5/16-18 UNC tap drill size
Use a letter F drill (0.2570 in, 6.53 mm) for 5/16-18 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 |
|---|---|---|---|---|
| D | 0.2460 | 6.25 | 92.1% | Tight: high tapping torque, tap breakage risk |
| E / 1/4" | 0.2500 | 6.35 | 86.6% | Tight: high tapping torque, tap breakage risk |
| 6.5 mm (metric drill) | 0.2559 | 6.5 | 78.4% | General purpose |
| F (chart drill) | 0.2570 | 6.53 | 76.9% | General purpose |
| G | 0.2610 | 6.63 | 71.4% | General purpose |
| 17/64" | 0.2656 | 6.75 | 65.0% | Hard or tough material, deep or blind holes |
| H | 0.2660 | 6.76 | 64.4% | Hard or tough material, deep or blind holes |
| Thread | Tap drill | Tap drill (mm) | Clearance close / free |
|---|---|---|---|
| #12-24 UNC | #16 (0.1770 in) | 4.5 mm | #2 / #1 |
| 1/4-20 UNC | #7 (0.2010 in) | 5.11 mm | F / H |
| 5/16-18 UNC (this page) | F (0.2570 in) | 6.53 mm | P / Q |
| 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" |
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 0.2847 in (7.23 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 5/16 screw: close fit letter P (0.3230 in, 8.2 mm), free fit letter Q (0.3320 in, 8.43 mm).
FAQ
What size drill for a 5/16-18 UNC tap?
The standard tap drill for 5/16-18 UNC is a letter F drill, 0.2570 in (6.53 mm), which gives about 77% thread.
What clearance hole for a 5/16 UNC screw?
Use letter P (0.3230 in, 8.2 mm) for a close fit or letter Q (0.3320 in, 8.43 mm) for a free fit. Clearance depends on diameter only, so it is the same for UNC and UNF 5/16 screws.
What metric drill can I use for 5/16-18 UNC?
The closest metric drill is 6.5 mm (0.2559 in), giving about 78% thread on 5/16-18 UNC.
What is the difference between 5/16-18 UNC and 5/16-24 UNF?
Both are 5/16 diameter. 5/16-18 UNC has 18 threads per inch and uses a F tap drill (0.2570 in); 5/16-24 UNF has 24 TPI and uses a I drill (0.2720 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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