Imperial Thread Size Chart: BSW, BSF, UNC and UNF
A workshop lookup chart for selected fractional bolt sizes, nominal metric diameters and BSW, BSF, UNC and UNF pitches, with a printable A4 PDF.
Look up threads per inch (TPI) for selected fractional nominal bolt diameters in the BSW, BSF, UNC and UNF series. Use the chart with the specified thread form; diameter and pitch alone do not establish a correct replacement.
Download the printable thread chart (PDF, A4, 1 page)
Print at actual size / 100% on A4. The reference uses readable text and a white background for economical workshop printing. Download size: 496 KB.
How to read this chart
The diameter is the nominal bolt or screw thread diameter in inches, not the bolt head, spanner opening or tapping-drill diameter. For a male thread, measure across its crests to help identify the nominal size. Actual manufactured or worn threads can measure below nominal.
TPI means threads per inch: a larger TPI value means a finer pitch. Where needed, pitch in millimetres can be calculated as 25.4 divided by TPI. The diameter conversions below use exactly 25.4 mm per inch, displayed to 0.001 mm with half-up rounding. That display precision is not a manufacturing tolerance.
A dash (-) means the combination is not included in this source-backed selection. It does not establish that no such standard or special thread exists. This is a selection of fractional sizes from 1/8 to 1 1/2 inches, not the complete size schedules. Numbered Unified screws, BA, special pitches and pipe threads are outside its scope.
The four series and their thread forms
- BSW: British Standard Whitworth, coarse series, 55-degree Whitworth form.
- BSF: British Standard Fine, fine series, 55-degree Whitworth form.
- UNC: Unified National Coarse, 60-degree Unified form.
- UNF: Unified National Fine, 60-degree Unified form.
The angle is the included angle between the thread flanks. Whitworth and Unified forms have different profiles. Equal nominal diameter and TPI do not make BSW/BSF and UNC/UNF interchangeable. A mismatched nut beginning to engage is not proof of a correct fit.
Fractional imperial thread lookup
| Nominal bolt diameter (in) | Nominal diameter (mm) | BSW TPI (55°) | BSF TPI (55°) | UNC TPI (60°) | UNF TPI (60°) |
|---|---|---|---|---|---|
| 1/8 | 3.175 | 40 | – | – | – |
| 3/16 | 4.763 | 24 | 32 | – | – |
| 7/32 | 5.556 | – | 28 | – | – |
| 1/4 | 6.350 | 20 | 26 | 20 | 28 |
| 9/32 | 7.144 | – | 26 | – | – |
| 5/16 | 7.938 | 18 | 22 | 18 | 24 |
| 3/8 | 9.525 | 16 | 20 | 16 | 24 |
| 7/16 | 11.113 | 14 | 18 | 14 | 20 |
| 1/2 | 12.700 | 12 | 16 | 13 | 20 |
| 9/16 | 14.288 | 12 | 16 | 12 | 18 |
| 5/8 | 15.875 | 11 | 14 | 11 | 18 |
| 11/16 | 17.463 | 11 | 14 | – | – |
| 3/4 | 19.050 | 10 | 12 | 10 | 16 |
| 7/8 | 22.225 | 9 | 11 | 9 | 14 |
| 1 | 25.400 | 8 | 10 | 8 | 12 |
| 1 1/8 | 28.575 | 7 | 9 | 7 | 12 |
| 1 1/4 | 31.750 | 7 | 9 | 7 | 12 |
| 1 3/8 | 34.925 | – | 8 | 6 | 12 |
| 1 1/2 | 38.100 | 6 | 8 | 6 | 12 |
Useful comparisons and limits
At 1/4 inch, BSW and UNC are both 20 TPI; BSF is 26 TPI and UNF is 28 TPI. The matching coarse pitches still use different thread forms. At 1/2 inch, the difference is visible in the pitch too: BSW is 12 TPI, UNC is 13 TPI, BSF is 16 TPI and UNF is 20 TPI.
Record the series as well as size and pitch, for example 1/2-inch BSW, 12 TPI. Confirm the drawing or original specification, thread form, condition and required fit before choosing a replacement. This chart does not provide tolerance classes, strength grades, tapping drills or a guarantee of compatibility.
Keep pipe threads separate. BSP and NPT designations use nominal pipe sizing; a fractional pipe designation is not that fraction of an inch measured across the thread. Do not identify a pipe thread using this bolt chart.
For the measurement process, read our existing guide to identifying BSW, BSF and UNC threads. For catalogue context, see our zinc-plated steel BSW Whitworth hex nuts, sold individually in their listed sizes. Check the product’s specification before ordering; chart coverage does not imply stock availability.
Sources and review
Data checked against manufacturer references on 14 October 2026. Revision 1.0. Each displayed TPI value was cross-checked between the BAER designation tables and Goliath’s independent catalogue chart. Metric conversions were calculated separately. The PDF and this HTML table share one source dataset.
- BAER BaerCoil BSW designation table – only thread designations are used, not insert installation dimensions.
- BAER BaerCoil BSF designation table – only thread designations are used, not insert installation dimensions.
- BAER BaerCoil UNC designation table – only thread designations are used, not insert installation dimensions.
- BAER BaerCoil UNF designation table – only thread designations are used, not insert installation dimensions.
- Goliath tooling catalogue, printed page 58 (PDF page 59) – independent pitch cross-check. Its tapping-drill recommendations are not reproduced here.
- GSR thread-cutting handbook, page 9 (PDF page 5) – thread-form angles for these four series.
- BSI: BS 84:2007, parallel screw threads of Whitworth form; ASME: B1.1, Unified inch screw threads – standard identification and scope. Full paid standard tables were not accessed for this reference.