1/2-20 UNF tap drill size
Use a 29/64" drill (0.4531 in, 11.51 mm) for 1/2-20 UNF. 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 |
|---|---|---|---|---|
| 11.5 mm (metric drill) | 0.4528 | 11.5 | 72.7% | General purpose |
| 29/64" (chart drill) | 0.4531 | 11.51 | 72.2% | General purpose |
| Thread | Tap drill | Tap drill (mm) | Clearance close / free |
|---|---|---|---|
| 3/8-24 UNF | Q (0.3320 in) | 8.43 mm | W / X |
| 7/16-20 UNF | 25/64" (0.3906 in) | 9.92 mm | 29/64" / 15/32" |
| 1/2-20 UNF (this page) | 29/64" (0.4531 in) | 11.51 mm | 33/64" / 17/32" |
| 9/16-18 UNF | 33/64" (0.5156 in) | 13.1 mm | 37/64" / 19/32" |
| 5/8-18 UNF | 37/64" (0.5781 in) | 14.68 mm | 41/64" / 21/32" |
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 0.4750 in (12.07 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 1/2 screw: close fit 33/64" (0.5156 in, 13.1 mm), free fit 17/32" (0.5313 in, 13.49 mm).
FAQ
What size drill for a 1/2-20 UNF tap?
The standard tap drill for 1/2-20 UNF is a 29/64" drill, 0.4531 in (11.51 mm), which gives about 72% thread.
What clearance hole for a 1/2 UNF screw?
Use 33/64" (0.5156 in, 13.1 mm) for a close fit or 17/32" (0.5313 in, 13.49 mm) for a free fit. Clearance depends on diameter only, so it is the same for UNC and UNF 1/2 screws.
What metric drill can I use for 1/2-20 UNF?
The closest metric drill is 11.5 mm (0.4528 in), giving about 73% thread on 1/2-20 UNF.
What is the difference between 1/2-20 UNF and 1/2-13 UNC?
Both are 1/2 diameter. 1/2-20 UNF has 20 threads per inch and uses a 29/64" tap drill (0.4531 in); 1/2-13 UNC has 13 TPI and uses a 27/64" drill (0.4219 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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