Thread pitch chart
Threads per inch for Unified (UNC and UNF) sizes and pitch in millimeters for metric threads. Use it to identify a thread, choose a tap, or confirm that a nut and bolt match.
Unified threads (threads per inch)
| Size | UNC (coarse) | UNF (fine) |
|---|---|---|
| #4 | 40 | 48 |
| #6 | 32 | 40 |
| #8 | 32 | 36 |
| #10 | 24 | 32 |
| 1/4 | 20 | 28 |
| 5/16 | 18 | 24 |
| 3/8 | 16 | 24 |
| 7/16 | 14 | 20 |
| 1/2 | 13 | 20 |
| 5/8 | 11 | 18 |
| 3/4 | 10 | 16 |
| 1 | 8 | 12 |
Metric threads (pitch in mm)
| Size | Coarse pitch | Fine pitch |
|---|---|---|
| M3 | 0.5 | 0.35 |
| M4 | 0.7 | 0.5 |
| M5 | 0.8 | 0.5 |
| M6 | 1.0 | 0.75 |
| M8 | 1.25 | 1.0 |
| M10 | 1.5 | 1.25 |
| M12 | 1.75 | 1.25 |
| M16 | 2.0 | 1.5 |
| M20 | 2.5 | 1.5 |
Inch threads are specified by threads per inch, where more threads means finer. Metric threads are specified by pitch, the distance between threads in millimeters, where a smaller pitch means finer. Coarse threads are the default for general use; fine threads give more holding power and finer adjustment but cross-thread more easily.
Need the tap drill for one of these?
The Tap Drill Calculator gives the exact drill size for any thread and pitch on this chart.
Coarse vs fine threads
Coarse threads (UNC, metric coarse) install quickly, tolerate dirt and damage, and are the standard choice for most work. Fine threads (UNF, metric fine) have a shallower helix, giving more thread engagement and resistance to loosening, which suits thin walls, fine adjustment, and vibration.
Reading a thread callout
An inch callout like 1/4-20 means 1/4 inch diameter with 20 threads per inch. A metric callout like M8 x 1.25 means 8 mm diameter with a 1.25 mm pitch. Matching both the diameter and the pitch is what makes a nut and bolt fit.
FAQ
How many threads per inch is a 1/4-20 bolt?
Twenty. The 20 is the thread count, making it a coarse UNC thread. The fine version is 1/4-28.
What is thread pitch?
For metric threads it is the distance in millimeters between adjacent threads; smaller is finer. Inch threads use threads per inch instead, where a higher number is finer.
Can I mix coarse and fine threads?
No. A coarse nut will not run onto a fine bolt of the same diameter. Both the diameter and the thread pitch must match.