---
type: "thread"
name: "1/4\"-28 UNF"
designation: "1/4-28 UNF"
series: "unf"
series_name: "Unified Fine (UNF)"
system: "unified"
size_label: "1/4"
tpi: 28
pitch_in: 0.035714
pitch_mm: 0.9071
basic_major_dia_in: 0.25
basic_major_dia_mm: 6.35
basic_pitch_dia_in: 0.2268
basic_pitch_dia_mm: 5.7607
basic_minor_dia_in: 0.2113
basic_minor_dia_mm: 5.367
tap_drill: "#3"
tap_drill_dia_in: 0.213
tap_drill_dia_mm: 5.4102
tap_drill_thread_pct: 79.8
clearance_normal_drill: "J"
tensile_stress_area_sqin: 0.036374
tensile_stress_area_sqmm: 23.467
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/1-4-28
---

# 1/4"-28 UNF Thread — Dimensions, Tap Drill & Clearance

The tap drill for 1/4"-28 UNF is #3 — 0.2130 in (5.410 mm) — which leaves 79.8% thread engagement with a cut tap. Basic pitch diameter is 0.2268 in (5.761 mm) and basic minor diameter is 0.2113 in (5.367 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**: 1/4-28 UNF
- **Threads per inch**: 28
- **Tap drill**: #3
- **Standard**: ASME B1.1 (reference values based on the ASME B1.1 system; not a compliance claim)

## Basic Thread Dimensions for 1/4"-28 UNF

| Property | Inch | Millimetre |
| --- | --- | --- |
| Basic major diameter | 0.2500 | 6.350 |
| Threads per inch | 28 | — |
| Pitch | 0.0357 | 0.907 |
| Basic pitch diameter | 0.2268 | 5.761 |
| Basic minor diameter | 0.2113 | 5.367 |
| External thread root diameter | 0.2075 | 5.271 |
| Sharp V thread height (H) | 0.0309 | 0.785 |

_Basic dimensions are the theoretical values before any tolerance class is applied._

## Tap Drill Sizes for 1/4"-28 UNF

Cut taps, from 50% to 85% thread engagement. The shop default is 75% — a #3 drill.

| Thread % | Target hole Ø (in) | Target hole Ø (mm) | Drill | Drill Ø (in) | Drill Ø (mm) | Actual thread % |
| --- | --- | --- | --- | --- | --- | --- |
| 50% | 0.2268 | 5.761 | #1 | 0.2280 | 5.791 | 47.4 |
| 60% | 0.2222 | 5.644 | #2 | 0.2210 | 5.613 | 62.5 |
| 65% | 0.2198 | 5.583 | 7/32 | 0.2188 | 5.556 | 67.4 |
| 70% | 0.2175 | 5.524 | 7/32 | 0.2188 | 5.556 | 67.4 |
| 75% (default) | 0.2152 | 5.466 | #3 | 0.2130 | 5.410 | 79.8 |
| 80% | 0.2129 | 5.408 | #3 | 0.2130 | 5.410 | 79.8 |
| 85% | 0.2106 | 5.349 | #4 | 0.2090 | 5.309 | 88.4 |

_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 1/4"-28 UNF

The through-hole a 1/4"-28 UNF fastener passes freely, not the tapped hole.

| Fit | Drill | Hole Ø (in) | Hole Ø (mm) | Use |
| --- | --- | --- | --- | --- |
| Close | H | 0.2677 | 6.800 | Located joints, dowelled assemblies |
| Normal | J | 0.2795 | 7.099 | General assembly — the default |
| Loose | M | 0.2953 | 7.501 | Slotted or adjustable joints, weldments |

_Clearance holes per ISO 273 (metric) and ASME B18.2.8 (inch)._

## Thread Class Limits for 1/4"-28 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 | 0.2490 | 6.325 | 0.2500 | 6.350 |
| Major Ø min | 0.2425 | 6.160 | 0.2435 | 6.185 |
| Pitch Ø max | 0.2258 | 5.735 | 0.2268 | 5.761 |
| Pitch Ø min | 0.2225 | 5.652 | 0.2243 | 5.697 |
| Pitch Ø tolerance | 0.0033 | 0.084 | 0.0025 | 0.064 |
| Minor Ø max | 0.2026 | 5.146 | 0.2036 | 5.171 |
| Minor Ø min | 0.1993 | 5.062 | 0.2011 | 5.108 |
| 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 | 0.2595 | 6.591 | 0.2584 | 6.563 |
| Major Ø min | 0.2500 | 6.350 | 0.2500 | 6.350 |
| Pitch Ø max | 0.2311 | 5.870 | 0.2300 | 5.842 |
| Pitch Ø min | 0.2268 | 5.761 | 0.2268 | 5.761 |
| Pitch Ø tolerance | 0.0043 | 0.109 | 0.0032 | 0.081 |
| Minor Ø max | 0.2198 | 5.583 | 0.2190 | 5.563 |
| Minor Ø min | 0.2113 | 5.367 | 0.2113 | 5.367 |
| Allowance | 0.0000 | 0.000 | 0.0000 | 0.000 |

## Tensile Stress Area of 1/4"-28 UNF

| Property | in² | mm² |
| --- | --- | --- |
| Tensile stress area | 0.03637 | 23.47 |

The effective cross-section used to size a 1/4"-28 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 1/4"-28 UNF

A 1/4 fastener is not always 1/4"-28 UNF. These are the other standard pitches cut on the same nominal size.

| Thread | Series | Pitch | Tap drill |
| --- | --- | --- | --- |
| [1/4"-32 UNEF](https://shopmath.org/threads/unef/1-4-32.md) | Unified Extra Fine (UNEF) | 32 TPI | 7/32 |
| [1/4"-20 UNC](https://shopmath.org/threads/unc/1-4-20.md) | Unified Coarse (UNC) | 20 TPI | #7 |

## Sources and Method for 1/4"-28 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 1/4"-28 UNF

### What is the tap drill size for 1/4"-28 UNF?

The tap drill for 1/4"-28 UNF is #3 — 0.2130 in (5.410 mm). That leaves 79.8% thread engagement with a cut tap.

### What is the pitch diameter of 1/4"-28 UNF?

The basic pitch diameter of 1/4"-28 UNF is 0.2268 in (5.761 mm). For a class 2B thread it runs 0.2268 in (5.761 mm) to 0.2311 in (5.870 mm).

### What is the minor diameter of 1/4"-28 UNF?

The basic minor diameter of 1/4"-28 UNF is 0.2113 in (5.367 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 1/4"-28 UNF?

1/4"-28 UNF has 28 threads per inch, a pitch of 0.0357 in (0.907 mm).

### What size clearance hole does a 1/4"-28 UNF fastener need?

close fit H (0.2677 in (6.800 mm)), normal fit J (0.2795 in (7.099 mm)), loose fit M (0.2953 in (7.501 mm)). The normal fit is the general-assembly default.

### What is the tensile stress area of 1/4"-28 UNF?

The tensile stress area of 1/4"-28 UNF is 0.03637 in² (23.47 mm²). Multiply it by the fastener material's allowable stress to get a load.

## Related pages

- Smaller in Unified Fine (UNF): [#12-28 UNF](https://shopmath.org/threads/unf/12-28.md) — tap drill #14
- Larger in Unified Fine (UNF): [5/16"-24 UNF](https://shopmath.org/threads/unf/5-16-24.md) — tap drill I
- 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/1-4-28) — data verified August 1, 2026. Reference data; verify against the governing standard before manufacture.*