---
type: "thread"
name: "1/4\"-20 UNC"
designation: "1/4-20 UNC"
series: "unc"
series_name: "Unified Coarse (UNC)"
system: "unified"
size_label: "1/4"
tpi: 20
pitch_in: 0.05
pitch_mm: 1.27
basic_major_dia_in: 0.25
basic_major_dia_mm: 6.35
basic_pitch_dia_in: 0.2175
basic_pitch_dia_mm: 5.5245
basic_minor_dia_in: 0.1959
basic_minor_dia_mm: 4.9759
tap_drill: "#7"
tap_drill_dia_in: 0.201
tap_drill_dia_mm: 5.1054
tap_drill_thread_pct: 75.4
clearance_normal_drill: "J"
tensile_stress_area_sqin: 0.031821
tensile_stress_area_sqmm: 20.53
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/1-4-20
---

# 1/4"-20 UNC Thread — Dimensions, Tap Drill & Clearance

The tap drill for 1/4"-20 UNC is #7 — 0.2010 in (5.105 mm) — which leaves 75.4% thread engagement with a cut tap. Basic pitch diameter is 0.2175 in (5.524 mm) and basic minor diameter is 0.1959 in (4.976 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**: 1/4-20 UNC
- **Threads per inch**: 20
- **Tap drill**: #7
- **Standard**: ASME B1.1 (reference values based on the ASME B1.1 system; not a compliance claim)

## Basic Thread Dimensions for 1/4"-20 UNC

| Property | Inch | Millimetre |
| --- | --- | --- |
| Basic major diameter | 0.2500 | 6.350 |
| Threads per inch | 20 | — |
| Pitch | 0.0500 | 1.270 |
| Basic pitch diameter | 0.2175 | 5.524 |
| Basic minor diameter | 0.1959 | 4.976 |
| External thread root diameter | 0.1905 | 4.839 |
| Sharp V thread height (H) | 0.0433 | 1.100 |

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

## Tap Drill Sizes for 1/4"-20 UNC

Cut taps, from 50% to 85% thread engagement. The shop default is 75% — a #7 drill.

| Thread % | Target hole Ø (in) | Target hole Ø (mm) | Drill | Drill Ø (in) | Drill Ø (mm) | Actual thread % |
| --- | --- | --- | --- | --- | --- | --- |
| 50% | 0.2175 | 5.524 | 7/32 | 0.2188 | 5.556 | 48.1 |
| 60% | 0.2110 | 5.359 | #3 | 0.2130 | 5.410 | 57.0 |
| 65% | 0.2078 | 5.278 | #4 | 0.2090 | 5.309 | 63.1 |
| 70% | 0.2045 | 5.194 | #6 | 0.2040 | 5.182 | 70.8 |
| 75% (default) | 0.2013 | 5.113 | #7 | 0.2010 | 5.105 | 75.4 |
| 80% | 0.1980 | 5.029 | #8 | 0.1990 | 5.055 | 78.5 |
| 85% | 0.1948 | 4.948 | #9 | 0.1960 | 4.978 | 83.1 |

_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"-20 UNC

The through-hole a 1/4"-20 UNC 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"-20 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 | 0.2489 | 6.322 | 0.2500 | 6.350 |
| Major Ø min | 0.2407 | 6.114 | 0.2419 | 6.144 |
| Pitch Ø max | 0.2164 | 5.497 | 0.2175 | 5.524 |
| Pitch Ø min | 0.2127 | 5.403 | 0.2147 | 5.453 |
| Pitch Ø tolerance | 0.0037 | 0.094 | 0.0028 | 0.071 |
| Minor Ø max | 0.1839 | 4.671 | 0.1850 | 4.699 |
| Minor Ø min | 0.1802 | 4.577 | 0.1823 | 4.630 |
| 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 | 0.2621 | 6.657 | 0.2609 | 6.627 |
| Major Ø min | 0.2500 | 6.350 | 0.2500 | 6.350 |
| Pitch Ø max | 0.2224 | 5.649 | 0.2212 | 5.619 |
| Pitch Ø min | 0.2175 | 5.524 | 0.2175 | 5.524 |
| Pitch Ø tolerance | 0.0048 | 0.122 | 0.0036 | 0.091 |
| Minor Ø max | 0.2074 | 5.268 | 0.2067 | 5.250 |
| Minor Ø min | 0.1959 | 4.976 | 0.1959 | 4.976 |
| Allowance | 0.0000 | 0.000 | 0.0000 | 0.000 |

## Tensile Stress Area of 1/4"-20 UNC

| Property | in² | mm² |
| --- | --- | --- |
| Tensile stress area | 0.03182 | 20.53 |

The effective cross-section used to size a 1/4"-20 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 1/4"-20 UNC

A 1/4 fastener is not always 1/4"-20 UNC. 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"-28 UNF](https://shopmath.org/threads/unf/1-4-28.md) | Unified Fine (UNF) | 28 TPI | #3 |

## Sources and Method for 1/4"-20 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 1/4"-20 UNC

### What is the tap drill size for 1/4"-20 UNC?

The tap drill for 1/4"-20 UNC is #7 — 0.2010 in (5.105 mm). That leaves 75.4% thread engagement with a cut tap.

### What is the pitch diameter of 1/4"-20 UNC?

The basic pitch diameter of 1/4"-20 UNC is 0.2175 in (5.524 mm). For a class 2B thread it runs 0.2175 in (5.524 mm) to 0.2224 in (5.649 mm).

### What is the minor diameter of 1/4"-20 UNC?

The basic minor diameter of 1/4"-20 UNC is 0.1959 in (4.976 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"-20 UNC?

1/4"-20 UNC has 20 threads per inch, a pitch of 0.0500 in (1.270 mm).

### What size clearance hole does a 1/4"-20 UNC 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"-20 UNC?

The tensile stress area of 1/4"-20 UNC is 0.03182 in² (20.53 mm²). Multiply it by the fastener material's allowable stress to get a load.

## Related pages

- Smaller in Unified Coarse (UNC): [#12-24 UNC](https://shopmath.org/threads/unc/12-24.md) — tap drill #16
- Larger in Unified Coarse (UNC): [5/16"-18 UNC](https://shopmath.org/threads/unc/5-16-18.md) — tap drill F
- 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/1-4-20) — data verified August 1, 2026. Reference data; verify against the governing standard before manufacture.*