What is the tap drill size for 9/16"-18 UNF?

The tap drill for 9/16"-18 UNF is 33/64 — 0.5156 in (13.097 mm). That leaves 65.0% thread engagement with a cut tap.

What is the pitch diameter of 9/16"-18 UNF?

The basic pitch diameter of 9/16"-18 UNF is 0.5264 in (13.371 mm). For a class 2B thread it runs 0.5264 in (13.371 mm) to 0.5323 in (13.520 mm).

What is the minor diameter of 9/16"-18 UNF?

The basic minor diameter of 9/16"-18 UNF is 0.5024 in (12.761 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 9/16"-18 UNF?

9/16"-18 UNF has 18 threads per inch, a pitch of 0.0556 in (1.411 mm).

What is the tensile stress area of 9/16"-18 UNF?

The tensile stress area of 9/16"-18 UNF is 0.20298 in² (130.96 mm²). Multiply it by the fastener material's allowable stress to get a load.

Why does the 9/16"-18 UNF tap drill here differ from the chart on the wall?

Computed 75% target 0.5084 in. Nearest standard drill is 33/64 (0.515625 in, off by 0.0073 in); the traditional published chart lists 1/2 (0.5 in, off by 0.0084 in, 86.6% thread). Both are shown; neither overrides the other.

Unified Fine (UNF) · 9/16-18 UNF

9/16"-18 UNF Thread — Dimensions, Tap Drill & Clearance

https://shopmath.org/threads/unf/9-16-18 · data verified August 1, 2026

The tap drill for 9/16"-18 UNF is 33/64 — 0.5156 in (13.097 mm) — which leaves 65.0% thread engagement with a cut tap. Basic pitch diameter is 0.5264 in (13.371 mm) and basic minor diameter is 0.5024 in (12.761 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: 9/16-18 UNF
TPI: 18
Standard: ASME B1.1

Tap drill · 65.0% thread · cut tap

33/64

Drill Ø: 0.5156in
Drill Ø: 13.097mm

Assumes 75% thread engagement with a cut tap and a class 2B hole. That drill actually leaves 65.0% thread.

Basic Thread Dimensions for 9/16"-18 UNF

Basic thread dimensions — 9/16"-18 UNF
Property	Inch	Millimetre
Basic major diameter	0.5625	14.287
Threads per inch	18	—
Pitch	0.0556	1.411
Basic pitch diameter	0.5264	13.371
Basic minor diameter	0.5024	12.761
External thread root diameter	0.4963	12.606
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.

Tap Drill Sizes for 9/16"-18 UNF

Cut taps, from 50% to 85% thread engagement. The shop default is 75% — a 33/64 drill. Deeper engagement means more holding strength and more torque on the tap.

Tap drill sizes by thread engagement — 9/16"-18 UNF, cut tap
Property	Target hole Ø		Nearest standard drill			Property
Thread %	in	mm	Drill	in	mm	Actual thread %
50%	0.5264	13.371	17/32	0.5313	13.494	43.3
60%	0.5192	13.188	33/64	0.5156	13.097	65.0
65%	0.5156	13.096	33/64	0.5156	13.097	65.0
70%	0.5120	13.005	33/64	0.5156	13.097	65.0
75%default	0.5084	12.913	33/64	0.5156	13.097	65.0
80%	0.5048	12.822	1/2	0.5000	12.700	86.6
85%	0.5012	12.730	1/2	0.5000	12.700	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.

Thread Class Limits for 9/16"-18 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 — 9/16"-18 UNF
Property	Class 2A		Class 3A
Limit	in	mm	in	mm
Major Ø max	0.5611	14.252	0.5625	14.287
Major Ø min	0.5524	14.031	0.5538	14.066
Pitch Ø max	0.5251	13.338	0.5264	13.371
Pitch Ø min	0.5205	13.221	0.5230	13.284
Pitch Ø tolerance	0.0045	0.114	0.0034	0.086
Minor Ø max	0.4890	12.421	0.4903	12.454
Minor Ø min	0.4844	12.304	0.4869	12.367
Allowance	0.0014	0.036	0.0000	0.000

Internal thread (nut / tapped hole) limits — 9/16"-18 UNF
Property	Class 2B		Class 3B
Limit	in	mm	in	mm
Major Ø max	0.5764	14.641	0.5750	14.605
Major Ø min	0.5625	14.287	0.5625	14.287
Pitch Ø max	0.5323	13.520	0.5308	13.482
Pitch Ø min	0.5264	13.371	0.5264	13.371
Pitch Ø tolerance	0.0059	0.150	0.0044	0.112
Minor Ø max	0.5150	13.081	0.5106	12.969
Minor Ø min	0.5024	12.761	0.5024	12.761
Allowance	0.0000	0.000	0.0000	0.000

Tensile Stress Area of 9/16"-18 UNF

The effective cross-section used to size a 9/16"-18 UNF fastener against a load. Multiply it by the material's allowable stress — the area itself carries no strength assumption.

Tensile stress area — 9/16"-18 UNF
Property	in²	mm²
Tensile stress area	0.20298	130.96

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

Other Thread Pitches in the Same Size as 9/16"-18 UNF

A 9/16 fastener is not always 9/16"-18 UNF. These are the other standard pitches cut on the same nominal size.

Other standard pitches at nominal size 9/16
Thread	Series	Pitch	Tap drill
9/16"-24 UNEF	Unified Extra Fine (UNEF)	24 TPI	33/64
9/16"-12 UNC	Unified Coarse (UNC)	12 TPI	31/64

Sources and Method for 9/16"-18 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 9/16"-18 UNF Reference

Citation

ShopMath. "9/16"-18 UNF Thread — Dimensions, Tap Drill & Clearance." ShopMath, verified August 1, 2026, https://shopmath.org/threads/unf/9-16-18

BibTeX

@misc{shopmath-threads-unf-9-16-18,
  title        = {9/16"-18 UNF Thread — Dimensions, Tap Drill & Clearance},
  author       = {{ShopMath}},
  year         = {2026},
  howpublished = {\url{https://shopmath.org/threads/unf/9-16-18}},
  note         = {Data verified 2026-08-01}
}

Permanent URL

https://shopmath.org/threads/unf/9-16-18

Common Questions About 9/16"-18 UNF

What is the tap drill size for 9/16"-18 UNF?: The tap drill for 9/16"-18 UNF is 33/64 — 0.5156 in (13.097 mm). That leaves 65.0% thread engagement with a cut tap.
What is the pitch diameter of 9/16"-18 UNF?: The basic pitch diameter of 9/16"-18 UNF is 0.5264 in (13.371 mm). For a class 2B thread it runs 0.5264 in (13.371 mm) to 0.5323 in (13.520 mm).
What is the minor diameter of 9/16"-18 UNF?: The basic minor diameter of 9/16"-18 UNF is 0.5024 in (12.761 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 9/16"-18 UNF?: 9/16"-18 UNF has 18 threads per inch, a pitch of 0.0556 in (1.411 mm).
What is the tensile stress area of 9/16"-18 UNF?: The tensile stress area of 9/16"-18 UNF is 0.20298 in² (130.96 mm²). Multiply it by the fastener material's allowable stress to get a load.
Why does the 9/16"-18 UNF tap drill here differ from the chart on the wall?: Computed 75% target 0.5084 in. Nearest standard drill is 33/64 (0.515625 in, off by 0.0073 in); the traditional published chart lists 1/2 (0.5 in, off by 0.0084 in, 86.6% thread). Both are shown; neither overrides the other.

All Unified Fine (UNF) sizes · Every thread series