#12-24 UNC tap drill size
Use a #16 drill (0.1770 in, 4.5 mm) for #12-24 UNC. It is the standard chart size and leaves about 72% thread, enough for full practical strength without excessive tapping torque.
| Drill | inch | mm | Thread % | Typical use |
|---|---|---|---|---|
| #18 | 0.1695 | 4.31 | 85.9% | Tight: high tapping torque, tap breakage risk |
| 11/64" | 0.1719 | 4.37 | 81.5% | General purpose |
| #17 | 0.1730 | 4.39 | 79.4% | General purpose |
| #16 (chart drill) | 0.1770 | 4.5 | 72.1% | General purpose |
| 4.5 mm (metric drill) | 0.1772 | 4.5 | 71.7% | General purpose |
| #15 | 0.1800 | 4.57 | 66.5% | Hard or tough material, deep or blind holes |
| #14 | 0.1820 | 4.62 | 62.8% | Hard or tough material, deep or blind holes |
| Thread | Tap drill | Tap drill (mm) | Clearance close / free |
|---|---|---|---|
| #8-32 UNC | #29 (0.1360 in) | 3.45 mm | #18 / #16 |
| #10-24 UNC | #25 (0.1495 in) | 3.8 mm | #9 / #7 |
| #12-24 UNC (this page) | #16 (0.1770 in) | 4.5 mm | #2 / #1 |
| 1/4-20 UNC | #7 (0.2010 in) | 5.11 mm | F / H |
| 5/16-18 UNC | F (0.2570 in) | 6.53 mm | P / Q |
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 0.1952 in (4.96 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 #12 screw: close fit #2 (0.2210 in, 5.61 mm), free fit #1 (0.2280 in, 5.79 mm).
FAQ
What size drill for a #12-24 UNC tap?
The standard tap drill for #12-24 UNC is a #16 drill, 0.1770 in (4.5 mm), which gives about 72% thread.
What clearance hole for a #12 UNC screw?
Use #2 (0.2210 in, 5.61 mm) for a close fit or #1 (0.2280 in, 5.79 mm) for a free fit. Clearance depends on diameter only, so it is the same for UNC and UNF #12 screws.
What metric drill can I use for #12-24 UNC?
The closest metric drill is 4.5 mm (0.1772 in), giving about 72% thread on #12-24 UNC.
What is the difference between #12-24 UNC and #12-28 UNF?
Both are #12 diameter. #12-24 UNC has 24 threads per inch and uses a #16 tap drill (0.1770 in); #12-28 UNF has 28 TPI and uses a #14 drill (0.1820 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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