NPT Thread Chart
Basic dimensions for NPT tapered pipe threads (ASME B1.20.1), from 1/16" to 6" — threads per inch, outside diameter, pitch diameter at the gauge plane, and tap drill size.
NPT (National Pipe Taper) is the standard tapered pipe thread used across North America for pipe fittings, hydraulics, pneumatics and plumbing. Its 60° threads taper at 1:16 (3/4" per foot of diameter change), so the joint seals by metal-to-metal interference as it's tightened — normally with PTFE tape or pipe dope, not a gasket or O-ring.
The nominal size doesn't match the actual thread diameter: a 1/2" NPT thread has an outside diameter of 0.840", not 0.500". The number is a legacy label tied to the bore of the iron pipe the fitting originally joined.
NPT is not compatible with BSP: BSP uses a 55° Whitworth thread angle instead of NPT's 60°, along with a different pitch series. Forcing an NPT fitting into a BSP port, or vice versa, can look like it threads in, but it won't seal reliably and can gall or crack the fitting — a costly mistake when importing or exporting hydraulic and pneumatic equipment.
| Size | TPI | OD (in) | OD (mm) | Pitch Ø at gauge plane (in) | Tap drill | Tap drill (mm) |
|---|---|---|---|---|---|---|
| 1/16 | 27 | 0.3125 | 7.94 | 0.28118 | C | 6.15 |
| 1/8 | 27 | 0.4050 | 10.29 | 0.37360 | R | 8.61 |
| 1/4 | 18 | 0.5400 | 13.72 | 0.49163 | 7/16 | 11.11 |
| 3/8 | 18 | 0.6750 | 17.15 | 0.62701 | 37/64 | 14.68 |
| 1/2 | 14 | 0.8400 | 21.34 | 0.77843 | 23/32 | 18.26 |
| 3/4 | 14 | 1.0500 | 26.67 | 0.98887 | 59/64 | 23.42 |
| 1 | 11.5 | 1.3150 | 33.40 | 1.23863 | 1-5/32 | 29.37 |
| 1-1/4 | 11.5 | 1.6600 | 42.16 | 1.58338 | 1-1/2 | 38.10 |
| 1-1/2 | 11.5 | 1.9000 | 48.26 | 1.82234 | 1-47/64 | 44.05 |
| 2 | 11.5 | 2.3750 | 60.33 | 2.29627 | 2-7/32 | 56.36 |
| 2-1/2 | 8 | 2.8750 | 73.03 | 2.76216 | 2-5/8 | 66.68 |
| 3 | 8 | 3.5000 | 88.90 | 3.38850 | 3-1/4 | 82.55 |
| 3-1/2 | 8 | 4.0000 | 101.60 | 3.88881 | 3-3/4 | 95.25 |
| 4 | 8 | 4.5000 | 114.30 | 4.38712 | 4-1/4 | 107.95 |
| 5 | 8 | 5.5630 | 141.30 | 5.44929 | 5-5/16 | 134.94 |
| 6 | 8 | 6.6250 | 168.28 | 6.50597 | 6-3/8 | 161.93 |
No thread size matches your search.
Values follow ASME B1.20.1 basic dimensions (outside diameter, TPI and pitch diameter at the gauge plane E1). Tap drill sizes are the commonly published values for average interference — adjust for tapping method and material. Data provided for reference — always verify against the current standard for critical or safety-related applications.
Understanding the columns
- Taper 1:16
- The rate of diameter change along the thread axis — 1 inch of diameter change per 16 inches of length (3/4 inch per foot) — which wedges the joint tight and produces the seal as it's tightened.
- Gauge plane
- The reference plane along the tapered thread, at the hand-tight engagement length, where the pitch diameter (E1) is defined and gauged. Because the thread tapers continuously, diameter elsewhere along its length is larger or smaller than at the gauge plane.
- NPT vs NPTF (Dryseal)
- NPT relies on sealant filling the small spiral clearance between crest and root. NPTF (ANSI B1.20.3) truncates the crests and roots differently so they interfere and crush together, sealing dry without added sealant — at the cost of tighter manufacturing tolerances. The two are dimensionally compatible.
- Nominal size vs actual OD
- The nominal size (e.g. "1/2 NPT") is a legacy label based on the historical bore of iron pipe, not the thread's actual outside diameter, which is always larger than the nominal number suggests.
Frequently asked questions
Can I mix NPT and BSP fittings?
No. NPT and BSP threads differ in thread angle (60° vs 55°) and generally in pitch, so their flanks don't mate correctly even when TPI and OD look similar. Forcing them together can produce a leak path or gouge/gall the threads, and any seal obtained is unreliable under pressure or vibration. Always use matched, standard-specific fittings, or a certified adapter, rather than the same tape or dope for both.
What is NPTF (Dryseal), and is it interchangeable with NPT?
NPTF is the "Dryseal" variant (ANSI B1.20.3) with modified crest/root truncation designed to crush and eliminate the spiral clearance path, sealing metal-to-metal without tape or sealant. NPT and NPTF can mechanically mate — they are dimensionally compatible — but guaranteed Dryseal performance requires an NPTF-to-NPTF joint, since the dry seal depends on both mating threads having the modified truncation. Treat any NPT/NPTF mixed joint as a standard NPT joint and use sealant.
Why don't two "1/2 inch" NPT and BSP fittings have the same actual diameter?
Nominal size in both standards is a legacy label tied to historical pipe bore, not the actual thread OD. NPT 1/2" has an OD of 0.840"; BSP 1/2" (based on Whitworth) has a different actual OD (~0.825"). Always match by measured OD/TPI/angle, never by nominal label alone.
How is pitch diameter defined for a tapered thread, and why does it matter?
Because NPT threads taper continuously, pitch diameter is only meaningful at a defined reference plane — the gauge plane (E1), the position corresponding to correct hand-tight makeup length (L1). Manufacturing and inspection gauges check E1 within a very tight tolerance band, since it governs both thread engagement and sealing interference.
References & further reading
- ASME B1.20.1 — Pipe Threads, General Purpose (Inch) — the standard this chart's dimensions are built from.
- National pipe thread — Wikipedia — background on NPT and its history.
- Swagelok — Thread and End Connection Identification Guide — a practical guide to identifying threads and fittings, including a comparison with BSP.
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