameter. For example, 1/2" NPT pipe has an actual OD of 0.840" (21.34 mm), and 1" NPT has an OD of 1.315" (33.40 mm). The nominal size originally referred to the approximate inside diameter of standard-weight pipe, but the name stuck even as wall thicknesses changed. Always use the OD column in this chart for the actual measurement.
Related Calculators
- NPT Thread Milling PDO Calculator — Bore dimensions and PDO for NPT thread milling
- Tap Drill Size Chart (UNC/UNF) — Tap drill sizes for straight threads
- Thread Milling PDO Calculator — PDO for straight threads (UNC, UNF, Metric)
- Pipe Schedule Chart — Pipe wall thickness and weight by schedule
References
- Oberg, E. et al. Machinery’s Handbook, 29th Edition, Industrial Press, 2012, pp. 1956–1968. American Pipe Threads, NPT basic dimensions per ANSI/ASME B1.20.1-1983 (R2006).
- ANSI/ASME B1.20.1 — Pipe Threads, General Purpose (Inch). Governing standard for NPT dimensions.
Data last verified: April 2026
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