Unified Coarse (UNC) · #12-24 UNC
#12-24 UNC Thread — Dimensions, Tap Drill & Clearance
https://shopmath.org/threads/unc/12-24 · data verified August 1, 2026
The tap drill for #12-24 UNC is #16 — 0.1770 in (4.496 mm) — which leaves 72.1% thread engagement with a cut tap. Basic pitch diameter is 0.1889 in (4.798 mm) and basic minor diameter is 0.1709 in (4.341 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
- #12-24 UNC
- TPI
- 24
- Standard
- ASME B1.1
Tap drill · 72.1% thread · cut tap
#16
- Drill Ø
- 0.1770in
- Drill Ø
- 4.496mm
Assumes 75% thread engagement with a cut tap and a class 2B hole. That drill actually leaves 72.1% thread.
Basic Thread Dimensions for #12-24 UNC
| Property | Inch | Millimetre |
|---|---|---|
| Basic major diameter | 0.2160 | 5.486 |
| Threads per inch | 24 | — |
| Pitch | 0.0417 | 1.058 |
| Basic pitch diameter | 0.1889 | 4.798 |
| Basic minor diameter | 0.1709 | 4.341 |
| External thread root diameter | 0.1664 | 4.227 |
| 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 #12-24 UNC
Cut taps, from 50% to 85% thread engagement. The shop default is 75% — a #16 drill. Deeper engagement means more holding strength and more torque on the tap.
| Property | Target hole Ø | Nearest standard drill | Property | |||
|---|---|---|---|---|---|---|
| Thread % | in | mm | Drill | in | mm | Actual thread % |
| 50% | 0.1889 | 4.798 | #12 | 0.1890 | 4.801 | 49.9 |
| 60% | 0.1835 | 4.661 | #13 | 0.1850 | 4.699 | 57.3 |
| 65% | 0.1808 | 4.592 | #15 | 0.1800 | 4.572 | 66.5 |
| 70% | 0.1781 | 4.524 | #16 | 0.1770 | 4.496 | 72.1 |
| 75%default | 0.1754 | 4.455 | #16 | 0.1770 | 4.496 | 72.1 |
| 80% | 0.1727 | 4.387 | #17 | 0.1730 | 4.394 | 79.4 |
| 85% | 0.1700 | 4.318 | #18 | 0.1695 | 4.305 | 85.9 |
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 #12-24 UNC
Manufacturing limits for classes 2B, 3B, 2A, 3A. A gauge reading outside these bands is a reject, not a judgement call.
| Property | Class 2A | Class 3A | ||
|---|---|---|---|---|
| Limit | in | mm | in | mm |
| Major Ø max | 0.2150 | 5.461 | 0.2160 | 5.486 |
| Major Ø min | 0.2078 | 5.278 | 0.2088 | 5.303 |
| Pitch Ø max | 0.1879 | 4.773 | 0.1889 | 4.798 |
| Pitch Ø min | 0.1845 | 4.686 | 0.1864 | 4.735 |
| Pitch Ø tolerance | 0.0034 | 0.086 | 0.0025 | 0.064 |
| Minor Ø max | 0.1609 | 4.087 | 0.1619 | 4.112 |
| Minor Ø min | 0.1575 | 4.000 | 0.1593 | 4.046 |
| Allowance | 0.0010 | 0.025 | 0.0000 | 0.000 |
| Property | Class 2B | Class 3B | ||
|---|---|---|---|---|
| Limit | in | mm | in | mm |
| Major Ø max | 0.2264 | 5.751 | 0.2253 | 5.723 |
| Major Ø min | 0.2160 | 5.486 | 0.2160 | 5.486 |
| Pitch Ø max | 0.1934 | 4.912 | 0.1923 | 4.884 |
| Pitch Ø min | 0.1889 | 4.798 | 0.1889 | 4.798 |
| Pitch Ø tolerance | 0.0044 | 0.112 | 0.0033 | 0.084 |
| Minor Ø max | 0.1807 | 4.590 | 0.1807 | 4.590 |
| Minor Ø min | 0.1709 | 4.341 | 0.1709 | 4.341 |
| Allowance | 0.0000 | 0.000 | 0.0000 | 0.000 |
Tensile Stress Area of #12-24 UNC
The effective cross-section used to size a #12-24 UNC fastener against a load. Multiply it by the material's allowable stress — the area itself carries no strength assumption.
| Property | in² | mm² |
|---|---|---|
| Tensile stress area | 0.02416 | 15.59 |
The formula this value comes from is in the sources section below.
Sources and Method for #12-24 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 to Cite This #12-24 UNC Reference
Citation
ShopMath. "#12-24 UNC Thread — Dimensions, Tap Drill & Clearance." ShopMath, verified August 1, 2026, https://shopmath.org/threads/unc/12-24
BibTeX
@misc{shopmath-threads-unc-12-24,
title = {#12-24 UNC Thread — Dimensions, Tap Drill & Clearance},
author = {{ShopMath}},
year = {2026},
howpublished = {\url{https://shopmath.org/threads/unc/12-24}},
note = {Data verified 2026-08-01}
}Permanent URL
https://shopmath.org/threads/unc/12-24
Common Questions About #12-24 UNC
What is the tap drill size for #12-24 UNC?
- The tap drill for #12-24 UNC is #16 — 0.1770 in (4.496 mm). That leaves 72.1% thread engagement with a cut tap.
What is the pitch diameter of #12-24 UNC?
- The basic pitch diameter of #12-24 UNC is 0.1889 in (4.798 mm). For a class 2B thread it runs 0.1889 in (4.798 mm) to 0.1934 in (4.912 mm).
What is the minor diameter of #12-24 UNC?
- The basic minor diameter of #12-24 UNC is 0.1709 in (4.341 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 #12-24 UNC?
- #12-24 UNC has 24 threads per inch, a pitch of 0.0417 in (1.058 mm).
What is the tensile stress area of #12-24 UNC?
- The tensile stress area of #12-24 UNC is 0.02416 in² (15.59 mm²). Multiply it by the fastener material's allowable stress to get a load.