Metric vs. SAE(Inch) Threads
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Metric vs. SAE(Inch) Threads
Metric vs. SAE(Inch) Threads.The major difference between metric and SAE(Inch) is in how threads are specified. When it comes to fasteners that use threading, an extra measurement called “thread pitch” is added after the diameter measurement. Examples of metric fasteners would be M6x1.0, M10x1.5, and M18x2.5; Examples of SAE(Inch) fasteners would be 1/4-20, 1/4-28, and 3/8-16.
Metric thread pitch is the distance between teeth. For example, an M18x2.5 thread means the diameter of the nut or bolt is 18 mm and that there are 2.5mm between the threads.
SAE(Inch) thread pitch is how many threads there are per inch. For example, a 1/4-20 thread means the diameter of the nut or bolt is 1/4 inch and that there are 20 threads per inch.
Table 1 – Standard Thread Pitch for Metric Bolts
| Bolt Diameter (mm) | Thread Pitch (distance between threads, mm) | ||
| Standard | Fine | Super Fine | |
| 2 | 0.4 | / | / |
| 2.5 | 0.45 | / | / |
| 3 | 0.5 | / | / |
| 4 | 0.7 | / | / |
| 5 | 0.8 | / | / |
| 6 | 1 | / | / |
| 7 | 1 | / | / |
| 8 | 1.25 | 1 | / |
| 10 | 1.5 | 1.25 | 1 |
| 12 | 1.75 | 1.5 | 1.25 |
| 14 | 2 | 1.5 | / |
| 16 | 2 | 1.5 | / |
| 18 | 2.5 | 2 | 1.5 |
| 20 | 2.5 | 2 | 1.5 |
| 22 | 2.5 | 2 | 1.5 |
| 24 | 3 | 2 | / |
| 27 | 3 | 2 | / |
| 30 | 3.5 | 2 | / |
| 33 | 3.5 | 2 | / |
| 36 | 4 | 3 | / |
Table 2 – Threads Per Inch (TPI) for SAE(Inch) Bolts and Machine Screws
| Bolt Size or Diameter | Threads Per Inch (TPI) | |
| Coarse Thread (UNC) | Fine Thread (UNF) | |
| #2 (0.0860″) | 56 | 64 |
| #3 (0.0990″) | 48 | 56 |
| #4 (0.1120″) | 40 | 48 |
| #5 (0.1250″) | 40 | 44 |
| #6 (0.1380″) | 32 | 40 |
| #8 (0.1640″) | 32 | 36 |
| #10 (0.1900″) | 24 | 32 |
| #12 (0.2160″) | 24 | 28 |
| 1/4 | 20 | 28 |
| 5/16 | 18 | 24 |
| 3/8 | 16 | 24 |
| 7/16 | 14 | 20 |
| 1/2 | 13 | 20 |
| 9/16 | 12 | 18 |
| 5/8 | 11 | 18 |
| 3/4 | 10 | 16 |
| 7/8 | 9 | 14 |
| 1 | 8 | 14 |
