Tap Drill Size Chart

1/2-20 Tap Drill Size

Tap drill

29/64″ 0.4531 in11.51 mm

Use a 29/64 in drill (0.4531 in, 11.51 mm) for 72.2% thread.

Fine thread 1/2-28? Use 15/32 in. (0.4688 in, 11.91 mm, 67.2% thread)

The drill size for a 1/2-20 tap is 29/64 in (0.4531 in / 11.51 mm). If you are wondering what size drill bit a 1/2-20 tap needs, that is the one: it leaves 72.2% of thread, which is the figure standard tap and drill charts are built around. Below are the other holes you might want for this size - a larger drill for tough material, the form-tap hole, and the clearance holes for the bolt that goes in.

Every drill for 1/2-20

Tap drills and clearance drills for 1/2-20 (UNF (Unified fine), 20 TPI). Percent of thread is the figure stored for each drill, which reproduces from the major diameter and TPI of 1/2-20.
Use Drill Decimal in mm % thread
Standard tap drill Cut tap, general use (standard chart drill) 29/64″ 0.4531 11.51 72.2%
Metric substitute Closest metric drill if you have no inch set 11.5 mm 0.4528 11.50 72.7%
Hard material Stainless, tool steel, anything that fights the tap 15/32″ 0.4688 11.91 48.0%
Form (roll) tap Theoretical 0.4779 in - starting point, check your tap maker's chart 31/64″ 0.4844 12.30 —

Clearance holes for a 1/2-20 bolt

A clearance hole is not tapped. It is the hole in the part the bolt passes through, so it is always larger than the thread.

Clearance holes for 1/2-20 per ASME B18.2.8. Clearance depends on the bolt diameter alone, so every 1/2 thread shares these figures.
Fit Drill Drill in Hole min in Hole max in Hole mm
Close 17/32″ 0.5312 0.531 0.538 13.49–13.67
Normal 9/16″ 0.5625 0.562 0.572 14.27–14.53
Loose 39/64″ 0.6094 0.609 0.625 15.47–15.88

1/2-20 dimensions

Nominal dimensions for 1/2-20. The basic internal minor diameter is the hole a 100% thread would start from, which is why no tap drill is ever that small.
Dimension Inch mm
Major diameter 0.500 12.70
Basic internal minor diameter 0.4459 11.33
Pitch (1 / TPI) 0.050 1.27
Threads per inch 20 TPI

What the hole looks like

Cross-section of a 1/2-20 tapped hole, drawn to scale The drilled hole is 29/64″, 0.4531 inches or 11.51 millimetres across. The tap cuts outward from that wall to the major diameter of 0.500 inches. A 100 percent thread would reach in to the basic minor diameter of 0.4459 inches, shown as a dashed line; the difference is why this drill gives 72.2% of thread. major 0.500 in drill 29/64″ · 0.4531 in dashed = 100% thread at 0.4459 in
1/2-20 in section, to scale. The accent line is the thread the tap cuts from a 29/64″ hole; the dashed line is where the crest would sit at 100% thread. The gap between them is the 27.8% of thread this drill leaves behind.

Work out a different hole for 1/2-20

Pre-filled with 1/2-20. Change the material, the tap type, or which drills you actually own, and it picks the closest real drill and tells you the percent of thread you end up with. The tables above are the published figures and do not depend on this.

The general-purpose target

Tap type
Drills you have

Best drill you have, cut tap, 70% target

29/64″0.4531 in11.51 mm72.2% thread

Theoretical hole 0.4545 in (11.55 mm)

One size down, deeper thread11.5 mm 0.4528 in · 72.7%

One size up, easier tapping11.6 mm 0.4567 in · 66.7%

For comparison, the published chart drill for 1/2-20 is 29/64″ at 72.2% thread.

Notes on 1/2-20

Middle pitch at this diameter
At 1/2 in the data holds 3 pitches: 1/2-13, 1/2-20, 1/2-28. 1/2-20 sits between 1/2-13 and 1/2-28.
Not the same as 1/2-13
1/2-13 and 1/2-20 share a 1/2 in major diameter but not a pitch, and they take different drills: 27/64 in for 1/2-13, 29/64 in for 1/2-20. The taps and the fasteners are not interchangeable.
What the standard drill gives you
29/64 in leaves 72.2% of thread (standard chart drill). That is in the normal 65-80% band most shop charts aim for.
Stainless, tool steel and other tough material
15/32 in opens the hole to 48.0% engagement. Less material for the tap to cut means less torque and less heat, at the cost of some thread depth.
Using a form (roll) tap
A form tap displaces metal instead of cutting it, so it needs a larger hole: 31/64 in, against a theoretical 0.4779 in. Treat that as a starting point and check your tap maker's chart.
Closest metric size
M12 x 1.75 is the nearest metric thread by diameter (12 mm against 12.7 mm). It is a planning reference only. The two are not interchangeable: the pitches differ, and a bolt from one will not run into a hole tapped for the other.

Related sizes

Same diameter, different pitch

Next size up and down

Closest metric size

M12 x 1.75 10.2 mm · 0.4016 in · 79.2% 12 mm major against 12.7 mm

Not interchangeable. M12 x 1.75 is the nearest size by diameter, nothing more. The pitches differ, so a M12 x 1.75 fastener will not run into a 1/2-20 hole and forcing one will strip both.

Guides for tapping 1/2-20

Longer reads on the decisions behind the numbers above.

All guides →

1/2-20 tap drill: common questions

What size drill bit do I need for a 1/2-20 tap?
A 29/64 in drill, 0.4531 in / 11.51 mm. That leaves 72.2% of thread, which is the standard chart figure for 1/2-20.
What is the drill size for a 1/2-20 tap in mm?
The 29/64 in tap drill measures 11.51 mm. If you only have metric drills, 11.5 mm is the closest, giving 72.7% thread.
What drill should I use for 1/2-20 in stainless steel?
Use 15/32 in instead of 29/64 in. It leaves 48.0% of thread rather than 72.2%, so there is less material for the tap to cut and less chance of snapping it.
Is 1/2-20 the same as 1/2-28?
No. Both are 1/2 in threads, but the pitch differs, so the drills differ: 29/64 in for 1/2-20 and 15/32 in for 1/2-28. The taps and the fasteners are not interchangeable.
What is the clearance hole for a 1/2-20 bolt?
For a normal fit, 9/16 in (0.5625 in), with the finished hole between 0.562 in and 0.572 in per ASME B18.2.8. A clearance hole is not tapped; the bolt passes straight through it.
A 4 × 6 in card with the drills for this size.