Fastener Dimensions and Thread Systems
Fasteners are produced in a wide range of diameters, lengths and thread forms. At the most basic level there are two main families of products: metric and imperial. Metric fasteners use millimetres for all dimensions, while imperial fasteners are based on inch‑derived thread systems such as Whitworth, UNC, UNF, BSF and BA.
Detailed dimensional data for individual products is provided on the dedicated Metric Dimensions and Imperial Dimensions pages. This section explains the general rules used to describe fastener size and introduces the main thread systems.
How Fastener Length Is Measured
For most headed fasteners (hex bolts, hex sets, socket caps, pan head screws, etc.), the length is measured from under the head to the end of the threaded shank. For headless products such as studs or allthread, the length is the overall length of the piece.
The general way of describing a fastener size is:
Diameter × Length
For example: M8 × 40 (metric) or 3/8" × 1 1/2" (imperial).
Metric and Imperial Products
Metric fasteners use the ISO metric thread system and are specified in millimetres. A typical designation is M10 × 50, where M10 is the nominal diameter and 50 is the length in millimetres. Thread pitch may be implicit (coarse) or stated explicitly for fine threads (for example M10 × 1.25).
Imperial fasteners use several different thread systems, each with its own history and conventions. Common examples include BSW (Whitworth), BSF, UNC, UNF and BA. Diameters are expressed in inches or fractional inches, and thread pitch is usually given in threads per inch (TPI).
The BA thread system uses a numbered size sequence that is counter‑intuitive compared with metric or other imperial threads. In BA, a larger number means a smaller screw. For example, 6 BA is smaller than 4 BA, and 8 BA is smaller again. The system is based on a geometric progression rather than a direct diameter measurement, which is why the numbering does not follow physical size in the way most users expect.
The development of the ISO metric thread is covered on the Metric Thread History page, while the evolution of Whitworth and other imperial threads is described on the Imperial Thread History page.
Principal Thread Systems
Although many thread forms exist, most engineering fasteners fall into a small number of standard systems. The table below summarises the key characteristics of the main thread families used on bolts, screws and studs.
| Thread System | Typical Designation | Units | Thread Angle | Pitch Expression | Typical Applications |
|---|---|---|---|---|---|
| ISO Metric | M10 × 1.5 | Millimetres | 60° | Pitch in mm (e.g. 1.5) | Modern machinery, construction, automotive, general engineering |
| UNC (Unified Coarse) | 3/8"–16 UNC | Inches | 60° | Threads per inch (TPI) | General engineering, structural, legacy equipment (primarily US‑origin) |
| UNF (Unified Fine) | 3/8"–24 UNF | Inches | 60° | Threads per inch (TPI) | Automotive, precision assemblies, where finer adjustment is required |
| BSW (Whitworth) | 3/8" BSW | Inches | 55° | Threads per inch (TPI) | Older British machinery, structural steelwork, restoration work |
| BSF (British Standard Fine) | 3/8" BSF | Inches | 55° | Threads per inch (TPI) | Older British automotive and engineering applications |
| BA (British Association) | 4 BA | Numbered series (geometric progression) | 47.5° | Numbered sizes (0 BA, 1 BA, etc.) | Small screws for instruments, electrical equipment and light mechanical assemblies. Note: BA sizing is counter‑intuitive — larger numbers indicate smaller screws. |
Where to Find Detailed Dimensions
This page provides a high‑level overview of how fastener sizes are described. For full dimensional data on metric parts — including head sizes, across‑flats dimensions, recesses and tolerances — please refer to: