What is the tap drill size for 3/8"-24 UNF?

The tap drill for 3/8"-24 UNF is Q — 0.3320 in (8.433 mm). That leaves 79.4% thread engagement with a cut tap.

What is the pitch diameter of 3/8"-24 UNF?

The basic pitch diameter of 3/8"-24 UNF is 0.3479 in (8.837 mm). For a class 2B thread it runs 0.3479 in (8.837 mm) to 0.3529 in (8.964 mm).

What is the minor diameter of 3/8"-24 UNF?

The basic minor diameter of 3/8"-24 UNF is 0.3299 in (8.380 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"-24 UNF?

3/8"-24 UNF has 24 threads per inch, a pitch of 0.0417 in (1.058 mm).

What size clearance hole does a 3/8"-24 UNF fastener need?

close fit W (0.3898 in (9.901 mm)), normal fit Y (0.4055 in (10.300 mm)), loose fit Z (0.4213 in (10.701 mm)). The normal fit is the general-assembly default.

What is the tensile stress area of 3/8"-24 UNF?

The tensile stress area of 3/8"-24 UNF is 0.08783 in² (56.66 mm²). Multiply it by the fastener material's allowable stress to get a load.

Unified Fine (UNF) · 3/8-24 UNF

3/8"-24 UNF Thread — Dimensions, Tap Drill & Clearance

https://shopmath.org/threads/unf/3-8-24 · data verified August 1, 2026

The tap drill for 3/8"-24 UNF is Q — 0.3320 in (8.433 mm) — which leaves 79.4% thread engagement with a cut tap. Basic pitch diameter is 0.3479 in (8.837 mm) and basic minor diameter is 0.3299 in (8.380 mm). Tapped-hole figures on this page assume a class 2B thread; drill sizes for 50–85% engagement are in the tap drill table below.

Series: Unified Fine (UNF)
Designation: 3/8-24 UNF
TPI: 24
Standard: ASME B1.1

Tap drill · 79.4% thread · cut tap

Drill Ø: 0.3320in
Drill Ø: 8.433mm
Clearance (normal): Y

Assumes 75% thread engagement with a cut tap and a class 2B hole. That drill actually leaves 79.4% thread.

Basic Thread Dimensions for 3/8"-24 UNF

Basic thread dimensions — 3/8"-24 UNF
Property	Inch	Millimetre
Basic major diameter	0.3750	9.525
Threads per inch	24	—
Pitch	0.0417	1.058
Basic pitch diameter	0.3479	8.837
Basic minor diameter	0.3299	8.380
External thread root diameter	0.3254	8.265
Sharp V thread height (H)	0.0361	0.917

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.

Tap Drill Sizes for 3/8"-24 UNF

Cut taps, from 50% to 85% thread engagement. The shop default is 75% — a Q drill. Deeper engagement means more holding strength and more torque on the tap.

Tap drill sizes by thread engagement — 3/8"-24 UNF, cut tap
Property	Target hole Ø		Nearest standard drill			Property
Thread %	in	mm	Drill	in	mm	Actual thread %
50%	0.3479	8.837	S	0.3480	8.839	49.9
60%	0.3425	8.700	11/32	0.3438	8.731	57.7
65%	0.3398	8.631	R	0.3390	8.611	66.5
70%	0.3371	8.562	R	0.3390	8.611	66.5
75%default	0.3344	8.494	Q	0.3320	8.433	79.4
80%	0.3317	8.425	Q	0.3320	8.433	79.4
85%	0.3290	8.357	21/64	0.3281	8.334	86.6

Target hole Ø is the theoretical hole for the stated engagement. The drill column is the nearest size in the standard number/letter/fractional/metric index, and actual thread % is what that real drill produces — the number that matters at the machine. Values are for cut taps. Tapped-hole figures assume a class 2B hole.

Clearance Hole Sizes for 3/8"-24 UNF

The through-hole a 3/8"-24 UNF fastener passes freely, not the tapped hole.

Clearance hole sizes — 3/8"-24 UNF
Property	Property	Hole Ø		Property
Fit	Drill	in	mm	Use
Close	W	0.3898	9.901	Located joints, dowelled assemblies
Normal	Y	0.4055	10.300	General assembly — the default
Loose	Z	0.4213	10.701	Slotted or adjustable joints, weldments

Clearance holes per ISO 273 (metric) and ASME B18.2.8 (inch). The drill column is the nearest standard drill at or above the specified hole diameter.

Thread Class Limits for 3/8"-24 UNF

Manufacturing limits for classes 2B, 3B, 2A, 3A. A gauge reading outside these bands is a reject, not a judgement call.

External thread (bolt / screw) limits — 3/8"-24 UNF
Property	Class 2A		Class 3A
Limit	in	mm	in	mm
Major Ø max	0.3739	9.497	0.3750	9.525
Major Ø min	0.3666	9.312	0.3678	9.342
Pitch Ø max	0.3468	8.809	0.3479	8.837
Pitch Ø min	0.3430	8.712	0.3451	8.765
Pitch Ø tolerance	0.0038	0.097	0.0029	0.074
Minor Ø max	0.3197	8.120	0.3209	8.151
Minor Ø min	0.3159	8.024	0.3180	8.077
Allowance	0.0011	0.028	0.0000	0.000

Internal thread (nut / tapped hole) limits — 3/8"-24 UNF
Property	Class 2B		Class 3B
Limit	in	mm	in	mm
Major Ø max	0.3860	9.804	0.3847	9.771
Major Ø min	0.3750	9.525	0.3750	9.525
Pitch Ø max	0.3529	8.964	0.3516	8.931
Pitch Ø min	0.3479	8.837	0.3479	8.837
Pitch Ø tolerance	0.0049	0.124	0.0037	0.094
Minor Ø max	0.3396	8.626	0.3372	8.565
Minor Ø min	0.3299	8.380	0.3299	8.380
Allowance	0.0000	0.000	0.0000	0.000

Tensile Stress Area of 3/8"-24 UNF

The effective cross-section used to size a 3/8"-24 UNF fastener against a load. Multiply it by the material's allowable stress — the area itself carries no strength assumption.

Tensile stress area — 3/8"-24 UNF
Property	in²	mm²
Tensile stress area	0.08783	56.66

The formula this value comes from is in the sources section below.

Other Thread Pitches in the Same Size as 3/8"-24 UNF

A 3/8 fastener is not always 3/8"-24 UNF. 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"-32 UNEF	Unified Extra Fine (UNEF)	32 TPI	11/32
3/8"-16 UNC	Unified Coarse (UNC)	16 TPI	5/16

Sources and Method for 3/8"-24 UNF

Basic dimensionscomputed

NBS Handbook H28 (1969) Part I §§6-7 (public domain, 17 U.S.C. 105); normative modern reference ASME B1.1; series membership: Series membership: threadlib UIS_thread.csv (BSD-3-Clause); cross-checked against NBS H28 (1969) Part I Table 2.21; clearance holes: ISO 273 (metric) and ASME B18.2.8 (inch) via FreeCAD FeatureHole.cpp (LGPL-2.1+), cross-checked against bd_warehouse clearance_hole_sizes.csv (Apache-2.0)

Verified against NBS H28 (1969) Part I, Table 2.21 (public domain); cross-checked against FreeCAD CAM thread tables (LGPL-2.1+) — 1064 checks passed at 0.0001 in

Computed from the standard's formulas, then checked against published tables.

Tap drill sizescomputed

Machinery's Handbook %-thread formula (geometry-derived); nearest-standard-drill-to-75% rule verified at 36/39 published chart rows in docs/research/data-sourcing.md §3.4

Verified against Wikipedia Unified Thread Standard, preferred cutting tap drill size columns (CC BY-SA 4.0) — 36/39 published UNC+UNF rows reproduced

Computed from the standard's formulas, then checked against published tables.

Class limitscomputed

NBS Handbook H28 (1969) Part I §§6-7 (public domain, 17 U.S.C. 105); normative modern reference ASME B1.1

Verified against NBS H28 (1969) Part I, Table 2.21 (public domain); cross-checked against FreeCAD CAM thread tables (LGPL-2.1+) — 1064 checks passed at 0.0001 in

Computed from the standard's formulas, then checked against published tables.

Formula

Tap drill hole diameter, inch threads

hole Ø  =  D − (%thread ÷ 100) × (1.29904 ÷ n)

  D = basic major diameter (in)
  n = threads per inch

1.29904 = 2 × 0.64952, twice the engaged thread height per side on the 60° Unified profile.

Actual thread engagement of the chosen drill

%thread  =  (D − drill Ø) × n ÷ 1.29904 × 100

The drill column rounds to a real drill; this is the engagement that drill actually leaves.

Tensile stress area, Unified inch threads

At  =  0.7854 × (D − 0.9743 ÷ n)²

ASME B1.1 / NBS H28 form. Reproduces the stored value exactly for this thread.

Assumptions

The headline tap drill assumes 75% thread engagement, the general shop default. Every other engagement from 50% to 85% is in the tap drill table — change the assumption and the number moves.
Tap drill figures are for cut taps. Drill diameters are nominal; a worn or run-out drill cuts oversize and reduces engagement further.
Tapped-hole figures assume a class 2B internal thread, the general-purpose fit.
Dataset scope: Le = 1D for UNC/UNF, 9P for UNEF — determined empirically against H28 Table 2.21, resolving data-sourcing.md open item 5. EXCLUDED: form (roll) tap drills, class 1A/1B limits, class 1AR allowances, constant-pitch UN series.

Data last verified

August 1, 2026

How this data is built and checked →

How to Cite This 3/8"-24 UNF Reference

Citation

ShopMath. "3/8"-24 UNF Thread — Dimensions, Tap Drill & Clearance." ShopMath, verified August 1, 2026, https://shopmath.org/threads/unf/3-8-24

BibTeX

@misc{shopmath-threads-unf-3-8-24,
  title        = {3/8"-24 UNF Thread — Dimensions, Tap Drill & Clearance},
  author       = {{ShopMath}},
  year         = {2026},
  howpublished = {\url{https://shopmath.org/threads/unf/3-8-24}},
  note         = {Data verified 2026-08-01}
}

Permanent URL

https://shopmath.org/threads/unf/3-8-24

Common Questions About 3/8"-24 UNF

What is the tap drill size for 3/8"-24 UNF?: The tap drill for 3/8"-24 UNF is Q — 0.3320 in (8.433 mm). That leaves 79.4% thread engagement with a cut tap.
What is the pitch diameter of 3/8"-24 UNF?: The basic pitch diameter of 3/8"-24 UNF is 0.3479 in (8.837 mm). For a class 2B thread it runs 0.3479 in (8.837 mm) to 0.3529 in (8.964 mm).
What is the minor diameter of 3/8"-24 UNF?: The basic minor diameter of 3/8"-24 UNF is 0.3299 in (8.380 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"-24 UNF?: 3/8"-24 UNF has 24 threads per inch, a pitch of 0.0417 in (1.058 mm).
What size clearance hole does a 3/8"-24 UNF fastener need?: close fit W (0.3898 in (9.901 mm)), normal fit Y (0.4055 in (10.300 mm)), loose fit Z (0.4213 in (10.701 mm)). The normal fit is the general-assembly default.
What is the tensile stress area of 3/8"-24 UNF?: The tensile stress area of 3/8"-24 UNF is 0.08783 in² (56.66 mm²). Multiply it by the fastener material's allowable stress to get a load.

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