What is the tap drill size for 3/8-18 NPT?

The tap drill for 3/8-18 NPT is 37/64 — 0.5781 in (14.684 mm). Taper pipe threads are tapped with a taper tap and sealed with compound or tape.

What is the pitch diameter of 3/8-18 NPT?

The basic pitch diameter of 3/8-18 NPT is 0.6270 in (15.926 mm).

What is the minor diameter of 3/8-18 NPT?

The basic minor diameter of 3/8-18 NPT is 0.5826 in (14.798 mm). That is the theoretical root of the internal thread, not the drilled hole size — see the tap drill table for the hole to drill.

What is the thread pitch of 3/8-18 NPT?

3/8-18 NPT has 18 threads per inch, a pitch of 0.0556 in (1.411 mm).

What is the hand-tight engagement length (L₁) of 3/8-18 NPT?

The hand-tight engagement length L₁ for 3/8-18 NPT is 0.2400 in (6.096 mm), on a pipe outside diameter of 0.6750 in (17.145 mm).

NPT Taper Pipe Thread · 3/8-18 NPT

3/8-18 NPT Thread — Dimensions, Tap Drill & Clearance

https://shopmath.org/threads/npt/3-8 · data verified August 1, 2026

The tap drill for 3/8-18 NPT taper pipe thread is 37/64 — 0.5781 in (14.684 mm). Basic pitch diameter is 0.6270 in (15.926 mm) and basic minor diameter is 0.5826 in (14.798 mm).

Series: NPT Taper Pipe Thread
Designation: 3/8-18 NPT
TPI: 18
Standard: ASME B1.20.1, ASME B1.20.3; public-domain primary FED-STD-H28/7A

Tap drill · taper pipe tap

37/64

Drill Ø: 0.5781in
Drill Ø: 14.684mm

NPT and NPTF holes are drilled straight and cut with a taper tap. The standard assumes a sealing compound or tape on the joint.

Basic Thread Dimensions for 3/8-18 NPT

Basic thread dimensions — 3/8-18 NPT
Property	Inch	Millimetre
Basic major diameter	0.6750	17.145
Threads per inch	18	—
Pitch	0.0556	1.411
Basic pitch diameter	0.6270	15.926
Basic minor diameter	0.5826	14.798
Sharp V thread height (H)	0.0481	1.222

Threads per inch has no millimetre counterpart — metric threads are specified by pitch instead. Basic dimensions are the theoretical values before any tolerance class is applied.

Taper Pipe Thread Geometry for 3/8-18 NPT

3/8-18 NPT is a taper thread: the diameter changes along the thread, so every diameter below is stated at a named plane. The taper is 0.7500 in per foot on diameter.

Taper pipe thread geometry — 3/8-18 NPT
Property	Inch	Millimetre
Pipe outside diameter	0.6750	17.145
Taper on diameter, per foot	0.7500	19.050
Pitch Ø at end of pipe (E₀)	0.6120	15.545
Pitch Ø at hand-tight plane (E₁)	0.6270	15.926
Pitch Ø at effective plane (E₂)	0.6375	16.192
Hand-tight engagement (L₁)	0.2400	6.096
Effective thread length (L₂)	0.4078	10.358
Overall external thread length (L₄)	0.6006	15.255
Thread height (h)	0.0444	1.129
Sharp V height (H)	0.0481	1.222
Minor Ø at hand-tight plane	0.5826	14.798

ASME B1.20.1 publishes these dimensions in inches only; the millimetre column is an exact conversion (1 in = 25.4 mm by definition). L₁ is the hand-tight engagement length and is tabulated in the standard, not derived.

Other Thread Pitches in the Same Size as 3/8-18 NPT

A 3/8 fastener is not always 3/8-18 NPT. These are the other standard pitches cut on the same nominal size.

Other standard pitches at nominal size 3/8
Thread	Series	Pitch	Tap drill
3/8-18 NPTF	NPTF Dryseal Pipe Thread	18 TPI	37/64

Sources and Method for 3/8-18 NPT

Basic dimensions · Taper pipe geometrycomputed+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

NPT threads are not a seal on their own — the standard assumes a sealing compound or tape on the joint.
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 3/8-18 NPT Reference

Citation

ShopMath. "3/8-18 NPT Thread — Dimensions, Tap Drill & Clearance." ShopMath, verified August 1, 2026, https://shopmath.org/threads/npt/3-8

BibTeX

@misc{shopmath-threads-npt-3-8,
  title        = {3/8-18 NPT Thread — Dimensions, Tap Drill & Clearance},
  author       = {{ShopMath}},
  year         = {2026},
  howpublished = {\url{https://shopmath.org/threads/npt/3-8}},
  note         = {Data verified 2026-08-01}
}

Permanent URL

https://shopmath.org/threads/npt/3-8

Common Questions About 3/8-18 NPT

What is the tap drill size for 3/8-18 NPT?: The tap drill for 3/8-18 NPT is 37/64 — 0.5781 in (14.684 mm). Taper pipe threads are tapped with a taper tap and sealed with compound or tape.
What is the pitch diameter of 3/8-18 NPT?: The basic pitch diameter of 3/8-18 NPT is 0.6270 in (15.926 mm).
What is the minor diameter of 3/8-18 NPT?: The basic minor diameter of 3/8-18 NPT is 0.5826 in (14.798 mm). That is the theoretical root of the internal thread, not the drilled hole size — see the tap drill table for the hole to drill.
What is the thread pitch of 3/8-18 NPT?: 3/8-18 NPT has 18 threads per inch, a pitch of 0.0556 in (1.411 mm).
What is the hand-tight engagement length (L₁) of 3/8-18 NPT?: The hand-tight engagement length L₁ for 3/8-18 NPT is 0.2400 in (6.096 mm), on a pipe outside diameter of 0.6750 in (17.145 mm).

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