| TPI stands for Threads Per Inch. This is simply a count of the number of threads per inch measured along the length of a fastener. TPI is used only with American fasteners. Metric Fasteners use a thread Pitch. In general smaller fasteners have finer threads, so the thread count is higher. For a table of standard US thread counts please see below£º
| Bolt Size |
Threads Per Inch (TPI) |
Coarse Thread
UNC |
Fine Thread
UNF |
| #2 |
56 |
- |
| #3 |
48 |
- |
| #4 |
40 |
- |
| #5 |
40 |
- |
| #6 |
32 |
- |
| #8 |
32 |
- |
| #10 |
24 |
32 |
| #12 |
24 |
- |
| 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 |
12* |
| 1-1/8 |
7 |
12 |
| 1-1/4 |
7 |
12 |
| 1-1/2 |
6 |
12 |
|
* 12tpi is the UNF standard however 14tpi is far more common in actual use. |
Metric fasteners are specified with a thread pitch instead of a thread count. The thread pitch is the distance between threads expressed in millimeters (measured along the length of the fastener). For example a thread pitch of 1.5 means that the distance between one thread and the next is 1.5mm. In general smaller fasteners have finer thread so they have lower thread pitch. For a table of standard metric thread pitchs please see below£º
Bolt
Diameter |
Thread Pitch |
| Standard |
Fine |
Extra or Super Fine |
JIS |
| 2 |
.4 |
- |
- |
.4 |
| 2.5 |
.45 |
- |
- |
.45 |
| 3 |
.5 |
- |
- |
.5 |
| 4 |
.7 |
- |
- |
.7 |
| 5 |
.8 |
- |
- |
.8 |
| 6 |
1.0 |
- |
- |
1.0 |
| 7 |
1.0 |
- |
- |
1.0 |
| 8 |
1.25 |
1 |
- |
1.0 |
| 10 |
1.5 |
1.25 |
1 |
1.25 |
| 12 |
1.75 |
1.5 |
1.25 |
1.25 |
| 14 |
2 |
1.5 |
- |
1.5 |
| 16 |
2 |
- |
- |
1.5 |
| 18 |
2.5 |
- |
- |
1.5 |
| 20 |
2.5 |
- |
- |
1.5 | |
| 
