Whitworth, BSF and AF Spanner Size Chart
Selected Whitworth and paired BS tool markings with across-flats dimensions in inches and millimetres. Includes a one-page printable A4 workshop reference.
Identify selected Whitworth tool markings, their paired BS markings and the corresponding nominal hexagon dimensions across flats. The chart covers 12 selected W sizes from 3/16 W to 1 W. It is a tool-marking reference, not a complete bolt-head standard.
Download the spanner size sheet (PDF, A4, 1 page)
Print at actual size / 100% on A4. Download size: 495 KiB.
How W, BSF and AF markings differ
W and BS/BSF markings refer to nominal bolt sizes under different head conventions. They do not directly state the width of the hexagon. The smaller-head BS/BSF convention gives a larger nominal bolt-size marking for the same tool size: for example, 1/4 W pairs with 5/16 BS, corresponding to 0.525 inch across flats. Later reduced-head BSW fasteners can follow the smaller-head convention too. Do not select a spanner from the bolt thread diameter alone.
AF means across flats: the distance between two opposite flat sides of a hexagonal nut or bolt head, not corner to corner. An inch AF tool is labelled by that dimension. A 1/4 AF tool denotes 0.250 inch AF; it is a different size from 1/4 W. Metric tools use the same across-flats idea with millimetre markings.
Whitworth and paired BS marking chart
Read the stamp on the tool, then compare the actual fastener with the nominal hexagon dimension. These are hexagon AF reference dimensions, not exact measured spanner jaw openings. Manufacturing clearance and tolerance, wear and head condition affect the fit. The table retains the sources’ BS label; BSF-style markings are explained above.
| W tool marking | Paired BS marking | Hexagon AF (in) | Hexagon AF (mm) |
|---|---|---|---|
| 3/16 W | 1/4 BS | 0.445 | 11.30 |
| 1/4 W | 5/16 BS | 0.525 | 13.34 |
| 5/16 W | 3/8 BS | 0.600 | 15.24 |
| 3/8 W | 7/16 BS | 0.710 | 18.03 |
| 7/16 W | 1/2 BS | 0.820 | 20.83 |
| 1/2 W | 9/16 BS | 0.920 | 23.37 |
| 9/16 W | 5/8 BS | 1.010 | 25.65 |
| 5/8 W | (11/16 BS)* | 1.100 | 27.94 |
| 11/16 W | 3/4 BS | 1.200 | 30.48 |
| 3/4 W | 7/8 BS | 1.300 | 33.02 |
| 7/8 W | 1 BS | 1.480 | 37.59 |
| 1 W | 1 1/8 BS | 1.670 | 42.42 |
* Parenthesised pairing: Samstag shows (11/16) in its BS column opposite 5/8 W; Konnect’s selected chart omits that pairing. Confirm the marking with the tool maker. Melform independently lists the 5/8 W hexagon AF dimension as 1.100 inch.
Metric values are calculated using exactly 25.4 mm per inch, rounded half up to 0.01 mm. For example, 0.525 inch is exactly 13.335 mm and displays as 13.34 mm here. This display rounding is not a dimensional tolerance.
Use the chart in the workshop
- Read the complete tool marking, including W, BS/BSF, AF or mm. The fraction alone is insufficient.
- Measure across opposite flats of the fastener and check its specified head convention. Older assemblies and replacement fasteners may have different head sizes.
- Check the tool maker’s specification and the actual fit before applying force. A rounded or worn head may not match its nominal size.
A nearby metric or inch AF size is not an interchangeable tool recommendation. The converted mm column describes the same nominal dimension; it does not name a metric spanner to use instead. This guide supplies no jaw-clearance limits, torque values or tool-strength ratings.
Spanner size also does not identify a thread series or pitch. For thread identification, see the imperial thread size chart.
Scope, sources and review
Data checked 17 October 2026. Revision 1.0. This selection excludes BA, pipe fittings, special heads and sizes outside the displayed rows. The 1/8 W entry is deliberately omitted: Melform gives 0.338 inch while Samstag gives 0.340 inch, and that difference remains unresolved. Absence from this selected chart does not mean a size never existed.
Melform’s own TW range supplies the inch hexagon dimensions and labels its sizes BS 2558 Grade B. Every displayed W/inch pair was checked against that range and Samstag’s separate table. Konnect corroborates 11 paired-marking/inch rows; its chart omits the parenthesised 5/8 W pairing. Millimetre conversions were recalculated independently rather than copied. No complete paid standard or spanner tolerance schedule was inspected, and this is not a claim of Workshop Standard machining or fit testing.
- Melform Engineers Tools: double-ended Whitworth box spanners, TW range, across-hexagon-flats inch column.
- Konnect Fastening Systems Conversion Charts: printed page 3, BS (Whitworth) BS/Whit/inch columns. Its nearest-fitting columns are not reproduced or recommended here.
- Samstag Whitworth Size and Number Reference: paired markings and inch values, including the parenthesised entry.
- NIST SI Units – Length: exact inch-to-millimetre basis.
- JRC Engineering: Whitworth and other British Threads: explanation of dual markings and reduced head sizes.
The original HTML chart and printable sheet are generated from one checked dataset. Supplier chart artwork is not reproduced.