1/2″-13 UNC Thread

Recommended tap drill 27/64″

Tap drill Ø 0.4219″ / 10.716 mm

Major Ø 0.5000″ / 12.7 mm

Threads per inch 13

The chart recommendation gives approximately 78.2% thread engagement — the conventional target for average materials. It is not the only valid choice; see the options below.

Specification

Designation	1/2″-13 UNC
Thread family	Unified National Coarse (UNC)
Nominal size	1/2
Basic major diameter	0.5000″ / 12.7 mm
Threads per inch / pitch	13 TPI (1.954 mm pitch)
Recommended tap drill	27/64″ (0.4219″ / 10.716 mm)

Tap drill options by thread engagement

Any of these standard drills produces a usable engagement for 1/2″-13 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
10.6 mm	0.4173	10.6	82.7%
10.7 mm	0.4213	10.7	78.8%
27/64″ — chart recommendation	0.4219	10.716	78.2%
10.8 mm	0.4252	10.8	74.9%
10.9 mm	0.4291	10.9	70.9%
11 mm	0.4331	11	67%
11.1 mm	0.4370	11.1	63%
7/16″	0.4375	11.112	62.5%

Nearest drills to the tap drill in other systems

Drill	Size	vs 27/64″	Resulting engagement
10.7 mm	0.4213″ / 10.7 mm	−0.016 mm	78.8%
Z	0.4130″ / 10.49 mm	−0.225 mm	87.1%

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	17/32″	0.5313	13.494
normal	9/16″	0.5625	14.287
loose	39/64″	0.6094	15.478

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)	3/4″
Head height	5/16″ (0.3125″)
Nut thickness	7/16″ (0.4375″)
Compatible nut	1/2-13 hex nut

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

Related threads

1/2″-20 UNF — same nominal size, 20 TPI (Unified National Fine (UNF)) · compare
7/16″-14 UNC — next size down in UNC
9/16″-12 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.