3/8″-16 UNC Thread

Recommended tap drill 5/16″

Tap drill Ø 0.3125″ / 7.938 mm

Major Ø 0.3750″ / 9.525 mm

Threads per inch 16

The chart recommendation gives approximately 77% thread engagement — the conventional target for average materials. It is not the only valid choice; see the options below.

Specification

Designation	3/8″-16 UNC
Thread family	Unified National Coarse (UNC)
Nominal size	3/8
Basic major diameter	0.3750″ / 9.525 mm
Threads per inch / pitch	16 TPI (1.587 mm pitch)
Recommended tap drill	5/16″ (0.3125″ / 7.938 mm)

Tap drill options by thread engagement

Any of these standard drills produces a usable engagement for 3/8″-16 UNC; smaller drills give stronger threads but higher tapping torque. Percentages use the conventional approximation (60° thread form).

★ = chart recommendation. A smaller drill leaves more of the thread height for the tap to form.

Drill	Decimal in	mm	Approx. engagement
7.8 mm	0.3071	7.8	83.6%
7.9 mm	0.3110	7.9	78.8%
5/16″ — chart recommendation	0.3125	7.938	77%
8 mm	0.3150	8	73.9%
O	0.3160	8.026	72.7%
8.1 mm	0.3189	8.1	69.1%
8.2 mm	0.3228	8.2	64.3%
P	0.3230	8.204	64%

Nearest drills to the tap drill in other systems

Drill	Size	vs 5/16″	Resulting engagement
7.9 mm	0.3110″ / 7.9 mm	−0.037 mm	78.8%
O	0.3160″ / 8.026 mm	+0.089 mm	72.7%

Clearance holes

For a bolt or screw of this size to pass through freely. Recommended drills per ASME B18.2.8: close, normal, and loose fits.

Schematic — proportions exaggerated for clarity; use the table values.

Fit	Drill / Ø	Decimal in	mm
close	25/64″	0.3906	9.922
normal	13/32″	0.4063	10.319
loose	27/64″	0.4219	10.716

Hex bolt & nut dimensions

For a standard hex cap screw (ASME B18.2.1) and finished hex nut (ASME B18.2.2) in this size.

Wrench / socket size (bolt head)	9/16″
Head height	15/64″ (0.2344″)
Nut thickness	21/64″ (0.3281″)
Compatible nut	3/8-16 hex nut

Basic (nominal) dimensions. See the wrench & socket size charts for all sizes and the metric ↔ SAE socket comparison.

Related threads

3/8″-24 UNF — same nominal size, 24 TPI (Unified National Fine (UNF)) · compare
5/16″-18 UNC — next size down in UNC
7/16″-14 UNC — next size up in UNC

Data provenance. ASME B1.1, Unified Inch Screw Threads (ASME). Machinery’s Handbook — tap drill charts (Industrial Press). ASME B18.2.8, Clearance Holes for Bolts, Screws, and Studs (ASME). ASME B18.2.1, Hex Cap Screws (head dimensions) (ASME). ASME B18.2.2, Finished Hex Nuts (ASME). Engagement percentages use the conventional approximation (engagement = (major − drill) ÷ (1.29904 × pitch)) and are guidance, not a tolerance specification. See all data sources.