#12-28 UNF tap drill size
Use a #14 drill (0.1820 in, 4.62 mm) for #12-28 UNF. It is the standard chart size and leaves about 73% thread, enough for full practical strength without excessive tapping torque.
| Drill | inch | mm | Thread % | Typical use |
|---|---|---|---|---|
| #16 | 0.1770 | 4.5 | 84.1% | General purpose |
| #15 | 0.1800 | 4.57 | 77.6% | General purpose |
| 4.6 mm (metric drill) | 0.1811 | 4.6 | 75.2% | General purpose |
| #14 (chart drill) | 0.1820 | 4.62 | 73.3% | General purpose |
| #13 | 0.1850 | 4.7 | 66.8% | Hard or tough material, deep or blind holes |
| 3/16" | 0.1875 | 4.76 | 61.4% | Hard or tough material, deep or blind holes |
| #12 | 0.1890 | 4.8 | 58.2% | Low engagement: check strength |
| Thread | Tap drill | Tap drill (mm) | Clearance close / free |
|---|---|---|---|
| #8-36 UNF | #29 (0.1360 in) | 3.45 mm | #18 / #16 |
| #10-32 UNF | #21 (0.1590 in) | 4.04 mm | #9 / #7 |
| #12-28 UNF (this page) | #14 (0.1820 in) | 4.62 mm | #2 / #1 |
| 1/4-28 UNF | #3 (0.2130 in) | 5.41 mm | F / H |
| 5/16-24 UNF | I (0.2720 in) | 6.91 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 0.1981 in (5.03 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.
UNF (fine) threads have more threads per inch than UNC, a larger tensile stress area and better resistance to loosening, but they strip more easily in soft material and are more sensitive to drill size.
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 #12-28 UNF tap?
The standard tap drill for #12-28 UNF is a #14 drill, 0.1820 in (4.62 mm), which gives about 73% thread.
What clearance hole for a #12 UNF 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 #12-28 UNF?
The closest metric drill is 4.6 mm (0.1811 in), giving about 75% thread on #12-28 UNF.
What is the difference between #12-28 UNF and #12-24 UNC?
Both are #12 diameter. #12-28 UNF has 28 threads per inch and uses a #14 tap drill (0.1820 in); #12-24 UNC has 24 TPI and uses a #16 drill (0.1770 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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