---
type: "thread"
name: "9/16\"-18 UNF"
designation: "9/16-18 UNF"
series: "unf"
series_name: "Unified Fine (UNF)"
system: "unified"
size_label: "9/16"
tpi: 18
pitch_in: 0.055556
pitch_mm: 1.4111
basic_major_dia_in: 0.5625
basic_major_dia_mm: 14.2875
basic_pitch_dia_in: 0.5264
basic_pitch_dia_mm: 13.3706
basic_minor_dia_in: 0.5024
basic_minor_dia_mm: 12.761
tap_drill: "33/64"
tap_drill_dia_in: 0.515625
tap_drill_dia_mm: 13.0969
tap_drill_thread_pct: 65
tensile_stress_area_sqin: 0.202981
tensile_stress_area_sqmm: 130.955
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/9-16-18
---

# 9/16"-18 UNF Thread — Dimensions, Tap Drill & Clearance

The tap drill for 9/16"-18 UNF is 33/64 — 0.5156 in (13.097 mm) — which leaves 65.0% thread engagement with a cut tap. Basic pitch diameter is 0.5264 in (13.371 mm) and basic minor diameter is 0.5024 in (12.761 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**: 9/16-18 UNF
- **Threads per inch**: 18
- **Tap drill**: 33/64
- **Standard**: ASME B1.1 (reference values based on the ASME B1.1 system; not a compliance claim)

## Basic Thread Dimensions for 9/16"-18 UNF

| Property | Inch | Millimetre |
| --- | --- | --- |
| Basic major diameter | 0.5625 | 14.287 |
| Threads per inch | 18 | — |
| Pitch | 0.0556 | 1.411 |
| Basic pitch diameter | 0.5264 | 13.371 |
| Basic minor diameter | 0.5024 | 12.761 |
| External thread root diameter | 0.4963 | 12.606 |
| Sharp V thread height (H) | 0.0481 | 1.222 |

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

## Tap Drill Sizes for 9/16"-18 UNF

Cut taps, from 50% to 85% thread engagement. The shop default is 75% — a 33/64 drill.

| Thread % | Target hole Ø (in) | Target hole Ø (mm) | Drill | Drill Ø (in) | Drill Ø (mm) | Actual thread % |
| --- | --- | --- | --- | --- | --- | --- |
| 50% | 0.5264 | 13.371 | 17/32 | 0.5313 | 13.494 | 43.3 |
| 60% | 0.5192 | 13.188 | 33/64 | 0.5156 | 13.097 | 65.0 |
| 65% | 0.5156 | 13.096 | 33/64 | 0.5156 | 13.097 | 65.0 |
| 70% | 0.5120 | 13.005 | 33/64 | 0.5156 | 13.097 | 65.0 |
| 75% (default) | 0.5084 | 12.913 | 33/64 | 0.5156 | 13.097 | 65.0 |
| 80% | 0.5048 | 12.822 | 1/2 | 0.5000 | 12.700 | 86.6 |
| 85% | 0.5012 | 12.730 | 1/2 | 0.5000 | 12.700 | 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._

> **This differs from the traditional chart.** Computed 75% target 0.5084 in. Nearest standard drill is 33/64 (0.515625 in, off by 0.0073 in); the traditional published chart lists 1/2 (0.5 in, off by 0.0084 in, 86.6% thread). Both are shown; neither overrides the other. Computed recommendation: 33/64 (0.5156 in, 65.0% thread). Traditional published chart: 1/2 (0.5000 in, 86.6% thread).

## Thread Class Limits for 9/16"-18 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 | 0.5611 | 14.252 | 0.5625 | 14.287 |
| Major Ø min | 0.5524 | 14.031 | 0.5538 | 14.066 |
| Pitch Ø max | 0.5251 | 13.338 | 0.5264 | 13.371 |
| Pitch Ø min | 0.5205 | 13.221 | 0.5230 | 13.284 |
| Pitch Ø tolerance | 0.0045 | 0.114 | 0.0034 | 0.086 |
| Minor Ø max | 0.4890 | 12.421 | 0.4903 | 12.454 |
| Minor Ø min | 0.4844 | 12.304 | 0.4869 | 12.367 |
| Allowance | 0.0014 | 0.036 | 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 | 0.5764 | 14.641 | 0.5750 | 14.605 |
| Major Ø min | 0.5625 | 14.287 | 0.5625 | 14.287 |
| Pitch Ø max | 0.5323 | 13.520 | 0.5308 | 13.482 |
| Pitch Ø min | 0.5264 | 13.371 | 0.5264 | 13.371 |
| Pitch Ø tolerance | 0.0059 | 0.150 | 0.0044 | 0.112 |
| Minor Ø max | 0.5150 | 13.081 | 0.5106 | 12.969 |
| Minor Ø min | 0.5024 | 12.761 | 0.5024 | 12.761 |
| Allowance | 0.0000 | 0.000 | 0.0000 | 0.000 |

## Tensile Stress Area of 9/16"-18 UNF

| Property | in² | mm² |
| --- | --- | --- |
| Tensile stress area | 0.20298 | 130.96 |

The effective cross-section used to size a 9/16"-18 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 9/16"-18 UNF

A 9/16 fastener is not always 9/16"-18 UNF. These are the other standard pitches cut on the same nominal size.

| Thread | Series | Pitch | Tap drill |
| --- | --- | --- | --- |
| [9/16"-24 UNEF](https://shopmath.org/threads/unef/9-16-24.md) | Unified Extra Fine (UNEF) | 24 TPI | 33/64 |
| [9/16"-12 UNC](https://shopmath.org/threads/unc/9-16-12.md) | Unified Coarse (UNC) | 12 TPI | 31/64 |

## Sources and Method for 9/16"-18 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 9/16"-18 UNF

### What is the tap drill size for 9/16"-18 UNF?

The tap drill for 9/16"-18 UNF is 33/64 — 0.5156 in (13.097 mm). That leaves 65.0% thread engagement with a cut tap.

### What is the pitch diameter of 9/16"-18 UNF?

The basic pitch diameter of 9/16"-18 UNF is 0.5264 in (13.371 mm). For a class 2B thread it runs 0.5264 in (13.371 mm) to 0.5323 in (13.520 mm).

### What is the minor diameter of 9/16"-18 UNF?

The basic minor diameter of 9/16"-18 UNF is 0.5024 in (12.761 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 9/16"-18 UNF?

9/16"-18 UNF has 18 threads per inch, a pitch of 0.0556 in (1.411 mm).

### What is the tensile stress area of 9/16"-18 UNF?

The tensile stress area of 9/16"-18 UNF is 0.20298 in² (130.96 mm²). Multiply it by the fastener material's allowable stress to get a load.

### Why does the 9/16"-18 UNF tap drill here differ from the chart on the wall?

Computed 75% target 0.5084 in. Nearest standard drill is 33/64 (0.515625 in, off by 0.0073 in); the traditional published chart lists 1/2 (0.5 in, off by 0.0084 in, 86.6% thread). Both are shown; neither overrides the other.

## Related pages

- Smaller in Unified Fine (UNF): [1/2"-20 UNF](https://shopmath.org/threads/unf/1-2-20.md) — tap drill 29/64
- Larger in Unified Fine (UNF): [5/8"-18 UNF](https://shopmath.org/threads/unf/5-8-18.md) — tap drill 37/64
- 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/9-16-18) — data verified August 1, 2026. Reference data; verify against the governing standard before manufacture.*