---
type: "thread"
name: "3/8\"-24 UNF"
designation: "3/8-24 UNF"
series: "unf"
series_name: "Unified Fine (UNF)"
system: "unified"
size_label: "3/8"
tpi: 24
pitch_in: 0.041667
pitch_mm: 1.0583
basic_major_dia_in: 0.375
basic_major_dia_mm: 9.525
basic_pitch_dia_in: 0.3479
basic_pitch_dia_mm: 8.8367
basic_minor_dia_in: 0.3299
basic_minor_dia_mm: 8.3795
tap_drill: "Q"
tap_drill_dia_in: 0.332
tap_drill_dia_mm: 8.4328
tap_drill_thread_pct: 79.4
clearance_normal_drill: "Y"
tensile_stress_area_sqin: 0.087828
tensile_stress_area_sqmm: 56.663
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/unf/3-8-24
---

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

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

## Basic Thread Dimensions for 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 |

_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.

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

## Clearance Hole Sizes for 3/8"-24 UNF

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

| Fit | Drill | Hole Ø (in) | Hole Ø (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)._

## 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

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

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

| Property | in² | mm² |
| --- | --- | --- |
| Tensile stress area | 0.08783 | 56.66 |

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.

## 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.

| Thread | Series | Pitch | Tap drill |
| --- | --- | --- | --- |
| [3/8"-32 UNEF](https://shopmath.org/threads/unef/3-8-32.md) | Unified Extra Fine (UNEF) | 32 TPI | 11/32 |
| [3/8"-16 UNC](https://shopmath.org/threads/unc/3-8-16.md) | Unified Coarse (UNC) | 16 TPI | 5/16 |

## Sources and Method for 3/8"-24 UNF

### 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 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.

## Related pages

- Smaller in Unified Fine (UNF): [5/16"-24 UNF](https://shopmath.org/threads/unf/5-16-24.md) — tap drill I
- Larger in Unified Fine (UNF): [7/16"-20 UNF](https://shopmath.org/threads/unf/7-16-20.md) — tap drill 25/64
- All Unified Fine (UNF) sizes: [Unified Fine (UNF) thread chart](https://shopmath.org/threads/unf.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/unf/3-8-24) — data verified August 1, 2026. Reference data; verify against the governing standard before manufacture.*