Verified by Rachel Mayfield, Supply Chain Analyst - August 2026
A #12-24 machine screw has a 0.216" (5.49 mm) major diameter and 24 threads per inch. The tap drill for a standard 75% thread is #16 (0.177"), the normal-fit clearance drill is #1 (0.228"), a pan head measures 0.425" across, and the matching machine screw nut takes a 7/16" wrench. The fine-thread version is #12-28 UNF.
#12-24 thread, tap drill and torque
| Property | #12-24 (UNC) | #12-28 (UNF) | Reference |
|---|
| Nominal major diameter | 0.216" (5.49 mm) | 0.216" (5.49 mm) | ASME B1.1 |
| Threads per inch | 24 TPI | 28 TPI | ASME B1.1 |
| Thread pitch | 1.06 mm | 0.91 mm | 1 / TPI |
| Tap drill (75% engagement) | #16 (0.177") | #14 (0.182") | Standard tap drill chart |
| Tensile stress area | 0.0242 in² | 0.0258 in² | ASME B1.1 |
| Typical torque, 18-8 stainless, dry | not listed in the common charts | not listed in the common charts | Published torque charts |
The number-size diameter comes from the ASME B1.1 formula 0.060" + 0.013" x screw number, so #12 is 0.060 + 0.156 = 0.216". Torque values are typical dry figures for 18-8 stainless or low-carbon steel screws from the common published charts, in inch-pounds; a screwdriver-driven fastener this size is normally tightened by feel and these numbers are a ceiling, not a target.
#12-24 clearance hole sizes
| Fit | Drill | Diameter | Reference |
|---|
| Close | #2 | 0.221" | ASME B18.2.8 |
| Normal | #1 | 0.228" | ASME B18.2.8 |
| Loose | D | 0.246" | ASME B18.2.8 |
Use the normal fit for general assembly. Close fit is for located parts where the screw is doing alignment duty; loose fit is for painted or plated parts, stacked tolerances and anything that has to go together in the field.
#12-24 screw head dimensions by head type
| Head type | Head diameter (max) | Head height | Drive | Reference |
|---|
| Pan head (slotted or Phillips) | 0.425" | 0.125" | Phillips #3 | ASME B18.6.3 |
| Flat countersunk head, 82° | 0.438" | 0.132" | Phillips #3 | ASME B18.6.3 |
ASME B18.3 does not list a #12 socket head cap screw. The series runs #0 to #10 and then jumps to 1/4. If a design calls for a socket head at this diameter, the usual substitution is #10-24 or 1/4-20, whichever the mating part allows.
Head diameters are maximums from the standard. Slotted and Phillips pan heads share the same envelope. 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.
Where #12 screws are used
Typical applications: older electrical fittings, marine hardware and some appliance work; increasingly replaced by 1/4-20. The machine screw nut for this size is 7/16" across flats (ASME B18.6.3).
Metric near-equivalent
The nearest metric size to #12 is M5.5 is the arithmetic match (5.49 mm); in practice M5 or M6. Near is not equal: a #12-24 will not run in a metric tapped hole and a metric screw will strip a #12 thread. For metric cards see MLC's M3, M4 and M5 screw dimension pages.
Other number sizes
#4-40, #6-32, #8-32, #10-24. Larger sizes move to fractional bolts: 1/4-20 and up. Full thread tables on the UNC/UNF thread chart and the tap drill and clearance hole chart.
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