Tap Drill Size Chart

9/16-18 Tap Drill Size

Tap drill

33/64″ 0.5156 in13.10 mm

Use a 33/64 in drill (0.5156 in, 13.10 mm) for 65.0% thread.

Fine thread 9/16-24? Use 33/64 in. (0.5156 in, 13.10 mm, 86.6% thread)

The drill size for a 9/16-18 tap is 33/64 in (0.5156 in / 13.10 mm). If you are wondering what size drill bit a 9/16-18 tap needs, that is the one: it leaves 65.0% 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 9/16-18

Tap drills and clearance drills for 9/16-18 (UNF (Unified fine), 18 TPI). Percent of thread is the figure stored for each drill, which reproduces from the major diameter and TPI of 9/16-18.
Use Drill Decimal in mm % thread
Standard tap drill Cut tap, general use (standard chart drill) 33/64″ 0.5156 13.10 65.0%
Metric substitute Closest metric drill if you have no inch set 12.9 mm 0.5079 12.90 75.7%
Hard material Stainless, tool steel, anything that fights the tap 33/64″ 0.5156 13.10 65.0%
Form (roll) tap Theoretical 0.5379 in - starting point, check your tap maker's chart 17/32″ 0.5312 13.49 —

Clearance holes for a 9/16-18 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.

9/16" is not listed in ASME B18.2.8 (it jumps from 1/2" to 5/8"), so no standard clearance hole is published for this size. Take it from your drawing.

9/16-18 dimensions

Nominal dimensions for 9/16-18. 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 0.5625 14.29
Basic internal minor diameter 0.5024 12.76
Pitch (1 / TPI) 0.0556 1.41
Threads per inch 18 TPI

What the hole looks like

Cross-section of a 9/16-18 tapped hole, drawn to scale The drilled hole is 33/64″, 0.5156 inches or 13.10 millimetres across. The tap cuts outward from that wall to the major diameter of 0.5625 inches. A 100 percent thread would reach in to the basic minor diameter of 0.5024 inches, shown as a dashed line; the difference is why this drill gives 65.0% of thread. major 0.5625 in drill 33/64″ · 0.5156 in dashed = 100% thread at 0.5024 in
9/16-18 in section, to scale. The accent line is the thread the tap cuts from a 33/64″ hole; the dashed line is where the crest would sit at 100% thread. The gap between them is the 35.0% of thread this drill leaves behind.

Work out a different hole for 9/16-18

Pre-filled with 9/16-18. 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

13 mm0.5118 in70.3% thread

Theoretical hole 0.512 in (13 mm)

One size down, deeper thread12.9 mm 0.5079 in · 75.7%

One size up, easier tapping33/64″ 0.5156 in · 65.0%

For comparison, the published chart drill for 9/16-18 is 33/64″ at 65.0% thread.

Notes on 9/16-18

Middle pitch at this diameter
At 9/16 in the data holds 3 pitches: 9/16-12, 9/16-18, 9/16-24. 9/16-18 sits between 9/16-12 and 9/16-24.
Not the same as 9/16-12
9/16-12 and 9/16-18 share a 9/16 in major diameter but not a pitch, and they take different drills: 31/64 in for 9/16-12, 33/64 in for 9/16-18. The taps and the fasteners are not interchangeable.
What the standard drill gives you
33/64 in leaves 65.0% of thread (standard chart drill). That is in the normal 65-80% band most shop charts aim for.
Hard material drill
The chart's hard-material drill for 9/16-18 is the same 33/64 in as the standard drill, at 65.0%. 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: 17/32 in, against a theoretical 0.5379 in. Treat that as a starting point and check your tap maker's chart.
Clearance holes
9/16" is not listed in ASME B18.2.8 (it jumps from 1/2" to 5/8"), so this page lists no clearance hole for it. Drill the clearance hole from your drawing, or measure a bolt and allow for the fit you need.
Closest metric size
M14 x 2 is the nearest metric thread by diameter (14 mm against 14.29 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

M14 x 2 12 mm · 0.4724 in · 77.0% 14 mm major against 14.29 mm

Not interchangeable. M14 x 2 is the nearest size by diameter, nothing more. The pitches differ, so a M14 x 2 fastener will not run into a 9/16-18 hole and forcing one will strip both.

Guides for tapping 9/16-18

Longer reads on the decisions behind the numbers above.

All guides →

9/16-18 tap drill: common questions

What size drill bit do I need for a 9/16-18 tap?
A 33/64 in drill, 0.5156 in / 13.10 mm. That leaves 65.0% of thread, which is the standard chart figure for 9/16-18.
What is the drill size for a 9/16-18 tap in mm?
The 33/64 in tap drill measures 13.10 mm. If you only have metric drills, 12.9 mm is the closest, giving 75.7% thread.
Is 9/16-18 the same as 9/16-24?
No. Both are 9/16 in threads, but the pitch differs, so the drills differ: 33/64 in for 9/16-18 and 33/64 in for 9/16-24. The taps and the fasteners are not interchangeable.
What is the clearance hole for a 9/16-18 bolt?
9/16" is not listed in ASME B18.2.8 (it jumps from 1/2" to 5/8"), so there is no standard clearance hole to quote for this size. Take the clearance hole from your drawing instead.
A 4 × 6 in card with the drills for this size.