---
type: "thread"
name: "5/16\"-18 UNC"
designation: "5/16-18 UNC"
series: "unc"
series_name: "Unified Coarse (UNC)"
system: "unified"
size_label: "5/16"
tpi: 18
pitch_in: 0.055556
pitch_mm: 1.4111
basic_major_dia_in: 0.3125
basic_major_dia_mm: 7.9375
basic_pitch_dia_in: 0.2764
basic_pitch_dia_mm: 7.0206
basic_minor_dia_in: 0.2524
basic_minor_dia_mm: 6.411
tap_drill: "F"
tap_drill_dia_in: 0.257
tap_drill_dia_mm: 6.5278
tap_drill_thread_pct: 76.9
clearance_normal_drill: "R"
tensile_stress_area_sqin: 0.05243
tensile_stress_area_sqmm: 33.826
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/5-16-18
---

# 5/16"-18 UNC Thread — Dimensions, Tap Drill & Clearance

The tap drill for 5/16"-18 UNC is F — 0.2570 in (6.528 mm) — which leaves 76.9% thread engagement with a cut tap. Basic pitch diameter is 0.2764 in (7.021 mm) and basic minor diameter is 0.2524 in (6.411 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**: 5/16-18 UNC
- **Threads per inch**: 18
- **Tap drill**: F
- **Standard**: ASME B1.1 (reference values based on the ASME B1.1 system; not a compliance claim)

## Basic Thread Dimensions for 5/16"-18 UNC

| Property | Inch | Millimetre |
| --- | --- | --- |
| Basic major diameter | 0.3125 | 7.938 |
| Threads per inch | 18 | — |
| Pitch | 0.0556 | 1.411 |
| Basic pitch diameter | 0.2764 | 7.021 |
| Basic minor diameter | 0.2524 | 6.411 |
| External thread root diameter | 0.2463 | 6.256 |
| 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 5/16"-18 UNC

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

| Thread % | Target hole Ø (in) | Target hole Ø (mm) | Drill | Drill Ø (in) | Drill Ø (mm) | Actual thread % |
| --- | --- | --- | --- | --- | --- | --- |
| 50% | 0.2764 | 7.021 | J | 0.2770 | 7.036 | 49.2 |
| 60% | 0.2692 | 6.838 | I | 0.2720 | 6.909 | 56.1 |
| 65% | 0.2656 | 6.746 | 17/64 | 0.2656 | 6.747 | 65.0 |
| 70% | 0.2620 | 6.655 | G | 0.2610 | 6.629 | 71.4 |
| 75% (default) | 0.2584 | 6.563 | F | 0.2570 | 6.528 | 76.9 |
| 80% | 0.2548 | 6.472 | F | 0.2570 | 6.528 | 76.9 |
| 85% | 0.2512 | 6.380 | E | 0.2500 | 6.350 | 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 5/16"-18 UNC

The through-hole a 5/16"-18 UNC fastener passes freely, not the tapped hole.

| Fit | Drill | Hole Ø (in) | Hole Ø (mm) | Use |
| --- | --- | --- | --- | --- |
| Close | P | 0.3268 | 8.301 | Located joints, dowelled assemblies |
| Normal | R | 0.3425 | 8.700 | General assembly — the default |
| Loose | T | 0.3583 | 9.101 | Slotted or adjustable joints, weldments |

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

## Thread Class Limits for 5/16"-18 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 | 0.3113 | 7.907 | 0.3125 | 7.938 |
| Major Ø min | 0.3026 | 7.686 | 0.3038 | 7.716 |
| Pitch Ø max | 0.2752 | 6.990 | 0.2764 | 7.021 |
| Pitch Ø min | 0.2712 | 6.888 | 0.2734 | 6.944 |
| Pitch Ø tolerance | 0.0040 | 0.102 | 0.0030 | 0.076 |
| Minor Ø max | 0.2391 | 6.073 | 0.2403 | 6.104 |
| Minor Ø min | 0.2351 | 5.971 | 0.2373 | 6.027 |
| Allowance | 0.0012 | 0.030 | 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 | 0.3258 | 8.275 | 0.3245 | 8.242 |
| Major Ø min | 0.3125 | 7.938 | 0.3125 | 7.938 |
| Pitch Ø max | 0.2817 | 7.155 | 0.2804 | 7.122 |
| Pitch Ø min | 0.2764 | 7.021 | 0.2764 | 7.021 |
| Pitch Ø tolerance | 0.0053 | 0.135 | 0.0039 | 0.099 |
| Minor Ø max | 0.2650 | 6.731 | 0.2630 | 6.680 |
| Minor Ø min | 0.2524 | 6.411 | 0.2524 | 6.411 |
| Allowance | 0.0000 | 0.000 | 0.0000 | 0.000 |

## Tensile Stress Area of 5/16"-18 UNC

| Property | in² | mm² |
| --- | --- | --- |
| Tensile stress area | 0.05243 | 33.83 |

The effective cross-section used to size a 5/16"-18 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 5/16"-18 UNC

A 5/16 fastener is not always 5/16"-18 UNC. These are the other standard pitches cut on the same nominal size.

| Thread | Series | Pitch | Tap drill |
| --- | --- | --- | --- |
| [5/16"-32 UNEF](https://shopmath.org/threads/unef/5-16-32.md) | Unified Extra Fine (UNEF) | 32 TPI | 9/32 |
| [5/16"-24 UNF](https://shopmath.org/threads/unf/5-16-24.md) | Unified Fine (UNF) | 24 TPI | I |

## Sources and Method for 5/16"-18 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 5/16"-18 UNC

### What is the tap drill size for 5/16"-18 UNC?

The tap drill for 5/16"-18 UNC is F — 0.2570 in (6.528 mm). That leaves 76.9% thread engagement with a cut tap.

### What is the pitch diameter of 5/16"-18 UNC?

The basic pitch diameter of 5/16"-18 UNC is 0.2764 in (7.021 mm). For a class 2B thread it runs 0.2764 in (7.021 mm) to 0.2817 in (7.155 mm).

### What is the minor diameter of 5/16"-18 UNC?

The basic minor diameter of 5/16"-18 UNC is 0.2524 in (6.411 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 5/16"-18 UNC?

5/16"-18 UNC has 18 threads per inch, a pitch of 0.0556 in (1.411 mm).

### What size clearance hole does a 5/16"-18 UNC fastener need?

close fit P (0.3268 in (8.301 mm)), normal fit R (0.3425 in (8.700 mm)), loose fit T (0.3583 in (9.101 mm)). The normal fit is the general-assembly default.

### What is the tensile stress area of 5/16"-18 UNC?

The tensile stress area of 5/16"-18 UNC is 0.05243 in² (33.83 mm²). Multiply it by the fastener material's allowable stress to get a load.

## Related pages

- Smaller in Unified Coarse (UNC): [1/4"-20 UNC](https://shopmath.org/threads/unc/1-4-20.md) — tap drill #7
- Larger in Unified Coarse (UNC): [3/8"-16 UNC](https://shopmath.org/threads/unc/3-8-16.md) — tap drill 5/16
- 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/5-16-18) — data verified August 1, 2026. Reference data; verify against the governing standard before manufacture.*