threadsizes

5/8-18 Tap Drill Size

Unified fine (UNF) · 18 threads per inch · ASME B1.1, class 2B

For a 5/8 tap: 5/8-11 coarse takes a 17/32" drill, 5/8-18 fine takes a 37/64" drill.

What size drill bit for a 5/8-18 tap?

37/64"fractional drill
0.5781inch
14.69mm
65%thread
No 37/64" drill? Use 14.5 mm (75%).

To scale section of the tapped hole

Section of a 5/8-18 tapped hole to scale: 37/64" (0.5781) drill, thread length 1.5 times the diameter. ⌀0.6250 P 0.0556 (18 TPI) DRILL 37/64" Thread 1.5 × D +5 P drill
Drawn from the 5/8-18 basic profile. Hatched area is the part; the thread runs 1.5 × D and the drill goes 5 pitches deeper for a plug tap.

Drill options % of thread

Drill options for 5/8-18
DrillInchmmThread2B
14.5 mm 0.5709 14.50 75% ✓
37/64"Use 0.5781 14.69 65% ✗

75% (orange tick) is the usual target. Hard or gummy metals like stainless are often tapped at 60 to 65% to spare the tap; aluminum, brass and plastic take 75% or a little more. The last column checks the hole against the 2B minor diameter limits (0.565 in to 0.578 in).

5/8-18 UNF ASME B1.1, class 2B

Major diameter0.6250 in / 15.88 mm
Threads per inch18 TPI (pitch 0.0556 in)
Pitch diameter (basic)0.5889 in
Minor diameter 2B0.5649–0.5775 in
Thread-forming tap drill19/32" 0.5938 in

Clearance hole ASME B18.2.8

Close21/32"0.656 in
Normal11/16"0.688 in
Loose47/64"0.734 in

For a screw that passes through the part. Normal fit is the default; close fit locates the part more accurately but leaves no room for hole position error.

Hole depth rule of thumb

  • 0.625 inthread length for full strength in steel (1 × D)
  • 0.938 inin cast iron or brass (1.5 × D), up to 1.250 in in aluminum
  • +0.278 inextra drill depth below the thread for a plug tap (5 pitches), +0.111 in with a bottoming tap

Shop notes

Here the chart and the limits disagree, and the limits win. The usual fractional drill for 5/8-18 is 37/64, at 0.5781 in. The 2B maximum minor diameter is 0.5775 in, so 37/64 leaves the hole a hair oversize, with only about 65% thread. It is a tiny miss and plenty of bolts have gone into holes like that, but it is not a hole that meets the standard.

The drill that does is 14.5 mm. It gives about 75% thread and lands inside the limits, and it is the only drill in the usual 55% to 85% range that does. If you have a choice, buy that one drill and keep it with the tap.

Fine threads at this size also mean big taps. A 5/8 tap in steel takes real effort, and the 14.5 mm hole asks for more of it than the oversize 37/64. Cutting oil and a tap wrench long enough to keep it square matter more than any drill choice here.

The coarse 5/8-11 is drilled 17/32, which is under the 0.5649 in minimum minor diameter for 5/8-18. A wrong drill on this size is an expensive tap.