3/8-16 UNC tap drill size
Use a 5/16" drill (0.3125 in, 7.94 mm) for 3/8-16 UNC. It is the standard chart size and leaves about 77% thread, enough for full practical strength without excessive tapping torque.
| Drill | inch | mm | Thread % | Typical use |
|---|---|---|---|---|
| N | 0.3020 | 7.67 | 89.9% | Tight: high tapping torque, tap breakage risk |
| 7.9 mm (metric drill) | 0.3110 | 7.9 | 78.8% | General purpose |
| 5/16" (chart drill) | 0.3125 | 7.94 | 77.0% | General purpose |
| O | 0.3160 | 8.03 | 72.7% | General purpose |
| P | 0.3230 | 8.2 | 64.0% | Hard or tough material, deep or blind holes |
| 21/64" | 0.3281 | 8.33 | 57.7% | Low engagement: check strength |
| Thread | Tap drill | Tap drill (mm) | Clearance close / free |
|---|---|---|---|
| 1/4-20 UNC | #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 (this page) | 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 | 27/64" (0.4219 in) | 10.72 mm | 33/64" / 17/32" |
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 0.3438 in (8.73 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 3/8 screw: close fit letter W (0.3860 in, 9.8 mm), free fit letter X (0.3970 in, 10.08 mm).
FAQ
What size drill for a 3/8-16 UNC tap?
The standard tap drill for 3/8-16 UNC is a 5/16" drill, 0.3125 in (7.94 mm), which gives about 77% thread.
What clearance hole for a 3/8 UNC screw?
Use letter W (0.3860 in, 9.8 mm) for a close fit or letter X (0.3970 in, 10.08 mm) for a free fit. Clearance depends on diameter only, so it is the same for UNC and UNF 3/8 screws.
What metric drill can I use for 3/8-16 UNC?
The closest metric drill is 7.9 mm (0.3110 in), giving about 79% thread on 3/8-16 UNC.
What is the difference between 3/8-16 UNC and 3/8-24 UNF?
Both are 3/8 diameter. 3/8-16 UNC has 16 threads per inch and uses a 5/16" tap drill (0.3125 in); 3/8-24 UNF has 24 TPI and uses a Q drill (0.3320 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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