Thread series · NPT

NPT Taper Pipe Thread Thread Chart — All Sizes, Dimensions & Tap Drills

Name: NPT Taper Pipe Thread thread dimensions and tap drill sizes
Creator: ShopMath

https://shopmath.org/threads/npt · data verified August 1, 2026

The NPT Taper Pipe Thread series covers 24 standard sizes, from 1/16-27 NPT to 24-8 NPT, and 24 of them carry a tap drill recommendation. American National Standard taper pipe thread, ASME B1.20.1 — sealing needs a compound or tape.

Sizes: 24
Smallest: 1/16-27 NPT
Largest: 24-8 NPT
Standard: ASME B1.20.1, ASME B1.20.3; public-domain primary FED-STD-H28/7A (1984)

Every NPT Thread Size — Dimensions and Tap Drills

24 sizes, ordered smallest to largest. Every row links to the full page for that thread, with class limits, clearance holes and tap drills across 50–85% engagement.

NPT Taper Pipe Thread thread sizes — basic dimensions and tap drills
Property	Property	Major Ø		Pitch Ø		Minor Ø		Tap drill
Thread	TPI	in	mm	in	mm	in	mm	Drill	in	mm
1/16-27 NPT	27	0.3125	7.938	0.2812	7.142	0.2516	6.391	E	0.2500	6.350
1/8-27 NPT	27	0.4050	10.287	0.3736	9.489	0.3440	8.738	11/32	0.3438	8.731
1/4-18 NPT	18	0.5400	13.716	0.4916	12.487	0.4472	11.359	7/16	0.4375	11.113
3/8-18 NPT	18	0.6750	17.145	0.6270	15.926	0.5826	14.798	37/64	0.5781	14.684
1/2-14 NPT	14	0.8400	21.336	0.7784	19.772	0.7213	18.321	23/32	0.7188	18.256
3/4-14 NPT	14	1.0500	26.670	0.9889	25.117	0.9317	23.665	59/64	0.9219	23.416
1-11.5 NPT	12	1.3150	33.401	1.2386	31.461	1.1691	29.695	1	1.0000	25.400
1 1/4-11.5 NPT	12	1.6600	42.164	1.5834	40.218	1.5138	38.450	1	1.0000	25.400
1 1/2-11.5 NPT	12	1.9000	48.260	1.8223	46.287	1.7528	44.521	1	1.0000	25.400
2-11.5 NPT	12	2.3750	60.325	2.2963	58.325	2.2267	56.558	1	1.0000	25.400
2 1/2-8 NPT	8	2.8750	73.025	2.7622	70.159	2.6622	67.620	1	1.0000	25.400
3-8 NPT	8	3.5000	88.900	3.3885	86.068	3.2885	83.528	1	1.0000	25.400
3 1/2-8 NPT	8	4.0000	101.600	3.8888	98.776	3.7888	96.236	1	1.0000	25.400
4-8 NPT	8	4.5000	114.300	4.3871	111.433	4.2871	108.892	1	1.0000	25.400
5-8 NPT	8	5.5630	141.300	5.4493	138.412	5.3493	135.872	1	1.0000	25.400
6-8 NPT	8	6.6250	168.275	6.5060	165.252	6.4060	162.712	1	1.0000	25.400
8-8 NPT	8	8.6250	219.075	8.5000	215.901	8.4000	213.360	1	1.0000	25.400
10-8 NPT	8	10.7500	273.050	10.6209	269.772	10.5209	267.231	1	1.0000	25.400
12-8 NPT	8	12.7500	323.850	12.6178	320.492	12.5178	317.952	1	1.0000	25.400
14-8 NPT	8	14.0000	355.600	13.8726	352.365	13.7726	349.824	1	1.0000	25.400
16-8 NPT	8	16.0000	406.400	15.8758	403.244	15.7758	400.705	1	1.0000	25.400
18-8 NPT	8	18.0000	457.200	17.8750	454.025	17.7750	451.485	1	1.0000	25.400
20-8 NPT	8	20.0000	508.000	19.8703	504.706	19.7703	502.166	1	1.0000	25.400
24-8 NPT	8	24.0000	609.600	23.8609	606.068	23.7609	603.527	1	1.0000	25.400

Basic dimensions are the theoretical values before a tolerance class is applied. Tap drills are for cut taps at the default engagement; per-size pages carry 50–85%.

Sources and Method for the NPT Thread Chart

NPT thread dimensionscomputed+compiled

ASME B1.20.1 taper pipe thread formulas; E0 and L2 reproduced to 5 decimals in docs/research/data-sourcing.md §3.3. Public-domain primary: FED-STD-H28/7A (1984).; pipe OD / tpi / L1 / L4: ASME B1.20.1 pipe OD / tpi / L1 / L4 via Wikipedia National pipe thread (CC BY-SA 4.0); OD and tpi cross-checked against FreeCAD FeatureHole.cpp NPT block (LGPL-2.1+)

Verified against Wikipedia National pipe thread standard-sizes table (CC BY-SA 4.0) — 98 checks passed to 1e-5 in

Part computed from the standard's formulas, part compiled from agreeing public sources.

Assumptions

Tap drill figures are for cut taps at the default thread engagement; each size page carries the full 50–85% range.
Dataset scope: L1 is tabulated, not derivable (data-sourcing.md §3.3). EXCLUDED: NPTF crest/root truncation limits (single-sourced), NPSM/NPSC/NPSF straight pipe threads, BSPT/BSPP.

Data last verified

August 1, 2026

How this data is built and checked →

How to Cite This NPT Thread Chart

Citation

ShopMath. "NPT Taper Pipe Thread Thread Chart — All Sizes, Dimensions & Tap Drills." ShopMath, verified August 1, 2026, https://shopmath.org/threads/npt

BibTeX

@misc{shopmath-threads-npt,
  title        = {NPT Taper Pipe Thread Thread Chart — All Sizes, Dimensions & Tap Drills},
  author       = {{ShopMath}},
  year         = {2026},
  howpublished = {\url{https://shopmath.org/threads/npt}},
  note         = {Data verified 2026-08-01}
}

Permanent URL

https://shopmath.org/threads/npt

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