Reference & Charts
Tap Drill Chart
The correct tap drill sizes for Unified inch threads (UNC coarse and UNF fine) at ~75% thread engagement — the shop-standard chart, with drill number/letter/fraction and decimal size for each screw size.
Last verified: 2026-07-09
Use these drill sizes before tapping a Unified inch thread for roughly 75% thread engagement — the common general-purpose default that balances thread strength against tapping torque and tap breakage. Sizes are given as the standard drill (number, letter, or fraction) and its decimal diameter in inches.
UNC — Coarse threads
| Size | Threads / in | Major Ø (in) | Tap drill | Tap drill Ø (in) |
|---|---|---|---|---|
| #4 | 40 | 0.112 | #43 | 0.089 |
| #6 | 32 | 0.138 | #36 | 0.1065 |
| #8 | 32 | 0.164 | #29 | 0.136 |
| #10 | 24 | 0.19 | #25 | 0.1495 |
| #12 | 24 | 0.216 | #16 | 0.177 |
| 1/4" | 20 | 0.25 | #7 | 0.201 |
| 5/16" | 18 | 0.3125 | F | 0.257 |
| 3/8" | 16 | 0.375 | 5/16 | 0.3125 |
| 7/16" | 14 | 0.4375 | U | 0.368 |
| 1/2" | 13 | 0.5 | 27/64 | 0.4219 |
| 9/16" | 12 | 0.5625 | 31/64 | 0.4844 |
| 5/8" | 11 | 0.625 | 17/32 | 0.5312 |
| 3/4" | 10 | 0.75 | 21/32 | 0.6562 |
UNF — Fine threads
| Size | Threads / in | Major Ø (in) | Tap drill | Tap drill Ø (in) |
|---|---|---|---|---|
| #4 | 48 | 0.112 | #42 | 0.0935 |
| #6 | 40 | 0.138 | #33 | 0.113 |
| #8 | 36 | 0.164 | #29 | 0.136 |
| #10 | 32 | 0.19 | #21 | 0.159 |
| 1/4" | 28 | 0.25 | #3 | 0.213 |
| 5/16" | 24 | 0.3125 | I | 0.272 |
| 3/8" | 24 | 0.375 | Q | 0.332 |
| 7/16" | 20 | 0.4375 | 25/64 | 0.3906 |
| 1/2" | 20 | 0.5 | 29/64 | 0.4531 |
| 9/16" | 18 | 0.5625 | 33/64 | 0.5156 |
| 5/8" | 18 | 0.625 | 37/64 | 0.5781 |
| 3/4" | 16 | 0.75 | 11/16 | 0.6875 |
Frequently asked
- What does 75% thread engagement mean?
- It's how much of the full theoretical thread depth is cut. ~75% is the common general-purpose target: it retains most of the thread's strength while noticeably lowering tapping torque and the risk of tap breakage compared with chasing 100%.
- Can I use a slightly different drill?
- Yes. A larger drill produces a lower % thread (easier tapping, weaker thread); a smaller drill produces a higher % thread (stronger, but much harder to tap and more likely to break the tap). For most work the 75% size here is the right default.
- Does this chart cover metric threads?
- No — these are Unified inch threads (UNC/UNF). A metric (ISO) tap drill chart is a separate reference.
Sources
- ASME B1.1 — Unified Inch Screw Threads (UN and UNR thread form)
- Standard machinist tap-drill sizing for ~75% thread engagement