1/4-28 UNF tap drill size
Use a #3 drill (0.2130 in, 5.41 mm) for 1/4-28 UNF. It is the standard chart size and leaves about 80% thread, enough for full practical strength without excessive tapping torque.
| Drill | inch | mm | Thread % | Typical use |
|---|---|---|---|---|
| #4 | 0.2090 | 5.31 | 88.4% | Tight: high tapping torque, tap breakage risk |
| 5.4 mm (metric drill) | 0.2126 | 5.4 | 80.6% | General purpose |
| #3 (chart drill) | 0.2130 | 5.41 | 79.8% | General purpose |
| 7/32" | 0.2188 | 5.56 | 67.4% | Hard or tough material, deep or blind holes |
| #2 | 0.2210 | 5.61 | 62.5% | Hard or tough material, deep or blind holes |
| Thread | Tap drill | Tap drill (mm) | Clearance close / free |
|---|---|---|---|
| #10-32 UNF | #21 (0.1590 in) | 4.04 mm | #9 / #7 |
| #12-28 UNF | #14 (0.1820 in) | 4.62 mm | #2 / #1 |
| 1/4-28 UNF (this page) | #3 (0.2130 in) | 5.41 mm | F / H |
| 5/16-24 UNF | I (0.2720 in) | 6.91 mm | P / Q |
| 3/8-24 UNF | Q (0.3320 in) | 8.43 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 0.2321 in (5.9 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 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 1/4-28 UNF tap?
The standard tap drill for 1/4-28 UNF is a #3 drill, 0.2130 in (5.41 mm), which gives about 80% thread.
What clearance hole for a 1/4 UNF 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 1/4-28 UNF?
The closest metric drill is 5.4 mm (0.2126 in), giving about 81% thread on 1/4-28 UNF.
What is the difference between 1/4-28 UNF and 1/4-20 UNC?
Both are 1/4 diameter. 1/4-28 UNF has 28 threads per inch and uses a #3 tap drill (0.2130 in); 1/4-20 UNC has 20 TPI and uses a #7 drill (0.2010 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)
Need it made?
Post the job free — verified makers quote you.
Post a CNC machining job Open the Tap & Drill Size Chart calculator