9/16-12 UNC tap drill size
The standard tap drill for 9/16-12 UNC is a 31/64" drill (0.4844 in, 12.3 mm), giving about 72% thread. At this size pilot-drill first and power-tap, thread-mill or single-point the thread.
| Drill | inch | mm | Thread % | Typical use |
|---|---|---|---|---|
| 15/32" | 0.4688 | 11.91 | 86.6% | Tight: high tapping torque, tap breakage risk |
| 12.3 mm (metric drill) | 0.4843 | 12.3 | 72.3% | General purpose |
| 31/64" (chart drill) | 0.4844 | 12.3 | 72.2% | General purpose |
| 1/2" | 0.5000 | 12.7 | 57.7% | Low engagement: check strength |
| Thread | Tap drill | Tap drill (mm) | Clearance close / free |
|---|---|---|---|
| 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" |
| 9/16-12 UNC (this page) | 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" |
| 3/4-10 UNC | 21/32" (0.6563 in) | 16.67 mm | 49/64" / 25/32" |
Practical notes
Drilling 31/64" (0.4844 in) is a heavy cut: pilot-drill, then open up, and check the hole size before tapping. An undersize hole is the usual cause of a seized tap at this size.
Use a tapping head, rigid tapping or thread milling on a machine. For one-off parts, single-point threading on a lathe or thread milling avoids the cost and risk of a large broken tap.
Thread-forming taps need about 0.5208 in (13.23 mm, D − 0.5 × pitch); that is approximate, so check the tap maker's chart.
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 9/16 screw: close fit 37/64" (0.5781 in, 14.68 mm), free fit 19/32" (0.5938 in, 15.08 mm).
FAQ
What size drill for a 9/16-12 UNC tap?
The standard tap drill for 9/16-12 UNC is a 31/64" drill, 0.4844 in (12.3 mm), which gives about 72% thread.
What clearance hole for a 9/16 UNC screw?
Use 37/64" (0.5781 in, 14.68 mm) for a close fit or 19/32" (0.5938 in, 15.08 mm) for a free fit. Clearance depends on diameter only, so it is the same for UNC and UNF 9/16 screws.
What metric drill can I use for 9/16-12 UNC?
The closest metric drill is 12.3 mm (0.4843 in), giving about 72% thread on 9/16-12 UNC.
What is the difference between 9/16-12 UNC and 9/16-18 UNF?
Both are 9/16 diameter. 9/16-12 UNC has 12 threads per inch and uses a 31/64" tap drill (0.4844 in); 9/16-18 UNF has 18 TPI and uses a 33/64" drill (0.5156 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 Metal fabrication job Open the Tap & Drill Size Chart calculator