Spec chart
Tap Drill Size Chart
Tap drill sizes for common UNC and UNF threads at 75% thread engagement, with drill decimal and metric equivalents. Filter by thread size and copy the drill you need.
| Thread size | Series | TPI | Tap drill | Drill (in) | Copy row |
|---|---|---|---|---|---|
| #4-40 | UNC | 40 | #43 | 0.089 | |
| #4-48 | UNF | 48 | #42 | 0.0935 | |
| #6-32 | UNC | 32 | #36 | 0.1065 | |
| #6-40 | UNF | 40 | #33 | 0.113 | |
| #8-32 | UNC | 32 | #29 | 0.136 | |
| #8-36 | UNF | 36 | #29 | 0.136 | |
| #10-24 | UNC | 24 | #25 | 0.1495 | |
| #10-32 | UNF | 32 | #21 | 0.159 | |
| #12-24 | UNC | 24 | #16 | 0.177 | |
| #12-28 | UNF | 28 | #14 | 0.182 | |
| 1/4"-20 | UNC | 20 | #7 | 0.201 | |
| 1/4"-28 | UNF | 28 | #3 | 0.213 | |
| 5/16"-18 | UNC | 18 | F | 0.257 | |
| 5/16"-24 | UNF | 24 | I | 0.272 | |
| 3/8"-16 | UNC | 16 | 5/16 | 0.3125 | |
| 3/8"-24 | UNF | 24 | Q | 0.332 | |
| 7/16"-14 | UNC | 14 | U | 0.368 | |
| 7/16"-20 | UNF | 20 | 25/64 | 0.3906 | |
| 1/2"-13 | UNC | 13 | 27/64 | 0.4219 | |
| 1/2"-20 | UNF | 20 | 29/64 | 0.4531 | |
| 9/16"-12 | UNC | 12 | 31/64 | 0.4844 | |
| 9/16"-18 | UNF | 18 | 33/64 | 0.5156 | |
| 5/8"-11 | UNC | 11 | 17/32 | 0.5312 | |
| 5/8"-18 | UNF | 18 | 37/64 | 0.5781 | |
| 3/4"-10 | UNC | 10 | 21/32 | 0.6562 | |
| 3/4"-16 | UNF | 16 | 11/16 | 0.6875 | |
| 7/8"-9 | UNC | 9 | 49/64 | 0.7656 | |
| 1"-8 | UNC | 8 | 7/8 | 0.875 |
Sizes assume roughly 75% thread engagement, the standard for general-purpose tapping in steel and aluminum. For harder materials, drop to 50–60% engagement by drilling one size larger.
Frequently asked
- What is a tap drill size?
- It is the hole diameter you drill before cutting internal threads with a tap. Too small and the tap binds or breaks; too large and the threads are weak.
- What thread engagement do these sizes use?
- About 75%, which balances thread strength against tapping effort for steel and aluminum. Drop to 50–60% (one drill size larger) for stainless or hardened stock.
- Can I use a metric drill for an inch tap?
- Yes — use the closest metric equivalent shown by toggling to metric. Pick the size nearest the decimal value; slightly larger is safer in hard materials.