Thread Types Comparison Chart
This chart compares the six thread systems most often seen on UK engineering drawings: metric coarse, Unified coarse and fine (UNC/UNF), British Standard Whitworth (BSW), British Standard Fine (BSF) and British Association (BA). Sizes are listed in order of nominal diameter, so if you have an unidentified fastener you can measure its outside diameter, find the nearest rows, and use the pitch to tell the systems apart. If your drawing calls for an imperial or legacy thread, send us the requirement and we can quote from the correct standard.
| Ø mm | Metric (coarse) | UNC | UNF | BSW | BSF | BA |
|---|---|---|---|---|---|---|
| 1.00 | M1 0.25mm | 14BA 0.23mm | ||||
| 1.10 | M1.1 0.25mm | |||||
| 1.20 | M1.2 0.25mm | |||||
| 1.30 | 12BA 0.28mm | |||||
| 1.40 | M1.4 0.30mm | |||||
| 1.59 | 1/16 60 tpi | |||||
| 1.60 | M1.6 0.35mm | |||||
| 1.70 | 10BA 0.35mm | |||||
| 1.80 | M1.8 0.35mm | |||||
| 2.00 | M2 0.40mm | |||||
| 2.18 | No.2 56 tpi | No.2 64 tpi | ||||
| 2.20 | 8BA 0.43mm | |||||
| 2.38 | 3/32 48 tpi | |||||
| 2.50 | M2.5 0.45mm | |||||
| 2.80 | 6BA 0.53mm | |||||
| 2.84 | No.4 40 tpi | No.4 48 tpi | ||||
| 3.00 | M3 0.50mm | |||||
| 3.18 | 1/8 40 tpi | |||||
| 3.20 | 5BA 0.59mm | |||||
| 3.50 | M3.5 0.60mm | |||||
| 3.51 | No.6 32 tpi | No.6 40 tpi | ||||
| 3.60 | 4BA 0.66mm | |||||
| 3.97 | 5/32 32 tpi | |||||
| 4.00 | M4 0.70mm | |||||
| 4.10 | 3BA 0.73mm | |||||
| 4.17 | No.8 32 tpi | No.8 36 tpi | ||||
| 4.50 | M4.5 0.75mm | |||||
| 4.70 | 2BA 0.81mm | |||||
| 4.76 | 3/16 24 tpi | 3/16 32 tpi | ||||
| 4.83 | No.10 24 tpi | No.10 32 tpi | ||||
| 5.00 | M5 0.80mm | |||||
| 5.30 | 1BA 0.90mm | |||||
| 5.49 | No.12 24 tpi | No.12 28 tpi | ||||
| 5.56 | 7/32 24 tpi | 7/32 28 tpi | ||||
| 6.00 | M6 1.00mm | 0BA 1.00mm | ||||
| 6.35 | 1/4 20 tpi | 1/4 28 tpi | 1/4 20 tpi | 1/4 26 tpi | ||
| 7.00 | M7 1.00mm | |||||
| 7.94 | 5/16 18 tpi | 5/16 24 tpi | 5/16 18 tpi | 5/16 22 tpi | ||
| 8.00 | M8 1.25mm | |||||
| 9.53 | 3/8 16 tpi | 3/8 24 tpi | 3/8 16 tpi | 3/8 20 tpi | ||
| 10.00 | M10 1.50mm | |||||
| 11.00 | M11 1.50mm | |||||
| 11.11 | 7/16 14 tpi | 7/16 20 tpi | 7/16 14 tpi | 7/16 18 tpi | ||
| 12.00 | M12 1.75mm | |||||
| 12.70 | 1/2 13 tpi | 1/2 20 tpi | 1/2 12 tpi | 1/2 16 tpi | ||
| 14.00 | M14 2.00mm | |||||
| 14.29 | 9/16 12 tpi | 9/16 18 tpi | 9/16 12 tpi | 9/16 16 tpi | ||
| 15.88 | 5/8 11 tpi | 5/8 18 tpi | 5/8 11 tpi | 5/8 14 tpi | ||
| 16.00 | M16 2.00mm | |||||
| 17.46 | 11/16 11 tpi | 11/16 14 tpi | ||||
| 18.00 | M18 2.50mm | |||||
| 19.05 | 3/4 10 tpi | 3/4 16 tpi | 3/4 10 tpi | 3/4 12 tpi | ||
| 20.00 | M20 2.50mm | |||||
| 22.00 | M22 2.50mm | |||||
| 22.23 | 7/8 9 tpi | 7/8 14 tpi | 7/8 9 tpi | 7/8 11 tpi | ||
| 24.00 | M24 3.00mm | |||||
| 25.40 | 1 8 tpi | 1 12 tpi | 1 8 tpi | 1 10 tpi | ||
| 27.00 | M27 3.00mm | |||||
| 28.58 | 1.1/8 7 tpi | 1.1/8 12 tpi | 1.1/8 7 tpi | 1.1/8 9 tpi | ||
| 30.00 | M30 3.50mm | |||||
| 31.75 | 1.1/4 7 tpi | 1.1/4 12 tpi | 1.1/4 7 tpi | 1.1/4 9 tpi | ||
| 33.00 | M33 3.50mm | |||||
| 34.93 | 1.3/8 6 tpi | 1.3/8 12 tpi | 1.3/8 6 tpi | 1.3/8 8 tpi | ||
| 36.00 | M36 4.00mm | |||||
| 38.10 | 1.1/2 6 tpi | 1.1/2 12 tpi | 1.1/2 6 tpi | 1.1/2 8 tpi | ||
| 39.00 | M39 4.00mm | |||||
| 41.28 | 1.5/8 5 tpi | 1.5/8 8 tpi | ||||
| 42.00 | M42 4.50mm | |||||
| 44.45 | 1.3/4 5 tpi | 1.3/4 5 tpi | 1.3/4 7 tpi | |||
| 45.00 | M45 4.50mm | |||||
| 48.00 | M48 5.00mm | |||||
| 50.80 | 2 4.5 tpi | 2 4.5 tpi | 2 7 tpi |
Ø mm is the nominal major (outside) diameter of the thread.
Imperial fractional sizes are shown in inches; UNC/UNF number sizes (No.2-No.12) follow the Unified number series.
Metric pitches shown are the coarse series. Fine-pitch metric threads also exist - if your drawing specifies a fine pitch, send us the requirement.
How do I identify an unknown thread?
Measure the outside diameter with callipers, then find the pitch with a thread pitch gauge or by measuring between crests - imperial threads are identified in threads per inch (TPI), metric and BA pitches in millimetres. Compare both measurements against the chart to find the closest match.
What is the difference between BSW and UNC threads?
BSW and UNC can have similar diameters and, at many sizes, the same threads per inch, but they are different thread forms - BSW uses a 55 degree angle and UNC 60 degrees, and pitches differ at some sizes (1/2" BSW is 12 TPI, 1/2" UNC is 13 TPI). They should not be assumed interchangeable: where a drawing specifies one, use the specified standard.
What is a BA thread?
British Association (BA) is a small-diameter thread series numbered from 0BA downwards - the higher the number, the smaller the thread, with 0BA at a nominal 6.0mm major diameter. BA threads were widely used on instrument, electrical and telecoms equipment and still appear on legacy drawings and repair work. A range of BA fasteners remains available.
Are BSW and BSF threads still available?
Yes - BSW and BSF fasteners can still be sourced, typically to order rather than from stock, for maintenance and restoration of older equipment where drawings specify these thread forms. Availability varies by size, material and product type, so send us your requirement and we can check what can be supplied.
How do I tell metric fine from metric coarse?
Check both diameter and pitch - for the same nominal diameter, a fine thread has a smaller pitch than the coarse standard. M10 coarse is M10 x 1.5, while M10 x 1.25 and M10 x 1.0 are common fine variants. When replacing an existing fastener, don't rely on diameter alone; check the pitch as well.
Can I use a UNF nut on a metric bolt?
No - UNF and metric are different systems and are not compatible, even where diameters appear very similar. A mismatched nut may start on the thread, but tightening can damage the threads, cause stripping or result in seizure. Always identify both diameter and pitch before selecting a replacement.
Need an imperial or legacy thread?
Send us the drawing callout or a sample — quoted within 24 hours.