Thread Dimensions
What's the difference between BSW, BSF, BA, UNC and UNF screw threads?
It’s easy to get confused with BSW, BSF, BA, UNC and UNF; after all, there are so many abbreviations. But we can assure you, after reading this; you’ll know the system better than Mr Whitworth himself!
Every thread, whether external (male) like a bolt or screw or internal (female) such as a nut, has a pitch and a diameter. The pitch is the distance between each of the threads measured in either the imperial or metric system. In some countries such as America, they will use TPI (threads per inch) which has the same underlying properties as pitch; hence, the terms can be used interchangeably. The difference between BSW or BSF, BA, and UNC or UNF is simply the thread angle and measurement system used for parts and components (often this varies between Brits, Americans and the international classification).
Below is a table of the abbreviated measurement terms as well as the thread angles used:
| Measurement System | Location Predominantly Used | Abbreviation and Thread Style | Thread Angle Size |
| Metric (mm) | Internationally | Coarse and Fine | 60° |
|
Imperial (in)
|
UN (Unified) is commonly used in the US and Canada
|
UNC: Unified Coarse |
60°
|
| UNF: Unified Fine | |||
|
BS (British Standard) is commonly used in Great Britain
|
BSW: British Standard Whitworth (Coarse) - devised and specified by Joseph Whitworth in 1841 |
50°
|
|
| BSF: British Standard Fine | |||
| BSP: British Standard Pipe (Fine - used extensively in plumbing) | |||
| BA: British Association (Coarse and Fine) | 47.5° |
To help understanding further, check out our diagram displaying the various diameters (major, minor and effective pitch) as well as the pitch angle and how these aspects are situated within internal and external threads.
Scroll or click below to find the thread diameter, coarse pitch, effective diameter and tap drill diameter for the following thread variations:
- Metric Coarse
- Metric Fine
- UNC Coarse (Unified Coarse)
- UNF Fine (Unified Fine)
- BSW Coarse (British Standard Whitworth Coarse)
- BSF Fine (British Standard Fine)
- BSP Fine (British Standard Pipe)
- BA Coarse and Fine (British Association Coarse and Fine)
Metric Coarse, Thread Angle 60°
| Metric Coarse, Thread Angle 60° | |||
| Metric Thread Diameter | Coarse Pitch (mm) | Effective Diameter (mm) | Tap Drill Size (mm) |
| M1 | 0.25 | 0.838 | 0.75 |
| M1.1 | 0.25 | 0.938 | 0.85 |
| M1.2 | 0.25 | 1.038 | 0.95 |
| M1.4 | 0.3 | 1.205 | 1.1 |
| M1.6 | 0.35 | 1.373 | 1.25 |
| M1.8 | 0.35 | 1.573 | 1.45 |
| M2 | 0.4 | 1.74 | 1.6 |
| M2.2 | 0.45 | 1.908 | 1.75 |
| M2.5 | 0.5 | 2.208 | 2.05 |
| M3 | 0.5 | 2.675 | 2.5 |
| M3.5 | 0.6 | 3.11 | 2.9 |
| M4 | 0.7 | 3.545 | 3.3 |
| M4.5 | 0.75 | 4.013 | 3.7 |
| M5 | 0.8 | 4.48 | 4.2 |
| M6 | 1 | 5.35 | 5 |
| M7 | 1 | 6.35 | 6 |
| M8 | 1.25 | 7.188 | 6.8 |
| M9 | 1.25 | 8.18 | 7.8 |
| M10 | 1.5 | 9.026 | 8.5 |
| M11 | 1.5 | 10.026 | 9.5 |
| M12 | 1.75 | 10.863 | 10.2 |
| M14 | 2 | 12.701 | 12 |
| M16 | 2 | 14.701 | 14 |
| M18 | 2.5 | 16.376 | 15.5 |
| M20 | 2.5 | 18.376 | 17.5 |
| M22 | 2.5 | 20.376 | 19.5 |
| M24 | 3 | 22.051 | 21 |
| M27 | 3 | 25.051 | 24 |
| M30 | 3.5 | 27.727 | 26.5 |
| M33 | 3.5 | 30.727 | 29.5 |
| M36 | 4 | 33.402 | 23 |
| M39 | 4 | 36.402 | 35 |
| M42 | 4.5 | 39.077 | 37.5 |
Metric Fine, Thread Angle 60°
| Metric Fine, Thread Angle 60° | |||
| Metric Thread Diameter | Fine Pitch (mm) | Effective Diameter (mm) | Tap Drill Size (mm) |
| M3 | 0.35 | 2.773 | 2.65 |
| M4 | 0.5 | 3.675 | 3.5 |
| M5 | 0.5 | 4.675 | 4.5 |
| M6 | 0.75 | 5.513 | 5.2 |
| M7 | 0.75 | 6.513 | 6.2 |
| M8 | 1 | 7.35 | 7 |
| M10 | 1.25 | 9.188 | 8.8 |
| M12 | 1.25 | 11.188 | 10.8 |
| M14 | 1.5 | 13.026 | 12.5 |
| M16 | 1.5 | 15.026 | 14.5 |
| M18 | 1.5 | 17.026 | 16.5 |
| M20 | 1.5 | 19.026 | 18.5 |
| M22 | 1.5 | 21.026 | 20.5 |
| M24 | 2 | 22.701 | 22 |
| M27 | 2 | 25.701 | 25 |
| M30 | 2 | 28.701 | 28 |
UNC Coarse, Thread Angle 60°
| UNC Coarse, Thread Angle 60° | |||
| UNC Thread Diameter (No./in) | Thread per Inch (TPI) | Effective Diameter (Inch) |
Tap Drill Size (Inch)
|
| No.1 | 64 | 0.0629 | 1.55 |
| No.2 | 56 | 0.0744 | 1.85 |
| No.3 | 48 | 0.0855 | 2.1 |
| No.4 | 40 | 0.0958 | 2.35 |
| No.5 | 40 | 0.1088 | 2.65 |
| No.6 | 32 | 0.1177 | 2.85 |
| No.8 | 32 | 0.1437 | 3.5 |
| No.10 | 24 | 0.1629 | 3.9 |
| No.12 | 24 | 0.1889 | 4.5 |
| 1/4 | 20 | 0.2175 | 5.1 |
| 5/16 | 18 | 0.2764 | 6.6 |
| 3/8 | 16 | 0.3344 | 8 |
| 7/16 | 14 | 0.3911 | 9.4 |
| 1/2 | 13 | 0.45 | 10.8 |
| 9/16 | 12 | 0.5084 | 12.2 |
| 5/8 | 11 | 0.566 | 13.5 |
| 3/4 | 10 | 0.685 | 16.5 |
| 7/8 | 9 | 0.8028 | 19.5 |
| 1 | 8 | 0.9188 | 22.25 |
| 1 1/8 | 7 | 1.0322 | 25 |
| 1 1/4 | 7 | 1.1572 | 28 |
| 1 3/8 | 6 | 1.2667 | 30.75 |
| 1 1/2 | 6 | 1.3917 | 34 |
| 1 3/4 | 5 | 1.6201 | 39.5 |
| 2 | 4 1/2 | 1.8557 | 45 |
| 2 1/4 | 4 1/2 | 2.1057 | 52 |
UNF Fine, Thread Angle 60°
| UNC Fine, Thread Angle 60° | |||
| UNC Thread Diameter (No./in) | Thread per Inch (TPI) | Effective Diameter (Inch) | Tap Drill Size (Inch) |
| No.0 | 80 | 0.0159 | 1.25 |
| No.1 | 72 | 0.064 | 1.55 |
| No.2 | 64 | 0.0759 | 1.9 |
| No.3 | 56 | 0.0874 | 2.15 |
| No.4 | 48 | 0.0985 | 2.4 |
| No.5 | 44 | 1.1102 | 2.7 |
| No.6 | 40 | 0.1218 | 2.95 |
| No.8 | 36 | 0.146 | 3.5 |
| No.10 | 32 | 0.1697 | 4.1 |
| No.12 | 28 | 0.1928 | 4.7 |
| 1/4 | 28 | 0.2268 | 5.5 |
| 5/16 | 24 | 0.2854 | 6.9 |
| 3/8 | 24 | 0.3479 | 8.5 |
| 7/16 | 20 | 0.405 | 9.9 |
| 1/2 | 20 | 0.4675 | 11.5 |
| 9/16 | 18 | 0.5264 | 12.9 |
| 5/8 | 18 | 0.5889 | 14.5 |
| 3/4 | 16 | 0.7094 | 17.5 |
| 7/8 | 14 | 0.8286 | 20.4 |
| 1 | 12 | 0.9459 | 23.25 |
| 1 1/8 | 12 | 1.0709 | 26.5 |
| 1 1/4 | 12 | 1.1959 | 29.5 |
| 1 3/8 | 12 | 0.3209 | 32.75 |
| 1 1/2 | 12 | 1.4459 | 36 |
BSW Coarse, Thread Angle 55°
| BSW Coarse, Thread Angle 55° | |||
| BSW Thread Diameter (Inch) | Threads per Inch (TPI) | Effective Diameter (inch) | Tap Drill Size (mm) |
| 1/16 | 60 | 0.0518 | 1.2 |
| 3/32 | 48 | 0.0805 | 1.85 |
| 1/8 | 40 | 0.109 | 2.55 |
| 5/32 | 32 | 0.1362 | 3.2 |
| 3/16 | 24 | 0.1608 | 3.7 |
| 7/32 | 24 | 0.1921 | 4.5 |
| 1/4 | 20 | 0.218 | 5.1 |
| 5/16 | 18 | 0.2769 | 6.5 |
| 3/8 | 16 | 0.335 | 7.9 |
| 7/16 | 14 | 0.3918 | 9.3 |
| 1/2 | 12 | 0.446 | 10.5 |
| 9/16 | 12 | 0.5091 | 12.1 |
| 5/8 | 11 | 0.5668 | 13.5 |
| 11/16 | 11 | 0.6293 | 15 |
| 3/4 | 10 | 0.686 | 16.25 |
| 13/16 | 10 | 0.7485 | 18 |
| 7/8 | 9 | 0.8039 | 19.25 |
| 15/16 | 9 | 0.8664 | 20.75 |
| 1 | 8 | 0.92 | 22 |
| 1 1/8 | 7 | 1.0335 | 24.75 |
| 1 1/4 | 7 | 1.1585 | 28 |
| 1 3/8 | 6 | 1.2683 | 30.25 |
| 1 1/2 | 6 | 1.3933 | 33.5 |
| 1 5/8 | 5 | 1.4969 | 35.5 |
| 1 3/4 | 5 | 1.6219 | 39 |
| 1 7/8 | 4 1/2 | 1.7327 | 41.5 |
| 2 | 4 1/2 | 1.8577 | 44.5 |
| 2 1/4 | 4 | 2.0899 | 51 |
| 2 1/2 | 4 | 2.3399 | 57 |
BSF Fine, Thread Angle 55°
| BSF Fine, Thread Angle 55° | |||
| BSW Thread Diameter | Threads per Inch | Effective Diameter | Tap Drill Size (mm) |
| 3/16 | 32 | 0.1675 | 4 |
| 7/32 | 28 | 0.1959 | 4.6 |
| 1/4 | 26 | 0.2254 | 5.3 |
| 5/16 | 22 | 0.2834 | 6.8 |
| 3/8 | 20 | 0.343 | 8.3 |
| 7/16 | 18 | 0.4019 | 9.7 |
| 1/2 | 16 | 0.46 | 11.1 |
| 9/16 | 16 | 0.5225 | 12.7 |
| 5/8 | 14 | 0.5793 | 14 |
| 11/16 | 14 | 0.6418 | 15.5 |
| 3/4 | 12 | 0.6966 | 16.75 |
| 7/8 | 11 | 0.8168 | 19.75 |
| 1 | 10 | 0.936 | 22.75 |
| 1 1/8 | 9 | 1.0539 | 25.5 |
| 1 1/4 | 9 | 1.1789 | 28.5 |
| 1 3/8 | 8 | 1.295 | 31.5 |
| 1 1/2 | 8 | 1.42 | 34.5 |
| 1 3/4 | 7 | 1.6585 | 41 |
| 2 | 7 | 1.9085 | 47 |
| 2 1/4 | 6 | 2.1433 | 53 |
| 2 1/2 | 6 | 2.3933 | 58 |
BSP Fine, Thread Angle 55°
| Tap Drill Size (mm) | ||||
| BSW Thread Diameter (Inch) | Threads per Inch (TPI) | Effective Diameter (Inch) | Rp Series (Parallel) (BSPP) | G Series (Fastening) (BSFF) |
| 1/16 | 28 | 0.2812 | 4 | 6.8 |
| 1/8 | 28 | 0.3601 | 4.6 | 8.8 |
| 1/4 | 19 | 0.4843 | 5.3 | 11.8 |
| 3/8 | 19 | 0.6223 | 6.8 | 15.25 |
| 1/2 | 14 | 0.7793 | 8.3 | 19 |
| 5/8 | 14 | 0.8563 | 9.7 | 21 |
| 3/4 | 14 | 0.9953 | 11.1 | 24.5 |
| 7/8 | 14 | 1.1433 | 12.7 | 28.25 |
| 1 | 11 | 1.2508 | 14 | 30.75 |
| 1 1/4 | 11 | 1.5918 | 15.5 | 39.5 |
| 1 1/2 | 11 | 1.8238 | 16.75 | 45.5 |
| 1 3/4 | 11 | 2.0578 | 19.75 | 51 |
| 2 | 11 | 2.2888 | 22.75 | 57 |
| 2 1/4 | 11 | 2.5288 | 25.5 | 62.7 |
| 2 1/2 | 11 | 2.9018 | 28.5 | 72.1 |
| 2 3/4 | 11 | 3.1518 | 31.5 | 78.5 |
| 3 | 11 | 3.4018 | 34.5 | 84.8 |
| 3 1/2 | 11 | 3.8918 | 41 | 97.3 |
| 4 | 11 | 4.3918 | 47 | 110 |
| 5 | 11 | 5.3918 | 53 | 135.4 |
| 6 | 11 | 6.3918 | 58 | 160.8 |
BA Coarse and Fine, Thread Angle 47.5°
| BA Coarse and Fine, Thread Angle 47.5° | |||
| BA (No.) | Pitch (Inch) | Effective Diameter (Inch) | Tap Drill Size (mm) |
| 0 | 0.0394 | 0.2126 | 5.1 |
| 1 | 0.0354 | 0.1874 | 4.5 |
| 2 | 0.0319 | 0.1659 | 4 |
| 3 | 0.0287 | 0.1441 | 3.4 |
| 4 | 0.026 | 0.1262 | 3 |
| 5 | 0.0232 | 0.112 | 2.65 |
| 6 | 0.0209 | 0.0976 | 2.3 |
| 7 | 0.0189 | 0.087 | 2.05 |
| 8 | 0.0169 | 0.0764 | 1.8 |
| 9 | 0.0154 | 0.0656 | 1.5 |
| 10 | 0.0138 | 0.0587 | 1.4 |
| 11 | 0.0122 | 0.0518 | 1.2 |
| 12 | 0.011 | 0.0445 | 1.05 |
| 13 | 0.0098 | 0.0413 | 0.98 |
| 14 | 0.0091 | 0.0339 | 0.8 |
| 15 | 0.0083 | 0.0305 | 0.7 |
| 16 | 0.0075 | 0.0266 | 0.6 |