---
type: "thread"
name: "#12-24 UNC"
designation: "#12-24 UNC"
series: "unc"
series_name: "Unified Coarse (UNC)"
system: "unified"
size_label: "#12"
tpi: 24
pitch_in: 0.041667
pitch_mm: 1.0583
basic_major_dia_in: 0.216
basic_major_dia_mm: 5.4864
basic_pitch_dia_in: 0.1889
basic_pitch_dia_mm: 4.7981
basic_minor_dia_in: 0.1709
basic_minor_dia_mm: 4.3409
tap_drill: "#16"
tap_drill_dia_in: 0.177
tap_drill_dia_mm: 4.4958
tap_drill_thread_pct: 72.1
tensile_stress_area_sqin: 0.024164
tensile_stress_area_sqmm: 15.59
internal_class_assumed: "2B"
standard: "ASME B1.1 (reference values based on the ASME B1.1 system; not a compliance claim)"
verified: "August 1, 2026"
url: https://shopmath.org/threads/unc/12-24
---

# #12-24 UNC Thread — Dimensions, Tap Drill & Clearance

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
- **Threads per inch**: 24
- **Tap drill**: #16
- **Standard**: ASME B1.1 (reference values based on the ASME B1.1 system; not a compliance claim)

## 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 |

_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.

| Thread % | Target hole Ø (in) | Target hole Ø (mm) | Drill | Drill Ø (in) | Drill Ø (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._

## 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.

### External thread (bolt / screw) limits

| Limit | Class 2A (in) | Class 2A (mm) | Class 3A (in) | Class 3A (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 |

### Internal thread (nut / tapped hole) limits

| Limit | Class 2B (in) | Class 2B (mm) | Class 3B (in) | Class 3B (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

| Property | in² | mm² |
| --- | --- | --- |
| Tensile stress area | 0.02416 | 15.59 |

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.

## Other Thread Pitches in the Same Size as #12-24 UNC

A #12 fastener is not always #12-24 UNC. These are the other standard pitches cut on the same nominal size.

| Thread | Series | Pitch | Tap drill |
| --- | --- | --- | --- |
| [#12-32 UNEF](https://shopmath.org/threads/unef/12-32.md) | Unified Extra Fine (UNEF) | 32 TPI | #13 |
| [#12-28 UNF](https://shopmath.org/threads/unf/12-28.md) | Unified Fine (UNF) | 28 TPI | #14 |

## Sources and Method for #12-24 UNC

### Basic dimensions

- **Source**: 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)
- **Method**: computed
- **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

### Tap drill sizes

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

### Class limits

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

### Formulas

**Tap drill hole diameter, inch threads**

```text
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**

```text
%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**

```text
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 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

## 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.

## Related pages

- Smaller in Unified Coarse (UNC): [#10-24 UNC](https://shopmath.org/threads/unc/10-24.md) — tap drill #25
- Larger in Unified Coarse (UNC): [1/4"-20 UNC](https://shopmath.org/threads/unc/1-4-20.md) — tap drill #7
- All Unified Coarse (UNC) sizes: [Unified Coarse (UNC) thread chart](https://shopmath.org/threads/unc.md)
- Every thread series: [Screw thread reference](https://shopmath.org/threads.md)
- Cross-series chart: [Tap drill chart](https://shopmath.org/charts/tap-drill.md)

---

*Source: [ShopMath](https://shopmath.org/threads/unc/12-24) — data verified August 1, 2026. Reference data; verify against the governing standard before manufacture.*