1/2-13 UNC tap drill size
Use a 27/64" drill (0.4219 in, 10.72 mm) for 1/2-13 UNC. It is the standard chart size and leaves about 78% thread, enough for full practical strength without excessive tapping torque.
| Drill | inch | mm | Thread % | Typical use |
|---|---|---|---|---|
| 13/32" | 0.4063 | 10.32 | 93.8% | Tight: high tapping torque, tap breakage risk |
| Z | 0.4130 | 10.49 | 87.1% | Tight: high tapping torque, tap breakage risk |
| 10.7 mm (metric drill) | 0.4213 | 10.7 | 78.8% | General purpose |
| 27/64" (chart drill) | 0.4219 | 10.72 | 78.2% | General purpose |
| 7/16" | 0.4375 | 11.11 | 62.5% | Hard or tough material, deep or blind holes |
| Thread | Tap drill | Tap drill (mm) | Clearance close / free |
|---|---|---|---|
| 3/8-16 UNC | 5/16" (0.3125 in) | 7.94 mm | W / X |
| 7/16-14 UNC | U (0.3680 in) | 9.35 mm | 29/64" / 15/32" |
| 1/2-13 UNC (this page) | 27/64" (0.4219 in) | 10.72 mm | 33/64" / 17/32" |
| 9/16-12 UNC | 31/64" (0.4844 in) | 12.3 mm | 37/64" / 19/32" |
| 5/8-11 UNC | 17/32" (0.5313 in) | 13.49 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 0.4615 in (11.72 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 1/2 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 1/2-13 UNC tap?
The standard tap drill for 1/2-13 UNC is a 27/64" drill, 0.4219 in (10.72 mm), which gives about 78% thread.
What clearance hole for a 1/2 UNC 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 1/2-13 UNC?
The closest metric drill is 10.7 mm (0.4213 in), giving about 79% thread on 1/2-13 UNC.
What is the difference between 1/2-13 UNC and 1/2-20 UNF?
Both are 1/2 diameter. 1/2-13 UNC has 13 threads per inch and uses a 27/64" tap drill (0.4219 in); 1/2-20 UNF has 20 TPI and uses a 29/64" drill (0.4531 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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