1/4-20 UNC tap drill size
Use a #7 drill (0.2010 in, 5.11 mm) for 1/4-20 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 |
|---|---|---|---|---|
| #9 | 0.1960 | 4.98 | 83.1% | General purpose |
| #8 | 0.1990 | 5.05 | 78.5% | General purpose |
| 5.1 mm (metric drill) | 0.2008 | 5.1 | 75.8% | General purpose |
| #7 (chart drill) | 0.2010 | 5.11 | 75.4% | General purpose |
| 13/64" | 0.2031 | 5.16 | 72.2% | General purpose |
| #6 | 0.2040 | 5.18 | 70.8% | General purpose |
| #5 | 0.2055 | 5.22 | 68.5% | Hard or tough material, deep or blind holes |
| Thread | Tap drill | Tap drill (mm) | Clearance close / free |
|---|---|---|---|
| #10-24 UNC | #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 (this page) | #7 (0.2010 in) | 5.11 mm | F / H |
| 5/16-18 UNC | F (0.2570 in) | 6.53 mm | P / Q |
| 3/8-16 UNC | 5/16" (0.3125 in) | 7.94 mm | W / X |
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 0.2250 in (5.72 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 1/4 screw: close fit letter F (0.2570 in, 6.53 mm), free fit letter H (0.2660 in, 6.76 mm).
FAQ
What size drill for a 1/4-20 UNC tap?
The standard tap drill for 1/4-20 UNC is a #7 drill, 0.2010 in (5.11 mm), which gives about 75% thread.
What clearance hole for a 1/4 UNC screw?
Use letter F (0.2570 in, 6.53 mm) for a close fit or letter H (0.2660 in, 6.76 mm) for a free fit. Clearance depends on diameter only, so it is the same for UNC and UNF 1/4 screws.
What metric drill can I use for 1/4-20 UNC?
The closest metric drill is 5.1 mm (0.2008 in), giving about 76% thread on 1/4-20 UNC.
What is the difference between 1/4-20 UNC and 1/4-28 UNF?
Both are 1/4 diameter. 1/4-20 UNC has 20 threads per inch and uses a #7 tap drill (0.2010 in); 1/4-28 UNF has 28 TPI and uses a #3 drill (0.2130 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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