#10-24 UNC tap drill size
Use a #25 drill (0.1495 in, 3.8 mm) for #10-24 UNC. It is the standard chart size and leaves about 75% thread, enough for full practical strength without excessive tapping torque.
| Drill | inch | mm | Thread % | Typical use |
|---|---|---|---|---|
| #27 | 0.1440 | 3.66 | 85.0% | General purpose |
| #26 | 0.1470 | 3.73 | 79.4% | General purpose |
| #25 (chart drill) | 0.1495 | 3.8 | 74.8% | General purpose |
| 3.8 mm (metric drill) | 0.1496 | 3.8 | 74.6% | General purpose |
| #24 | 0.1520 | 3.86 | 70.2% | General purpose |
| #23 | 0.1540 | 3.91 | 66.5% | Hard or tough material, deep or blind holes |
| 5/32" | 0.1563 | 3.97 | 62.4% | Hard or tough material, deep or blind holes |
| Thread | Tap drill | Tap drill (mm) | Clearance close / free |
|---|---|---|---|
| #6-32 UNC | #36 (0.1065 in) | 2.71 mm | #27 / #25 |
| #8-32 UNC | #29 (0.1360 in) | 3.45 mm | #18 / #16 |
| #10-24 UNC (this page) | #25 (0.1495 in) | 3.8 mm | #9 / #7 |
| #12-24 UNC | #16 (0.1770 in) | 4.5 mm | #2 / #1 |
| 1/4-20 UNC | #7 (0.2010 in) | 5.11 mm | F / H |
Practical notes
Spiral-point taps suit through holes; spiral-flute taps lift chips out of blind holes. In a blind hole, drill a few threads deeper than the full-thread depth you need.
Typical engagement lengths: about 1 × diameter in steel, 1.5 × in cast iron or brass and 2 × in aluminium. In tough stainless or hardened steel a slightly larger drill (60–70% thread) cuts torque and tap breakage with little loss of strength.
Thread-forming (roll) taps need a larger hole, roughly 0.1692 in (4.3 mm, D − 0.5 × pitch). Check the tap maker's chart; forming suits ductile metals such as aluminium, mild steel and brass, not cast iron.
UNC (coarse) is the default Unified series: quicker to assemble, more tolerant of dirt and damage, and better in soft materials such as aluminium and cast iron.
Clearance drills for a #10 screw: close fit #9 (0.1960 in, 4.98 mm), free fit #7 (0.2010 in, 5.11 mm).
FAQ
What size drill for a #10-24 UNC tap?
The standard tap drill for #10-24 UNC is a #25 drill, 0.1495 in (3.8 mm), which gives about 75% thread.
What clearance hole for a #10 UNC screw?
Use #9 (0.1960 in, 4.98 mm) for a close fit or #7 (0.2010 in, 5.11 mm) for a free fit. Clearance depends on diameter only, so it is the same for UNC and UNF #10 screws.
What metric drill can I use for #10-24 UNC?
The closest metric drill is 3.8 mm (0.1496 in), giving about 75% thread on #10-24 UNC.
What is the difference between #10-24 UNC and #10-32 UNF?
Both are #10 diameter. #10-24 UNC has 24 threads per inch and uses a #25 tap drill (0.1495 in); #10-32 UNF has 32 TPI and uses a #21 drill (0.1590 in). The threads are not interchangeable.
Sources: ASME B1.1 — Unified inch screw threads (UN and UNR form) · Machinery's Handbook — tap drill sizes and percentage-of-thread formula · ASME B94.11M — twist drills (number and letter drill sizes) · Common machinist clearance-drill chart (close / free fit)
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