1/2″-20 UNF Thread

Recommended tap drill 29/64″

Tap drill Ø 0.4531″ / 11.509 mm

Major Ø 0.5000″ / 12.7 mm

Threads per inch 20

The chart recommendation gives approximately 72.2% thread engagement — the conventional target for average materials. It is not the only valid choice; see the options below.

Specification

Designation	1/2″-20 UNF
Thread family	Unified National Fine (UNF)
Nominal size	1/2
Basic major diameter	0.5000″ / 12.7 mm
Threads per inch / pitch	20 TPI (1.27 mm pitch)
Recommended tap drill	29/64″ (0.4531″ / 11.509 mm)

Tap drill options by thread engagement

Any of these standard drills produces a usable engagement for 1/2″-20 UNF; 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
11.3 mm	0.4449	11.3	84.9%
11.4 mm	0.4488	11.4	78.8%
11.5 mm	0.4528	11.5	72.7%
29/64″ — chart recommendation	0.4531	11.509	72.2%
11.6 mm	0.4567	11.6	66.7%
11.7 mm	0.4606	11.7	60.6%

Nearest drills to the tap drill in other systems

Drill	Size	vs 29/64″	Resulting engagement
11.5 mm	0.4528″ / 11.5 mm	−0.009 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	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	1/2-20 hex nut

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

Related threads

1/2″-13 UNC — same nominal size, 13 TPI (Unified National Coarse (UNC)) · compare
7/16″-20 UNF — next size down in UNF
9/16″-18 UNF — next size up in UNF

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.