Tap Drill Size Chart

5/8-18 Tap Drill Size

Tap drill

37/64″ 0.5781 in14.68 mm

Use a 37/64 in drill (0.5781 in, 14.68 mm) for 65.0% thread.

Fine thread 5/8-24? Use 37/64 in. (0.5781 in, 14.68 mm, 86.6% thread)

The drill size for a 5/8-18 tap is 37/64 in (0.5781 in / 14.68 mm). If you are wondering what size drill bit a 5/8-18 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 5/8-18

Tap drills and clearance drills for 5/8-18 (UNF (Unified fine), 18 TPI). Percent of thread is the figure stored for each drill, which reproduces from the major diameter and TPI of 5/8-18.
Use Drill Decimal in mm % thread
Standard tap drill Cut tap, general use (standard chart drill) 37/64″ 0.5781 14.68 65.0%
Metric substitute Closest metric drill if you have no inch set 14.5 mm 0.5709 14.50 75.0%
Hard material Stainless, tool steel, anything that fights the tap 37/64″ 0.5781 14.68 65.0%
Form (roll) tap Theoretical 0.6004 in - starting point, check your tap maker's chart 19/32″ 0.5938 15.08 —

Clearance holes for a 5/8-18 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.

Clearance holes for 5/8-18 per ASME B18.2.8. Clearance depends on the bolt diameter alone, so every 5/8 thread shares these figures.
Fit Drill Drill in Hole min in Hole max in Hole mm
Close 21/32″ 0.6562 0.656 0.663 16.66–16.84
Normal 11/16″ 0.6875 0.688 0.698 17.48–17.73
Loose 47/64″ 0.7344 0.734 0.754 18.64–19.15

5/8-18 dimensions

Nominal dimensions for 5/8-18. 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 0.625 15.88
Basic internal minor diameter 0.5649 14.35
Pitch (1 / TPI) 0.0556 1.41
Threads per inch 18 TPI

What the hole looks like

Cross-section of a 5/8-18 tapped hole, drawn to scale The drilled hole is 37/64″, 0.5781 inches or 14.68 millimetres across. The tap cuts outward from that wall to the major diameter of 0.625 inches. A 100 percent thread would reach in to the basic minor diameter of 0.5649 inches, shown as a dashed line; the difference is why this drill gives 65.0% of thread. major 0.625 in drill 37/64″ · 0.5781 in dashed = 100% thread at 0.5649 in
5/8-18 in section, to scale. The accent line is the thread the tap cuts from a 37/64″ 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 5/8-18

Pre-filled with 5/8-18. 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

14.6 mm0.5748 in69.6% thread

Theoretical hole 0.5745 in (14.59 mm)

One size down, deeper thread14.5 mm 0.5709 in · 75.0%

One size up, easier tapping37/64″ 0.5781 in · 65.0%

For comparison, the published chart drill for 5/8-18 is 37/64″ at 65.0% thread.

Notes on 5/8-18

Middle pitch at this diameter
At 5/8 in the data holds 3 pitches: 5/8-11, 5/8-18, 5/8-24. 5/8-18 sits between 5/8-11 and 5/8-24.
Not the same as 5/8-11
5/8-11 and 5/8-18 share a 5/8 in major diameter but not a pitch, and they take different drills: 17/32 in for 5/8-11, 37/64 in for 5/8-18. The taps and the fasteners are not interchangeable.
What the standard drill gives you
37/64 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 5/8-18 is the same 37/64 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: 19/32 in, against a theoretical 0.6004 in. Treat that as a starting point and check your tap maker's chart.
Closest metric size
M16 x 2 is the nearest metric thread by diameter (16 mm against 15.88 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

M16 x 2 14 mm · 0.5512 in · 77.0% 16 mm major against 15.88 mm

Not interchangeable. M16 x 2 is the nearest size by diameter, nothing more. The pitches differ, so a M16 x 2 fastener will not run into a 5/8-18 hole and forcing one will strip both.

Guides for tapping 5/8-18

Longer reads on the decisions behind the numbers above.

All guides →

5/8-18 tap drill: common questions

What size drill bit do I need for a 5/8-18 tap?
A 37/64 in drill, 0.5781 in / 14.68 mm. That leaves 65.0% of thread, which is the standard chart figure for 5/8-18.
What is the drill size for a 5/8-18 tap in mm?
The 37/64 in tap drill measures 14.68 mm. If you only have metric drills, 14.5 mm is the closest, giving 75.0% thread.
Is 5/8-18 the same as 5/8-24?
No. Both are 5/8 in threads, but the pitch differs, so the drills differ: 37/64 in for 5/8-18 and 37/64 in for 5/8-24. The taps and the fasteners are not interchangeable.
What is the clearance hole for a 5/8-18 bolt?
For a normal fit, 11/16 in (0.6875 in), with the finished hole between 0.688 in and 0.698 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.