What is the tap drill size for 1"-8 UNC?

The tap drill for 1"-8 UNC is 7/8 — 0.8750 in (22.225 mm). That leaves 77.0% thread engagement with a cut tap.

What is the pitch diameter of 1"-8 UNC?

The basic pitch diameter of 1"-8 UNC is 0.9188 in (23.337 mm). For a class 2B thread it runs 0.9188 in (23.337 mm) to 0.9276 in (23.561 mm).

What is the minor diameter of 1"-8 UNC?

The basic minor diameter of 1"-8 UNC is 0.8647 in (21.963 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 1"-8 UNC?

1"-8 UNC has 8 threads per inch, a pitch of 0.1250 in (3.175 mm).

What size clearance hole does a 1"-8 UNC fastener need?

close fit 1 (1.0315 in (26.200 mm)), normal fit 1 (1.0945 in (27.800 mm)), loose fit 1 (1.1575 in (29.401 mm)). The normal fit is the general-assembly default.

What is the tensile stress area of 1"-8 UNC?

The tensile stress area of 1"-8 UNC is 0.60574 in² (390.80 mm²). Multiply it by the fastener material's allowable stress to get a load.

Unified Coarse (UNC) · 1-8 UNC

1"-8 UNC Thread — Dimensions, Tap Drill & Clearance

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

The tap drill for 1"-8 UNC is 7/8 — 0.8750 in (22.225 mm) — which leaves 77.0% thread engagement with a cut tap. Basic pitch diameter is 0.9188 in (23.337 mm) and basic minor diameter is 0.8647 in (21.963 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 Coarse (UNC)
Designation: 1-8 UNC
TPI: 8
Standard: ASME B1.1

Tap drill · 77.0% thread · cut tap

7/8

Drill Ø: 0.8750in
Drill Ø: 22.225mm
Clearance (normal): 1

Assumes 75% thread engagement with a cut tap and a class 2B hole. That drill actually leaves 77.0% thread.

Basic Thread Dimensions for 1"-8 UNC

Basic thread dimensions — 1"-8 UNC
Property	Inch	Millimetre
Basic major diameter	1.0000	25.400
Threads per inch	8	—
Pitch	0.1250	3.175
Basic pitch diameter	0.9188	23.337
Basic minor diameter	0.8647	21.963
External thread root diameter	0.8512	21.620
Sharp V thread height (H)	0.1083	2.751

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 1"-8 UNC

Cut taps, from 50% to 85% thread engagement. The shop default is 75% — a 7/8 drill. Deeper engagement means more holding strength and more torque on the tap.

Tap drill sizes by thread engagement — 1"-8 UNC, cut tap
Property	Target hole Ø		Nearest standard drill			Property
Thread %	in	mm	Drill	in	mm	Actual thread %
50%	0.9188	23.337	59/64	0.9219	23.416	48.1
60%	0.9026	22.926	29/32	0.9063	23.019	57.7
65%	0.8945	22.720	57/64	0.8906	22.622	67.4
70%	0.8863	22.512	57/64	0.8906	22.622	67.4
75%default	0.8782	22.306	7/8	0.8750	22.225	77.0
80%	0.8701	22.101	7/8	0.8750	22.225	77.0
85%	0.8620	21.895	55/64	0.8594	21.828	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 1"-8 UNC

The through-hole a 1"-8 UNC fastener passes freely, not the tapped hole.

Clearance hole sizes — 1"-8 UNC
Property	Property	Hole Ø		Property
Fit	Drill	in	mm	Use
Close	1	1.0315	26.200	Located joints, dowelled assemblies
Normal	1	1.0945	27.800	General assembly — the default
Loose	1	1.1575	29.401	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 1"-8 UNC

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 — 1"-8 UNC
Property	Class 2A		Class 3A
Limit	in	mm	in	mm
Major Ø max	0.9980	25.349	1.0000	25.400
Major Ø min	0.9830	24.968	0.9850	25.019
Pitch Ø max	0.9168	23.287	0.9188	23.337
Pitch Ø min	0.9100	23.114	0.9137	23.208
Pitch Ø tolerance	0.0068	0.173	0.0051	0.130
Minor Ø max	0.8356	21.224	0.8376	21.275
Minor Ø min	0.8288	21.052	0.8326	21.148
Allowance	0.0020	0.051	0.0000	0.000

Internal thread (nut / tapped hole) limits — 1"-8 UNC
Property	Class 2B		Class 3B
Limit	in	mm	in	mm
Major Ø max	1.0268	26.081	1.0246	26.025
Major Ø min	1.0000	25.400	1.0000	25.400
Pitch Ø max	0.9276	23.561	0.9254	23.505
Pitch Ø min	0.9188	23.337	0.9188	23.337
Pitch Ø tolerance	0.0088	0.224	0.0066	0.168
Minor Ø max	0.8897	22.598	0.8797	22.344
Minor Ø min	0.8647	21.963	0.8647	21.963
Allowance	0.0000	0.000	0.0000	0.000

Tensile Stress Area of 1"-8 UNC

The effective cross-section used to size a 1"-8 UNC fastener against a load. Multiply it by the material's allowable stress — the area itself carries no strength assumption.

Tensile stress area — 1"-8 UNC
Property	in²	mm²
Tensile stress area	0.60574	390.80

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

Other Thread Pitches in the Same Size as 1"-8 UNC

A 1 fastener is not always 1"-8 UNC. These are the other standard pitches cut on the same nominal size.

Other standard pitches at nominal size 1
Thread	Series	Pitch	Tap drill
1"-20 UNEF	Unified Extra Fine (UNEF)	20 TPI	61/64
1"-12 UNF	Unified Fine (UNF)	12 TPI	59/64

Sources and Method for 1"-8 UNC

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 1"-8 UNC Reference

Citation

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

BibTeX

@misc{shopmath-threads-unc-1-8,
  title        = {1"-8 UNC Thread — Dimensions, Tap Drill & Clearance},
  author       = {{ShopMath}},
  year         = {2026},
  howpublished = {\url{https://shopmath.org/threads/unc/1-8}},
  note         = {Data verified 2026-08-01}
}

Permanent URL

https://shopmath.org/threads/unc/1-8

Common Questions About 1"-8 UNC

What is the tap drill size for 1"-8 UNC?: The tap drill for 1"-8 UNC is 7/8 — 0.8750 in (22.225 mm). That leaves 77.0% thread engagement with a cut tap.
What is the pitch diameter of 1"-8 UNC?: The basic pitch diameter of 1"-8 UNC is 0.9188 in (23.337 mm). For a class 2B thread it runs 0.9188 in (23.337 mm) to 0.9276 in (23.561 mm).
What is the minor diameter of 1"-8 UNC?: The basic minor diameter of 1"-8 UNC is 0.8647 in (21.963 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 1"-8 UNC?: 1"-8 UNC has 8 threads per inch, a pitch of 0.1250 in (3.175 mm).
What size clearance hole does a 1"-8 UNC fastener need?: close fit 1 (1.0315 in (26.200 mm)), normal fit 1 (1.0945 in (27.800 mm)), loose fit 1 (1.1575 in (29.401 mm)). The normal fit is the general-assembly default.
What is the tensile stress area of 1"-8 UNC?: The tensile stress area of 1"-8 UNC is 0.60574 in² (390.80 mm²). Multiply it by the fastener material's allowable stress to get a load.

All Unified Coarse (UNC) sizes · Every thread series