A #10-32 machine screw has a 0.190" (4.826 mm) major diameter and 32 threads per inch, which makes it the fine-thread (UNF) version of the #10 size. The tap drill for a standard 75% thread is #21 (0.1590"), a pan head measures 0.373" across and the machine screw nut takes a 3/8" spanner. The coarse-thread version is #10-24 UNC, which shares every head dimension and clearance hole but needs a different tap.
#10-32 thread and tap drill
| Property |
#10-32 (UNF) |
#10-24 (UNC) |
Reference |
| Nominal major diameter | 0.190" (4.826 mm) | 0.190" (4.826 mm) | ASME B1.1 |
| Threads per inch | 32 TPI | 24 TPI | ASME B1.1 |
| Thread pitch | 0.7938 mm | 1.0583 mm | 25.4 / TPI |
| Tap drill (75% engagement) | #21 (0.1590", 4.04 mm) | #25 (0.1495") | Standard tap drill chart |
| Tensile stress area | 0.0200 in² | 0.0175 in² | ASME B1.1 |
The number-size diameter comes from the ASME B1.1 formula 0.060" + 0.013" x screw number, so #10 is 0.060 + 0.130 = 0.190". The fine thread has a pitch of 0.794 mm against 1.058 mm for the coarse thread. That leaves more metal in the core, so the tensile stress area is 14% larger and the tap drill is bigger: #21 for #10-32 against #25 for #10-24.
#10-32 or #10-24: how to tell them apart
Both are the same diameter, so calipers on the thread will not separate them. Count the threads over a measured length, or use a thread pitch gauge: 32 crests per inch is #10-32, 24 is #10-24. A #10-32 screw will start in a #10-24 nut for a turn or so and then bind, and forcing it strips the thread.
The fine thread is the usual choice in thin material and in tapped holes in metal panels, because more threads engage in a short length. The coarse thread is quicker to assemble and more tolerant of dirt, paint and damaged threads.
#10-32 clearance hole sizes
| Fit |
Drill |
Hole diameter |
Basis |
| Close | #5 | 0.206" | ASME B18.2.8 |
| Normal | #2 | 0.221" | ASME B18.2.8 |
| Loose | B | 0.238" | ASME B18.2.8 |
| Close fit | #9 | 0.196" | Traditional drill chart |
| Free fit | #7 | 0.201" | Traditional drill chart |
Two sets of clearance drills are in use for #10 screws and they are not the same. ASME B18.2.8 gives #5, #2 and B for close, normal and loose fits. Older workshop drill charts give the tighter #9 and #7 as close and free fits. Clearance holes are sized to the screw diameter, not the thread, so #10-32 and #10-24 use the same drills.
#10-32 screw head dimensions by head type
| Head type |
Head diameter (max) |
Head height |
Drive |
Reference |
| Pan head, slotted | 0.373" | 0.110" | slot | ASME B18.6.3 |
| Pan head, Phillips | 0.373" | 0.133" | Phillips #2 | ASME B18.6.3 |
| Flat countersunk head, 82° | 0.362" | 0.116" | Phillips #2 | ASME B18.6.3 |
| Socket head cap screw | 0.312" | 0.190" | hex key 5/32" | ASME B18.3 |
| Button head socket screw | 0.361" | 0.101" | hex key 1/8" | ASME B18.3 |
Head diameters are maximums from the standard and are the same for both thread pitches. A Phillips pan head is taller than a slotted one: 0.133" against 0.110". The 82-degree flat head is the US standard. The 90-degree countersink used on metric screws to ISO 7046 will not seat an 82-degree head properly, which matters when imperial screws go into metric-drilled parts.
Metric near-equivalent
The nearest metric size to #10 is M5: 4.83 mm against 5.00 mm. Near is not equal. A #10-32 will not run in an M5 tapped hole and an M5 screw will strip a #10-32 thread. For the metric size see the M5 screw dimensions page.
Other number sizes
#4-40, #6-32, #8-32, #10-24, #12-24. Larger sizes move to fractional bolts: 1/4-20 and up. Full thread tables are on the UNC/UNF thread chart and the tap drill and clearance hole chart.
References
- ASME B1.1 - Unified Inch Screw Threads
- ASME B18.6.3 - Machine Screws, Tapping Screws and Machine Screw Nuts (Inch Series)
- ASME B18.3 - Socket Cap, Shoulder, Set Screws and Hex Keys (Inch Series)
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