What is the tap drill size for 20-8 NPT?

The tap drill for 20-8 NPT is 1 — 1.0000 in (25.400 mm). Taper pipe threads are tapped with a taper tap and sealed with compound or tape.

What is the pitch diameter of 20-8 NPT?

The basic pitch diameter of 20-8 NPT is 19.8703 in (504.706 mm).

What is the minor diameter of 20-8 NPT?

The basic minor diameter of 20-8 NPT is 19.7703 in (502.166 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 20-8 NPT?

20-8 NPT has 8 threads per inch, a pitch of 0.1250 in (3.175 mm).

What is the hand-tight engagement length (L₁) of 20-8 NPT?

The hand-tight engagement length L₁ for 20-8 NPT is 2.1250 in (53.975 mm), on a pipe outside diameter of 20.0000 in (508.000 mm).

NPT Taper Pipe Thread · 20-8 NPT

20-8 NPT Thread — Dimensions, Tap Drill & Clearance

https://shopmath.org/threads/npt/20 · data verified August 1, 2026

The tap drill for 20-8 NPT taper pipe thread is 1 — 1.0000 in (25.400 mm). Basic pitch diameter is 19.8703 in (504.706 mm) and basic minor diameter is 19.7703 in (502.166 mm).

Series: NPT Taper Pipe Thread
Designation: 20-8 NPT
TPI: 8
Standard: ASME B1.20.1, ASME B1.20.3; public-domain primary FED-STD-H28/7A

Tap drill · taper pipe tap

Drill Ø: 1.0000in
Drill Ø: 25.400mm

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 20-8 NPT

Basic thread dimensions — 20-8 NPT
Property	Inch	Millimetre
Basic major diameter	20.0000	508.000
Threads per inch	8	—
Pitch	0.1250	3.175
Basic pitch diameter	19.8703	504.706
Basic minor diameter	19.7703	502.166
Sharp V thread height (H)	0.1082	2.750

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 20-8 NPT

20-8 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 — 20-8 NPT
Property	Inch	Millimetre
Pipe outside diameter	20.0000	508.000
Taper on diameter, per foot	0.7500	19.050
Pitch Ø at end of pipe (E₀)	19.7375	501.332
Pitch Ø at hand-tight plane (E₁)	19.8703	504.706
Pitch Ø at effective plane (E₂)	19.9156	505.857
Hand-tight engagement (L₁)	2.1250	53.975
Effective thread length (L₂)	2.8500	72.390
Overall external thread length (L₄)	3.2837	83.406
Thread height (h)	0.1000	2.540
Sharp V height (H)	0.1082	2.750
Minor Ø at hand-tight plane	19.7703	502.166

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.

Sources and Method for 20-8 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 20-8 NPT Reference

Citation

ShopMath. "20-8 NPT Thread — Dimensions, Tap Drill & Clearance." ShopMath, verified August 1, 2026, https://shopmath.org/threads/npt/20

BibTeX

@misc{shopmath-threads-npt-20,
  title        = {20-8 NPT Thread — Dimensions, Tap Drill & Clearance},
  author       = {{ShopMath}},
  year         = {2026},
  howpublished = {\url{https://shopmath.org/threads/npt/20}},
  note         = {Data verified 2026-08-01}
}

Permanent URL

https://shopmath.org/threads/npt/20

Common Questions About 20-8 NPT

What is the tap drill size for 20-8 NPT?: The tap drill for 20-8 NPT is 1 — 1.0000 in (25.400 mm). Taper pipe threads are tapped with a taper tap and sealed with compound or tape.
What is the pitch diameter of 20-8 NPT?: The basic pitch diameter of 20-8 NPT is 19.8703 in (504.706 mm).
What is the minor diameter of 20-8 NPT?: The basic minor diameter of 20-8 NPT is 19.7703 in (502.166 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 20-8 NPT?: 20-8 NPT has 8 threads per inch, a pitch of 0.1250 in (3.175 mm).
What is the hand-tight engagement length (L₁) of 20-8 NPT?: The hand-tight engagement length L₁ for 20-8 NPT is 2.1250 in (53.975 mm), on a pipe outside diameter of 20.0000 in (508.000 mm).

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