Every inch-size fastener has a thread pitch — the number of threads packed into one inch, written as the second number in a size like ½–13 (½″ diameter, 13 threads per inch). Most sizes come in two pitches: a coarse series (UNC) that’s quicker to run and more forgiving, and a fine series (UNF) that holds a little more strength and resists loosening under vibration. If you’re cutting your own threads, the tap drill column tells you which hole to drill first — sized for a strong, standard thread (around 70–75%; some fine sizes run a little lower).
Nominal diameter. Numbered sizes (#4–#12) are small gauges; the rest are fractions of an inch.
How tightly the threads are spaced. Higher number = finer thread.
The hole to drill before tapping, for a clean, strong thread (about 70–75%). Drill size shown with its decimal.
| Size | Threads per inch | Major dia. (in) | Tap drill | Tap drill (in) |
|---|---|---|---|---|
| #4 | 40 | 0.112 | #43 | 0.0890 |
| #6 | 32 | 0.138 | #36 | 0.1065 |
| #8 | 32 | 0.164 | #29 | 0.1360 |
| #10 | 24 | 0.190 | #25 | 0.1495 |
| #12 | 24 | 0.216 | #16 | 0.1770 |
| ¼″ | 20 | 0.250 | #7 | 0.2010 |
| 5⁄16″ | 18 | 0.3125 | F | 0.2570 |
| ⅜″ | 16 | 0.375 | 5⁄16 | 0.3125 |
| 7⁄16″ | 14 | 0.4375 | U | 0.3680 |
| ½″ | 13 | 0.500 | 27⁄64 | 0.4219 |
| 9⁄16″ | 12 | 0.5625 | 31⁄64 | 0.4844 |
| 5⁄8″ | 11 | 0.625 | 17⁄32 | 0.5312 |
| ¾″ | 10 | 0.750 | 21⁄32 | 0.6562 |
| 7⁄8″ | 9 | 0.875 | 49⁄64 | 0.7656 |
| 1″ | 8 | 1.000 | 7⁄8 | 0.8750 |
| 1⅛″ | 7 | 1.125 | 63⁄64 | 0.9844 |
| 1¼″ | 7 | 1.250 | 1 7⁄64 | 1.1094 |
| 1⅜″ | 6 | 1.375 | 1 7⁄32 | 1.2188 |
| 1½″ | 6 | 1.500 | 1 11⁄32 | 1.3438 |
Which pitch do I have? If you’re not sure whether a bolt is coarse or fine, count the threads against a ruler over one inch, or bring it to either counter — we’ll gauge it in a second.
About the tap drill. The sizes here are the standard tap drills, aiming for roughly a 70–75% thread (a little less on some fine sizes) — what most jobs want: nearly full holding strength without making the tap fight the hole. Going to a slightly larger drill (a lower thread percentage) makes tapping easier in hard material; a smaller drill gives a fuller thread but works the tap harder.
Need a pitch that isn’t here? Extra-fine (UNEF), 8-pitch, and special threads exist too — and our machine shop cuts threads we don’t stock. Just ask.
