UNC Tap Drill Sizes
UNC (Unified National Coarse) is the default thread series in the US. If a drawing says “1/2-13” — that’s UNC. Coarse threads are stronger, more forgiving of dirty conditions, and easier to assemble than fine threads.
| Thread Size | TPI | Tap Drill | Decimal (in) | 75% Thread | Clearance Drill |
|---|---|---|---|---|---|
| #1-64 | 64 | #53 | 0.0595 | 0.0595 | #46 (0.081) |
| #2-56 | 56 | #50 | 0.0700 | 0.0700 | #43 (0.089) |
| #3-48 | 48 | #47 | 0.0785 | 0.0781 | #37 (0.104) |
| #4-40 | 40 | #43 | 0.0890 | 0.0890 | #32 (0.116) |
| #5-40 | 40 | #38 | 0.1015 | 0.1015 | #30 (0.129) |
| #6-32 | 32 | #36 | 0.1065 | 0.1065 | #27 (0.144) |
| #8-32 | 32 | #29 | 0.1360 | 0.1360 | #18 (0.170) |
| #10-24 | 24 | #25 | 0.1495 | 0.1495 | #9 (0.196) |
| #12-24 | 24 | #16 | 0.1770 | 0.1770 | #2 (0.221) |
| 1/4-20 | 20 | #7 | 0.2010 | 0.2010 | 17/64 (0.266) |
| 5/16-18 | 18 | F | 0.2570 | 0.2570 | 21/64 (0.328) |
| 3/8-16 | 16 | 5/16 | 0.3125 | 0.3125 | 25/64 (0.391) |
| 7/16-14 | 14 | U | 0.3680 | 0.3680 | 29/64 (0.453) |
| 1/2-13 | 13 | 27/64 | 0.4219 | 0.4219 | 17/32 (0.531) |
| 9/16-12 | 12 | 31/64 | 0.4844 | 0.4844 | 19/32 (0.594) |
| 5/8-11 | 11 | 17/32 | 0.5312 | 0.5312 | 21/32 (0.656) |
| 3/4-10 | 10 | 21/32 | 0.6562 | 0.6562 | 25/32 (0.781) |
| 7/8-9 | 9 | 49/64 | 0.7656 | 0.7656 | 29/32 (0.906) |
| 1-8 | 8 | 7/8 | 0.8750 | 0.8750 | 1-1/16 (1.063) |
| 1-1/8-7 | 7 | 63/64 | 0.9844 | 0.9844 | 1-3/16 (1.188) |
| 1-1/4-7 | 7 | 1-7/64 | 1.1094 | 1.1094 | 1-5/16 (1.313) |
| 1-1/2-6 | 6 | 1-11/32 | 1.3438 | 1.3438 | 1-9/16 (1.563) |
What is 75% Thread Engagement?
A 75% thread gives you about 95% of the strength of a full-form thread — with much less risk of tap breakage. This is the industry standard for most applications. Going to 100% thread costs significantly more (smaller drill, higher torque, more broken taps) and gains almost nothing.
When to Use UNC vs. UNF
- UNC (Coarse) — general purpose, thick parts, softer materials, dirty environments
- UNF (Fine) — thin walls, high vibration, precision adjustment (see UNF chart)
Related: UNF Tap Drill Sizes | UNC Thread Dimensions | Master Drill Size Chart