#10-32 UNF tap drill size
Use a #21 drill (0.1590 in, 4.04 mm) for #10-32 UNF. It is the standard chart size and leaves about 76% thread, enough for full practical strength without excessive tapping torque.
| Drill | inch | mm | Thread % | Typical use |
|---|---|---|---|---|
| #23 | 0.1540 | 3.91 | 88.7% | Tight: high tapping torque, tap breakage risk |
| 5/32" | 0.1563 | 3.97 | 83.1% | General purpose |
| #22 | 0.1570 | 3.99 | 81.3% | General purpose |
| 4.0 mm (metric drill) | 0.1575 | 4.0 | 80.1% | General purpose |
| #21 (chart drill) | 0.1590 | 4.04 | 76.4% | General purpose |
| #20 | 0.1610 | 4.09 | 71.4% | General purpose |
| #19 | 0.1660 | 4.22 | 59.1% | Low engagement: check strength |
| Thread | Tap drill | Tap drill (mm) | Clearance close / free |
|---|---|---|---|
| #6-40 UNF | #33 (0.1130 in) | 2.87 mm | #27 / #25 |
| #8-36 UNF | #29 (0.1360 in) | 3.45 mm | #18 / #16 |
| #10-32 UNF (this page) | #21 (0.1590 in) | 4.04 mm | #9 / #7 |
| #12-28 UNF | #14 (0.1820 in) | 4.62 mm | #2 / #1 |
| 1/4-28 UNF | #3 (0.2130 in) | 5.41 mm | F / H |
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 0.1744 in (4.43 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 #10 screw: close fit #9 (0.1960 in, 4.98 mm), free fit #7 (0.2010 in, 5.11 mm).
FAQ
What size drill for a #10-32 UNF tap?
The standard tap drill for #10-32 UNF is a #21 drill, 0.1590 in (4.04 mm), which gives about 76% thread.
What clearance hole for a #10 UNF screw?
Use #9 (0.1960 in, 4.98 mm) for a close fit or #7 (0.2010 in, 5.11 mm) for a free fit. Clearance depends on diameter only, so it is the same for UNC and UNF #10 screws.
What metric drill can I use for #10-32 UNF?
The closest metric drill is 4.0 mm (0.1575 in), giving about 80% thread on #10-32 UNF.
What is the difference between #10-32 UNF and #10-24 UNC?
Both are #10 diameter. #10-32 UNF has 32 threads per inch and uses a #21 tap drill (0.1590 in); #10-24 UNC has 24 TPI and uses a #25 drill (0.1495 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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