Tap Drill Size Chart

1/2-28 Tap Drill Size

Tap drill

15/32″ 0.4688 in11.91 mm

Use a 15/32 in drill (0.4688 in, 11.91 mm) for 67.2% thread.

The drill size for a 1/2-28 tap is 15/32 in (0.4688 in / 11.91 mm). If you are wondering what size drill bit a 1/2-28 tap needs, that is the one: it leaves 67.2% 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 1/2-28

Tap drills and clearance drills for 1/2-28 (UNEF (Unified extra fine), 28 TPI). Percent of thread is the figure stored for each drill, which reproduces from the major diameter and TPI of 1/2-28.
Use Drill Decimal in mm % thread
Standard tap drill Cut tap, general use (nearest to 75% (computed)) 15/32″ 0.4688 11.91 67.2%
Metric substitute Closest metric drill if you have no inch set 11.8 mm 0.4646 11.80 76.3%
Hard material Stainless, tool steel, anything that fights the tap 15/32″ 0.4688 11.91 67.2%
Form (roll) tap Theoretical 0.4842 in - starting point, check your tap maker's chart 31/64″ 0.4844 12.30 —

Clearance holes for a 1/2-28 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 1/2-28 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

1/2-28 dimensions

Nominal dimensions for 1/2-28. 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 0.4613 11.72
Pitch (1 / TPI) 0.0357 0.91
Threads per inch 28 TPI

What the hole looks like

Cross-section of a 1/2-28 tapped hole, drawn to scale The drilled hole is 15/32″, 0.4688 inches or 11.91 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 0.4613 inches, shown as a dashed line; the difference is why this drill gives 67.2% of thread. major 0.500 in drill 15/32″ · 0.4688 in dashed = 100% thread at 0.4613 in
1/2-28 in section, to scale. The accent line is the thread the tap cuts from a 15/32″ hole; the dashed line is where the crest would sit at 100% thread. The gap between them is the 32.8% of thread this drill leaves behind.

Work out a different hole for 1/2-28

Pre-filled with 1/2-28. 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

11.9 mm0.4685 in67.9% thread

Theoretical hole 0.4675 in (11.88 mm)

One size down, deeper thread11.8 mm 0.4646 in · 76.3%

One size up, easier tapping15/32″ 0.4688 in · 67.2%

For comparison, the published chart drill for 1/2-28 is 15/32″ at 67.2% thread.

Notes on 1/2-28

Finest pitch at this diameter
1/2-28 is the finest of the standard pitches at 1/2 in. Fine threads suit thin walls and joints that see vibration, but the hole size matters more: the window between a weak thread and a broken tap is narrower than on 1/2-13.
Not the same as 1/2-13
1/2-13 and 1/2-28 share a 1/2 in major diameter but not a pitch, and they take different drills: 27/64 in for 1/2-13, 15/32 in for 1/2-28. The taps and the fasteners are not interchangeable.
What the standard drill gives you
15/32 in leaves 67.2% of thread (nearest to 75% (computed)). That is in the normal 65-80% band most shop charts aim for.
Hard material drill
The chart's hard-material drill for 1/2-28 is the same 15/32 in as the standard drill, at 67.2%. There is no larger drill in the set that stays in a sensible range at this pitch.
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 0.4842 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 1/2-28 hole and forcing one will strip both.

Guides for tapping 1/2-28

Longer reads on the decisions behind the numbers above.

All guides →

1/2-28 tap drill: common questions

What size drill bit do I need for a 1/2-28 tap?
A 15/32 in drill, 0.4688 in / 11.91 mm. That leaves 67.2% of thread, which is the standard chart figure for 1/2-28.
What is the drill size for a 1/2-28 tap in mm?
The 15/32 in tap drill measures 11.91 mm. If you only have metric drills, 11.8 mm is the closest, giving 76.3% thread.
Is 1/2-28 the same as 1/2-13?
No. Both are 1/2 in threads, but the pitch differs, so the drills differ: 15/32 in for 1/2-28 and 27/64 in for 1/2-13. The taps and the fasteners are not interchangeable.
What is the clearance hole for a 1/2-28 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.