The tap drill for 10-32 UNF is #21 (0.1590 in, 4.04 mm), which gives about 76% of full thread depth. The exact 75% hole is 0.1596 in (4.05 mm). The table lists every number, letter, fractional and metric drill that gives between 55% and 90% thread, so you can work from the bits you own.
| Drill | Type | in | mm | Thread | Note |
|---|---|---|---|---|---|
| 3.9 mm | metric | 0.1535 | 3.90 | 90% | deep, hard to tap |
| #23 | number | 0.1540 | 3.91 | 89% | deep, hard to tap |
| 5/32" | fraction | 0.1562 | 3.97 | 83% | deep, hard to tap |
| #22 | number | 0.1570 | 3.99 | 81% | deep, hard to tap |
| 4 mm | metric | 0.1575 | 4.00 | 80% | deep, hard to tap |
| #21 | number | 0.1590 | 4.04 | 76% | chart drill |
| #20 | number | 0.1610 | 4.09 | 71% | |
| 4.1 mm | metric | 0.1614 | 4.10 | 70% | |
| 4.2 mm | metric | 0.1654 | 4.20 | 61% | easier to tap |
| #19 | number | 0.1660 | 4.22 | 59% | easier to tap |
About the 10-32 thread
10-32 is the fine #10 thread (UNF). It is the thread on the green grounding screws used in metal electrical boxes, on many 19-inch equipment rack rails and cage nuts in North America, and on a great deal of instrument, optical and pneumatic hardware: most small air-tool and pneumatic fittings sold as “10-32” use this thread. Fine threads are chosen where the wall is thin, where adjustment needs to be precise, or where vibration might loosen a coarse thread.
#21 is the drill on nearly every chart. It is only 0.010 in larger than the #25 used for 10-24, so it is easy to grab the wrong one; label the two drills if you keep both threads in the same drawer.
10-32 thread dimensions
| Dimension | Value | Also |
|---|---|---|
| Nominal diameter | 0.1900 in | 4.83 mm |
| Threads per inch | 32 | pitch 0.0312 in (0.794 mm) |
| Basic pitch diameter | 0.1697 in | 4.310 mm |
| Basic minor diameter (nut) | 0.1562 in | 3.967 mm |
| Tensile stress area | 0.0200 in² | 12.90 mm² |
| #10-24 UNC (same diameter): tap drill | #25 (0.1495 in) | 75% thread |
Clearance hole for #10
The tapped hole is in the part that holds the thread; the part being clamped gets a clearance hole the bolt passes through. For #10 a normal fit is #5 (0.2055 in). Never drill the clearance part with the tap drill: the screw will not pass.
| Fit | Drill | Diameter |
|---|---|---|
| Close fit | #9 | 0.1960 in |
| Normal fit | #5 | 0.2055 in |
| Loose fit | #2 | 0.2210 in |
How deep to drill a blind 10-32 hole
| Part material | Full thread wanted | Drill depth (full diameter) | Depth to drill tip (118°) |
|---|---|---|---|
| Steel or cast iron (1 × diameter) | 0.190 in | 0.409 in (about 13/32″) | 0.457 in (about 15/32″) |
| Aluminium or brass (2 × diameter) | 0.380 in | 0.599 in (about 19/32″) | 0.647 in (about 21/32″) |
Wrench size for #10
- Machine screw hex nut: 3/8 in wrench
- Socket head cap screw: 5/32 in hex key
SAE bolt grades start at 1/4 in, so there is no graded torque for #10 machine screws; tighten to the maker’s figure or until snug. Over-tightening is the usual way small tapped threads strip.
Threads easily mistaken for 10-32
| Thread | Diameter (in) | Diameter (mm) | Pitch |
|---|---|---|---|
| #10-32 UNF (this page) | 0.1900 | 4.83 | 32 TPI (0.79 mm) |
| #10-24 UNC | 0.1900 | 4.83 | 24 TPI (1.06 mm) |
| M5 x 0.8 | 0.1969 | 5.00 | 0.8 mm (31.7 TPI) |
Tips for tapping 10-32
- A 10-32 screw in a 10-24 hole will start and stop. If you have a box of unknown #10 screws, test them in a known nut first.
- Rack rails: confirm the thread before tapping. Older North American racks are often 10-32 or 12-24; European racks use square holes and M6 cage nuts.
- The fine pitch means each turn advances only 0.031 in, so a 10-32 screw takes longer to drive but resists loosening better.
- Pneumatic 10-32 ports need a smooth, square face for the O-ring or gasket; spot-face the hole after tapping.
Frequently asked questions
What size drill bit for a 10-32 tap?
#21 (0.1590 in), which gives about 76% thread. The exact 75% size is 0.1596 in (4.05 mm).
How deep should a blind 10-32 hole be?
For a full thread of 1 × diameter in steel, drill about 0.409 in (about 13/32″) at full diameter with a plug tap, or 0.457 in (about 15/32″) to the drill tip; in aluminium, about 0.599 in (about 19/32″).
Is 10-32 the same as M5?
No. M5 x 0.8 is 0.197 in in diameter against 0.190 in, and 0.8 mm pitch is about 31.75 TPI, so the threads drift out of step. An M5 screw may start in a 10-32 hole and then bind.
What drill do I use for a 10-32 tap in mm?
A 4.0 mm drill (0.157 in) is the nearest metric size to #21 (0.159 in) and gives a slightly deeper thread, about 80%.
Sources and method
Every dimension and percentage on this page is computed from the formulas listed, not copied from another chart. Values are basic (nominal); real parts are made to tolerance classes (2A/2B for inch, 6g/6H for metric).
- ASME B1.1 and FED-STD-H28/2B (Unified inch), ISO 68-1 and ISO 261 (metric): basic thread profile and stress area formulas.
- Machinery’s Handbook (Industrial Press): percentage-of-thread relation used by our tap drill size calculator.
- ASME B94.11M twist drill series (number, letter, fractional); 0.1 mm metric drill steps.
- ISO 273 metric clearance holes; inch close, normal and loose fits as published in the ASME B18.2.8 clearance classes.
- SAE J429 and ISO 898-1 proof strengths; T = K × D × F torque relation (see our bolt torque chart for the full method).
- Wrench and hex key sizes: ASME B18.2.1, ASME B18.3, ASME B18.6.3, ISO 4014/4017, DIN 933 and ISO 4762 values as widely published.
More thread and fastener tools
- All thread and fastener charts
- UNC and UNF thread chart
- Metric thread pitch chart
- Tap drill chart (inch and metric)
- Clearance hole chart
- Nail size chart
- Wood screw size chart
- Lag screw pilot hole chart
- Bolt torque chart
- Wrench size chart
- Drill bit size chart
- Thread engagement chart
- Nail gun nail sizes by gauge
- Screw and nail size calculator
- Tap drill size calculator
- Clearance hole size calculator
- Tapped hole depth calculator
Last reviewed: 24 October 2026. How we build and check our data: methodology and corrections. Spotted an error? Please let us know.