Convert inch thread to metric system
Whoever reads this blog often or has already dealt with the thread topic, knows that there are many types of threads and that these can be divided into inch threads (UNC, UNF, BSW, BSF etc.) and metric threads (M, MF etc.) in a first step.
Inch threads are very common in Europe, especially in the automotive industry, in the sanitary sector (water and gas installations) and computer technology. Again and again, confusion arises due to the customs information.
In this blog article we would therefore like to convert inch threads and clarify special features. Let’s get started!
Inch thread is not equal to inch thread
As already mentioned, thread types can be divided into inch threads and metric threads. However, the inch threads also differ. For example, UNC and UNF are American threads, BSW and BSF are English threads. These are both specified in inch/inch, but have different flank angles and thread pitches. It is therefore important to understand that they are not interchangeable.
Another important point is the distinction between screw threads and pipe threads. Inch pipe threads cannot be converted using the table below due to historical circumstances! Since one inch/inch does not correspond to 25.4 mm for pipe threads! Rather 33 mm.
You can read more information and differences in this blog post:
und
How is an inch thread converted for screws?
If you want to convert an inch screw thread, you must have the following simple formula in mind:
1.00 inch = 25.40 mm <=> 2.54 cm
Warning: This conversion formula is only suitable for screw threads, but not for pipe threads!
Since in metal processing sizes are always given in millimeters, we will follow this unwritten law in the further course of this article!
Inch threads are always shown as a fraction (1/4″) and never as a decimal number (0.25 inch) on the thread cutters.
Thread sizes smaller than 1/4″ are represented as a natural number (0-12), such as UNC No.3.
Conversion table inch thread in millimetres (mm)
| Inch | Decimal Inch | Decimal mm |
| 3/16 | 0,1875 | 4,7625 |
| 1/4 | 0,2500 | 6,3500 |
| 5/16 | 0,3125 | 7,9375 |
| 3/8 | 0,3750 | 9,5250 |
| 7/16 | 0,4375 | 11,1125 |
| 1/2 | 0,5000 | 12,7000 |
| 5/8 | 0,6250 | 15,8750 |
| 3/4 | 0,7500 | 19,0500 |
| 7/8 | 0,8750 | 22,2250 |
| 1″ | 1,0000 | 25,4000 |
| 1. 1/4 | 1,2500 | 31,7500 |
| 1. 1/2 | 1,5000 | 38,1000 |
| 1. 3/4 | 1,7500 | 44,4500 |
| 2″ | 2,0000 | 50,8000 |
| 2. 1/4 | 2,2500 | 57,1500 |
| 2. 1/2 | 2,5000 | 63,5000 |
| 2. 3/4 | 2,7500 | 69,8500 |
| 3″ | 3,0000 | 76,2000 |
| 3. 1/4 | 3,2500 | 82,5500 |
| 3. 1/2 | 3,5000 | 88,9000 |
| 3. 3/4 | 3,7500 | 95,2500 |
| 4″ | 4,0000 | 101,6000 |
| 4. 1/2 | 4,5000 | 114,3000 |
| 5″ | 5,0000 | 127,0000 |
| 5. 1/2 | 5,5000 | 139,7000 |
| 6″ | 6,0000 | 152,4000 |
| 6. 1/2 | 6,5000 | 165,1000 |
| 7″ | 7,0000 | 177,8000 |
| 7. 1/2 | 7,5000 | 190,5000 |
| 8″ | 8,0000 | 203,2000 |
| 8. 1/2 | 8,5000 | 215,9000 |
| 9″ | 9,0000 | 228,6000 |
| 9. 1/2 | 9,5000 | 241,3000 |
| 10″ | 10,0000 | 254,0000 |
| 10. 1/2 | 10,5000 | 266,7000 |
| 11″ | 11,0000 | 279,4000 |
| 11. 1/2 | 11,5000 | 292,1000 |
| 12″ | 12,0000 | 304,8000 |
| 12. 1/2 | 12,5000 | 317,5000 |
| 13″ | 13,0000 | 330,2000 |
| 13. 1/2 | 13,5000 | 342,9000 |
| 14″ | 14,0000 | 355,6000 |
| 14. 1/2 | 14,5000 | 368,3000 |
| 15″ | 15,0000 | 381,0000 |
