What is the tap drill size for 1/2 inch NPT?

45/64 in (0.703 in). It is a taper pipe thread drill — the tap cuts the taper, so the drill is a straight hole sized to the thread's minor diameter in the range where the thread engages.

What is the taper on an NPT thread?

1:16 — a diameter change of 3/4 inch per foot of length, or 1/16 inch per inch. The included angle is 1.7899° (about 1°47').

Is 1/2 NPT half an inch in diameter?

No. A 1/2 in NPT thread has an outside diameter of 0.840 in on the pipe. The nominal size refers to the nominal bore of the pipe it fits, and pipe sizes have not matched actual bores for decades — the outside diameter is a fixed size per nominal size.

What is the difference between NPT and NPTF?

NPTF (Dryseal) has modified crest and root dimensions that interfere to seal without a sealant. NPT requires a sealant such as PTFE tape or pipe dope. They are close enough in basic dimensions to thread together but should not be mixed where sealing matters.

How far should an NPT fitting be tightened?

Typically two to three turns past hand-tight for small sizes, reducing to about 1.5 to 2.5 turns for 1 in and above. Beyond that there is a real risk of splitting the fitting — thin-wall and plastic fittings especially. The L1 and L2 columns show the engagement lengths these turns correspond to.

NPT Pipe Thread Size Chart — Dimensions, L1/L2 and Tap Drill

Sources and scope for this page: ASME B1.20.1-2013, covering all dimensional data on this page: taper, pitch diameters, L1/L2 engagement lengths and tap drill recommendations. No independent source check is recorded for this page: the content rests on the sources listed and on the working shown on the page. Full sources, values and limits on this page.

Page last changed 2026-11-08 · based on ASME B1.20.1-2013

Quick Answer

NPT is a tapered pipe thread with a 1:16 taper — 3/4 inch per foot on diameter. The pitch diameter is specified at a gauge plane, not along the whole thread, so the hand-tight (L1) and wrench makeup (L2) lengths matter as much as the diameter. 1/2-14 NPT: 0.840 in outside diameter, 0.75843 in pitch diameter, tap drill 45/64 in.

Find a size

NPT Thread Dimensions (ASME B1.20.1)

Nominal pipe size^[1]	TPI^[1]	Pitch in^[2]	Outside dia in^[1]	Pitch dia in^[1]	L1 in^[1]	E1 in^[1]	L2 in^[1]	E2 in^[1]	Tap drill^[3]
1/16 #	27	0.03704	0.3125	0.27118	0.1600	0.28118	0.2611	0.28750	C (0.242)
1/8 #	27	0.03704	0.405	0.36351	0.1615	0.37360	0.2639	0.38000	Q (0.332)
1/4 #	18	0.05556	0.540	0.47739	0.2278	0.49163	0.4018	0.50250	7/16 (0.438)
3/8 #	18	0.05556	0.675	0.61201	0.2400	0.62701	0.4078	0.63750	9/16 (0.562)
1/2 #	14	0.07143	0.840	0.75843	0.3200	0.77843	0.5337	0.79179	45/64 (0.703)
3/4 #	14	0.07143	1.050	0.96768	0.3390	0.98887	0.5457	1.00179	29/32 (0.906)
1 #	11 1/2	0.08696	1.315	1.21363	0.4000	1.23863	0.6828	1.25630	1 9/64 (1.141)
1 1/4 #	11 1/2	0.08696	1.660	1.55713	0.4200	1.58338	0.7068	1.60130	1 31/64 (1.484)
1 1/2 #	11 1/2	0.08696	1.900	1.79609	0.4200	1.82234	0.7235	1.84130	1 23/32 (1.719)
2 #	11 1/2	0.08696	2.375	2.26902	0.4360	2.29627	0.7565	2.31630	2 3/16 (2.188)
2 1/2 #	8	0.12500	2.875	2.71953	0.6820	2.76216	1.1375	2.79062	2 39/64 (2.609)
3 #	8	0.12500	3.500	3.34062	0.7660	3.38850	1.2000	3.41562	3 15/64 (3.234)
3 1/2 #	8	0.12500	4.000	3.83750	0.8210	3.88881	1.2500	3.91562	—
4 #	8	0.12500	4.500	4.33438	0.8440	4.38712	1.3000	4.41562	—
5 #	8	0.12500	5.563	5.39073	0.9370	5.44929	1.4063	5.47862	—
6 #	8	0.12500	6.625	6.44609	0.9580	6.50597	1.5125	6.54062	—
8 #	8	0.12500	8.625	8.43359	1.0630	8.50003	1.7125	8.54062	—
10 #	8	0.12500	10.750	10.54531	1.2100	10.62094	1.9250	10.66562	—
12 #	8	0.12500	12.750	12.53281	1.3600	12.61781	2.1250	12.66562	—
14 #	8	0.12500	14.000	13.77500	1.5620	13.87262	2.2500	13.91562	—
16 #	8	0.12500	16.000	15.76250	1.8120	15.87575	2.4500	15.91562	—
18 #	8	0.12500	18.000	17.75000	2.0000	17.87500	2.6500	17.91562	—
20 #	8	0.12500	20.000	19.73750	2.1250	19.87031	2.8500	19.91562	—
24 #	8	0.12500	24.000	23.71250	2.3750	23.86094	3.2500	23.91562	—

Pitch diameter is specified at the gauge plane, which for an external thread is at the hand-tight engagement length L1 from the large end. L1 is the length of thread engaged by hand; L2 is the additional length available for wrench makeup. Tap drill sizes are for NPT (drill without reamer) and apply to the common sizes only.

What Makes NPT Different from Other Threads

NPT is a tapered thread, and that changes how every dimension is read:

1:16 taper — the diameter changes by 3/4 in per foot of length. There is no single "the diameter" of the thread; it depends where you measure.
Gauge plane — pitch diameter is specified at a defined plane, not along the whole thread. For an external thread that plane sits at L1 from the large end.
Sealing is by interference — the taper forces the flanks together as the joint is made up, so NPT seals on the thread itself, and a sealant or tape is normally still required.
Nominal size is not a diameter — a 1/2 in NPT has a 0.840 in outside diameter. The nominal size refers to the nominal bore of the pipe, and has not matched the actual bore for decades.

This is why NPT dimensions cannot be computed the way unified machine screw threads can — the taper, the gauge plane and the engagement lengths are all separately specified.

L1 and L2: Why Two Lengths

Because the thread is tapered, how far it is screwed in determines the diameter at which it seats. ASME B1.20.1 therefore specifies two lengths:

L1 — hand-tight engagement. The length of thread engaged when the joint is made up by hand, before any wrench is applied. The pitch diameter at the gauge plane is defined here, and E1 is the diameter at the end of L1.
L2 — wrench makeup. The additional length available to be taken up with a wrench. E2 is the diameter at the end of L2.

Wrench makeup is what produces the seal. Too little and the joint leaks; too much and the fitting can split, particularly in thin-wall or plastic fittings — which is why NPT joints are made up to a specified turns-from-hand-tight rather than "as tight as it goes".

NPT, NPTF and NPS

NPT — the general-purpose tapered thread covered here. Requires a sealant.
NPTF (Dryseal) — a tapered thread with a modified root and crest that interfere to seal metal-to-metal without sealant. Not interchangeable with NPT for sealing purposes, though the basic dimensions are close.
NPS — the straight (parallel) version, used with a gasket or O-ring rather than sealing on the thread. NPS and NPT have the same pitch and pitch diameters but different forms at the crest/root.

Mixing these is a common source of leaks: an NPT male into an NPS female will thread together but cannot seal on the thread, because neither has the taper that produces interference.

Frequently Asked Questions

What is the tap drill size for 1/2 inch NPT?: 45/64 in (0.703 in). It is a taper pipe thread drill — the tap cuts the taper, so the drill is a straight hole sized to the thread's minor diameter in the range where the thread engages.
What is the taper on an NPT thread?: 1:16 — a diameter change of 3/4 inch per foot of length, or 1/16 inch per inch. The included angle is 1.7899° (about 1°47').
Is 1/2 NPT half an inch in diameter?: No. A 1/2 in NPT thread has an outside diameter of 0.840 in on the pipe. The nominal size refers to the nominal bore of the pipe it fits, and pipe sizes have not matched actual bores for decades — the outside diameter is a fixed size per nominal size.
What is the difference between NPT and NPTF?: NPTF (Dryseal) has modified crest and root dimensions that interfere to seal without a sealant. NPT requires a sealant such as PTFE tape or pipe dope. They are close enough in basic dimensions to thread together but should not be mixed where sealing matters.
How far should an NPT fitting be tightened?: Typically two to three turns past hand-tight for small sizes, reducing to about 1.5 to 2.5 turns for 1 in and above. Beyond that there is a real risk of splitting the fitting — thin-wall and plastic fittings especially. The L1 and L2 columns show the engagement lengths these turns correspond to.

Value Sources

References and calculation methods used for this page are listed below. Numbered markers connect table columns where present. A reference listing does not itself establish that every value has been independently checked.

#	Source	Type	Revision / method
[1]	ASME B1.20.1 — Pipe Threads, General Purpose (Inch)	reference	ASME B1.20.1-2013 — source
[2]	Pitch computed from threads per inch	derived	P = 1 / TPI. Verified at build time against the tabulated pitch column.
[3]	NPT tap drill sizes (drill without reamer)	reference	n/a — drill recommendation table, not a separately versioned standard — source
[4]	MIL-P-7105 — Pipe Threads, General Purpose (free public-domain mirror)	reference	n/a — withdrawn, superseded by ASME B1.20.1 — source

Data Sources

Standard	Revision	What it covers on this page
ASME B1.20.1 — Pipe Threads, General Purpose (Inch)	ASME B1.20.1-2013	all dimensional data on this page: taper, pitch diameters, L1/L2 engagement lengths and tap drill recommendations

Verification scope

No page-specific independent source review is recorded here. The listed references and formulas explain the basis of the compilation; automated consistency checks do not verify the original sources or establish product conformity.

Derived values — the following values on this page are calculated, not taken directly from the standard:

Value	How it is derived
Pitch	P = 1 / TPI, verified against the tabulated pitch column on every build.

NPT dimensions are tabulated rather than computed — the taper, gauge plane and engagement lengths are separately specified and cannot be derived from a thread form formula. Tap drill sizes are given for the common sizes only.

Accuracy and use. Use the source notes and recorded verification scope above to assess each value. Calculated values follow the stated derivations; compilation values may lack a confirmed original document or product condition. No warranty, express or implied, is made as to the accuracy or completeness of this information, and no liability is accepted for any loss or damage arising from its use. Engineering reference data is provided for guidance in preliminary work — before a value is used for design, fabrication or acceptance testing, verify it against the current revision of the governing standard and against your own inspection. The user assumes all risk and responsibility in connection with the use of this information.

Cite This Page

Read the source notes, calculation methods and verification scope above before reusing a value. To cite this page:

Plain text

Engineering Reference. "NPT Pipe Thread Sizes." https://refmanual.com/charts/npt-thread-sizes/ (accessed 2026-11-09). Data from ASME B1.20.1 — Pipe Threads, General Purpose (Inch), ASME B1.20.1-2013.

BibTeX

@misc{npt-thread-sizes,
  title        = {NPT Pipe Thread Sizes},
  author       = {{Engineering Reference}},
  howpublished = {\url{https://refmanual.com/charts/npt-thread-sizes/}},
  note         = {Accessed: 2026-11-09},
  source       = {ASME B1.20.1 — Pipe Threads, General Purpose (Inch), ASME B1.20.1-2013}
}

RIS (EndNote, Zotero, Mendeley)

TY  - ELEC
TI  - NPT Pipe Thread Sizes
AU  - Engineering Reference
PB  - Engineering Reference
UR  - https://refmanual.com/charts/npt-thread-sizes/
PY  - 2026
DA  - 2026-11-09
N1  - Source: ASME B1.20.1 — Pipe Threads, General Purpose (Inch), ASME B1.20.1-2013
ER  -

Each row in the tables above also has a permanent link — hover a row and use the # link to cite a single value rather than the whole page.