How many threads per inch a given size carries — and the drill to use before you tap it. Standard Unified inch sizes, coarse and fine.
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).
How to read the chart
Size
Nominal diameter. Numbered sizes (#4–#12) are small gauges; the rest are fractions of an inch.
Threads per inch (TPI)
How tightly the threads are spaced. Higher number = finer thread.
Tap drill
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) |
|---|---|---|---|---|
| #1 | 64 | 0.073 | — | — |
| #2 | 56 | 0.086 | — | — |
| #3 | 48 | 0.099 | — | — |
| #4 | 40 | 0.112 | #43 | 0.0890 |
| #5 | 40 | 0.125 | — | — |
| #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 |
| 1¾″ | 5 | 1.750 | — | — |
| 2″ | 4½ | 2.000 | — | — |
| 2¼″ | 4½ | 2.250 | — | — |
| 2½″ | 4 | 2.500 | — | — |
| 2¾″ | 4 | 2.750 | — | — |
| 3″ | 4 | 3.000 | — | — |
| 3¼″ | 4 | 3.250 | — | — |
| 3½″ | 4 | 3.500 | — | — |
| 3¾″ | 4 | 3.750 | — | — |
| 4″ | 4 | 4.000 | — | — |
Special pitches: extra-fine and constant-pitch series
Besides coarse and fine, ASME B1.1 has an extra-fine series (UNEF) and constant-pitch series — the same threads per inch at every size (12UN, 16UN, 20UN, 28 UN, 32UN). They show up on larger, thin-walled or adjusting parts. Tap drills for these aren’t listed here — give us a call and we’ll size the drill.
| Size | Threads per inch | Major dia. (in) | Tap drill | Tap drill (in) |
|---|---|---|---|---|
| #12 | 32 | 0.216 | — | — |
| 1/4″ | 32 | 0.250 | — | — |
| 5/16″ | 32 | 0.312 | — | — |
| 3/8″ | 32 | 0.375 | — | — |
| 1/2″ | 28 | 0.500 | — | — |
| 9/16″ | 24 | 0.562 | — | — |
| 5/8″ | 24 | 0.625 | — | — |
| 3/4″ | 20 | 0.750 | — | — |
| 7/8″ | 20 | 0.875 | — | — |
| 15/16″ | 20 | 0.938 | — | — |
| 1″ | 20 | 1.000 | — | — |
| 1-1/8″ | 18 | 1.125 | — | — |
| 1-3/16″ | 18 | 1.188 | — | — |
| 1-1/4″ | 18 | 1.250 | — | — |
| 1-5/16″ | 18 | 1.312 | — | — |
| 1-3/8″ | 18 | 1.375 | — | — |
| Size | Threads per inch | Major dia. (in) | Tap drill | Tap drill (in) |
|---|---|---|---|---|
| 3/4″ | 12 | 0.750 | — | — |
| 7/8″ | 12 | 0.875 | — | — |
| 1-1/16″ | 12 | 1.062 | — | — |
| 1-3/16″ | 12 | 1.188 | — | — |
| 1-5/16″ | 12 | 1.312 | — | — |
| 1-7/16″ | 12 | 1.438 | — | — |
| 1-9/16″ | 12 | 1.562 | — | — |
| 1-5/8″ | 12 | 1.625 | — | — |
| 1-11/16″ | 12 | 1.688 | — | — |
| 1-3/4″ | 12 | 1.750 | — | — |
| 1-7/8″ | 12 | 1.875 | — | — |
| 2″ | 12 | 2.000 | — | — |
| 2-1/8″ | 12 | 2.125 | — | — |
| 2-1/4″ | 12 | 2.250 | — | — |
| 2-3/8″ | 12 | 2.375 | — | — |
| 2-1/2″ | 12 | 2.500 | — | — |
| 2-5/8″ | 12 | 2.625 | — | — |
| 2-3/4″ | 12 | 2.750 | — | — |
| 3″ | 12 | 3.000 | — | — |
| Size | Threads per inch | Major dia. (in) | Tap drill | Tap drill (in) |
|---|---|---|---|---|
| 1/2″ | 16 | 0.500 | — | — |
| 11/16″ | 16 | 0.688 | — | — |
| 13/16″ | 16 | 0.812 | — | — |
| 1″ | 16 | 1.000 | — | — |
| 1-1/16″ | 16 | 1.062 | — | — |
| 1-1/8″ | 16 | 1.125 | — | — |
| 1-1/4″ | 16 | 1.250 | — | — |
| 1-1/2″ | 16 | 1.500 | — | — |
| 1-9/16″ | 16 | 1.562 | — | — |
| 2″ | 16 | 2.000 | — | — |
| 2-1/8″ | 16 | 2.125 | — | — |
| 2-3/8″ | 16 | 2.375 | — | — |
| 2-5/8″ | 16 | 2.625 | — | — |
| Size | Threads per inch | Major dia. (in) | Tap drill | Tap drill (in) |
|---|---|---|---|---|
| 5/16″ | 20 | 0.312 | — | — |
| 9/16″ | 20 | 0.562 | — | — |
| 5/8″ | 20 | 0.625 | — | — |
| 1-9/16″ | 20 | 1.562 | — | — |
| Size | Threads per inch | Major dia. (in) | Tap drill | Tap drill (in) |
|---|---|---|---|---|
| 3/8″ | 28 | 0.375 | — | — |
| 5/8″ | 28 | 0.625 | — | — |
| 1-1/4″ | 28 | 1.250 | — | — |
| Size | Threads per inch | Major dia. (in) | Tap drill | Tap drill (in) |
|---|---|---|---|---|
| 1/2″ | 32 | 0.500 | — | — |
| 3/4″ | 32 | 0.750 | — | — |
A few plain-English pointers
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.