---
type: "thread"
name: "1/2\"-20 UNF"
designation: "1/2-20 UNF"
series: "unf"
series_name: "Unified Fine (UNF)"
system: "unified"
size_label: "1/2"
tpi: 20
pitch_in: 0.05
pitch_mm: 1.27
basic_major_dia_in: 0.5
basic_major_dia_mm: 12.7
basic_pitch_dia_in: 0.4675
basic_pitch_dia_mm: 11.8745
basic_minor_dia_in: 0.4459
basic_minor_dia_mm: 11.3259
tap_drill: "29/64"
tap_drill_dia_in: 0.453125
tap_drill_dia_mm: 11.5094
tap_drill_thread_pct: 72.2
clearance_normal_drill: "9/16"
tensile_stress_area_sqin: 0.159953
tensile_stress_area_sqmm: 103.195
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/1-2-20
---

# 1/2"-20 UNF Thread — Dimensions, Tap Drill & Clearance

The tap drill for 1/2"-20 UNF is 29/64 — 0.4531 in (11.509 mm) — which leaves 72.2% thread engagement with a cut tap. Basic pitch diameter is 0.4675 in (11.874 mm) and basic minor diameter is 0.4459 in (11.326 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**: 1/2-20 UNF
- **Threads per inch**: 20
- **Tap drill**: 29/64
- **Standard**: ASME B1.1 (reference values based on the ASME B1.1 system; not a compliance claim)

## Basic Thread Dimensions for 1/2"-20 UNF

| Property | Inch | Millimetre |
| --- | --- | --- |
| Basic major diameter | 0.5000 | 12.700 |
| Threads per inch | 20 | — |
| Pitch | 0.0500 | 1.270 |
| Basic pitch diameter | 0.4675 | 11.874 |
| Basic minor diameter | 0.4459 | 11.326 |
| External thread root diameter | 0.4405 | 11.189 |
| 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 1/2"-20 UNF

Cut taps, from 50% to 85% thread engagement. The shop default is 75% — a 29/64 drill.

| Thread % | Target hole Ø (in) | Target hole Ø (mm) | Drill | Drill Ø (in) | Drill Ø (mm) | Actual thread % |
| --- | --- | --- | --- | --- | --- | --- |
| 50% | 0.4675 | 11.874 | 15/32 | 0.4688 | 11.906 | 48.1 |
| 60% | 0.4610 | 11.709 | 15/32 | 0.4688 | 11.906 | 48.1 |
| 65% | 0.4578 | 11.628 | 29/64 | 0.4531 | 11.509 | 72.2 |
| 70% | 0.4545 | 11.544 | 29/64 | 0.4531 | 11.509 | 72.2 |
| 75% (default) | 0.4513 | 11.463 | 29/64 | 0.4531 | 11.509 | 72.2 |
| 80% | 0.4480 | 11.379 | 29/64 | 0.4531 | 11.509 | 72.2 |
| 85% | 0.4448 | 11.298 | 7/16 | 0.4375 | 11.113 | 96.2 |

_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 1/2"-20 UNF

The through-hole a 1/2"-20 UNF fastener passes freely, not the tapped hole.

| Fit | Drill | Hole Ø (in) | Hole Ø (mm) | Use |
| --- | --- | --- | --- | --- |
| Close | 17/32 | 0.5315 | 13.500 | Located joints, dowelled assemblies |
| Normal | 9/16 | 0.5630 | 14.300 | General assembly — the default |
| Loose | 39/64 | 0.6102 | 15.499 | Slotted or adjustable joints, weldments |

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

## Thread Class Limits for 1/2"-20 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 | 0.4987 | 12.667 | 0.5000 | 12.700 |
| Major Ø min | 0.4906 | 12.461 | 0.4919 | 12.494 |
| Pitch Ø max | 0.4662 | 11.841 | 0.4675 | 11.874 |
| Pitch Ø min | 0.4620 | 11.735 | 0.4643 | 11.793 |
| Pitch Ø tolerance | 0.0043 | 0.109 | 0.0032 | 0.081 |
| Minor Ø max | 0.4338 | 11.018 | 0.4350 | 11.049 |
| Minor Ø min | 0.4295 | 10.909 | 0.4318 | 10.968 |
| Allowance | 0.0013 | 0.033 | 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 | 0.5128 | 13.025 | 0.5114 | 12.990 |
| Major Ø min | 0.5000 | 12.700 | 0.5000 | 12.700 |
| Pitch Ø max | 0.4731 | 12.017 | 0.4717 | 11.981 |
| Pitch Ø min | 0.4675 | 11.874 | 0.4675 | 11.874 |
| Pitch Ø tolerance | 0.0056 | 0.142 | 0.0042 | 0.107 |
| Minor Ø max | 0.4574 | 11.618 | 0.4537 | 11.524 |
| Minor Ø min | 0.4459 | 11.326 | 0.4459 | 11.326 |
| Allowance | 0.0000 | 0.000 | 0.0000 | 0.000 |

## Tensile Stress Area of 1/2"-20 UNF

| Property | in² | mm² |
| --- | --- | --- |
| Tensile stress area | 0.15995 | 103.19 |

The effective cross-section used to size a 1/2"-20 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 1/2"-20 UNF

A 1/2 fastener is not always 1/2"-20 UNF. These are the other standard pitches cut on the same nominal size.

| Thread | Series | Pitch | Tap drill |
| --- | --- | --- | --- |
| [1/2"-28 UNEF](https://shopmath.org/threads/unef/1-2-28.md) | Unified Extra Fine (UNEF) | 28 TPI | 15/32 |
| [1/2"-13 UNC](https://shopmath.org/threads/unc/1-2-13.md) | Unified Coarse (UNC) | 13 TPI | 27/64 |

## Sources and Method for 1/2"-20 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 1/2"-20 UNF

### What is the tap drill size for 1/2"-20 UNF?

The tap drill for 1/2"-20 UNF is 29/64 — 0.4531 in (11.509 mm). That leaves 72.2% thread engagement with a cut tap.

### What is the pitch diameter of 1/2"-20 UNF?

The basic pitch diameter of 1/2"-20 UNF is 0.4675 in (11.874 mm). For a class 2B thread it runs 0.4675 in (11.874 mm) to 0.4731 in (12.017 mm).

### What is the minor diameter of 1/2"-20 UNF?

The basic minor diameter of 1/2"-20 UNF is 0.4459 in (11.326 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 1/2"-20 UNF?

1/2"-20 UNF has 20 threads per inch, a pitch of 0.0500 in (1.270 mm).

### What size clearance hole does a 1/2"-20 UNF fastener need?

close fit 17/32 (0.5315 in (13.500 mm)), normal fit 9/16 (0.5630 in (14.300 mm)), loose fit 39/64 (0.6102 in (15.499 mm)). The normal fit is the general-assembly default.

### What is the tensile stress area of 1/2"-20 UNF?

The tensile stress area of 1/2"-20 UNF is 0.15995 in² (103.19 mm²). Multiply it by the fastener material's allowable stress to get a load.

## Related pages

- Smaller in Unified Fine (UNF): [7/16"-20 UNF](https://shopmath.org/threads/unf/7-16-20.md) — tap drill 25/64
- Larger in Unified Fine (UNF): [9/16"-18 UNF](https://shopmath.org/threads/unf/9-16-18.md) — tap drill 33/64
- 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/1-2-20) — data verified August 1, 2026. Reference data; verify against the governing standard before manufacture.*