Use a 17/32-Inch Drill for a 5/8-11 Cutting Tap

Use a 17/32-inch drill bit for a conventional 5/8-11 UNC cutting tap unless the tap manufacturer or job specification gives a different hole size. The bit’s nominal diameter is 0.53125 inch, or 13.49375 mm.

A 13.5 mm bit is the closest practical metric substitute. At 0.53150 inch, it is only about 0.00025 inch (0.00625 mm) larger than 17/32 inch.

Setup	Hole size	Decimal equivalent	Metric equivalent
Common fractional size for a 5/8-11 cutting tap	17/32 in	0.53125 in	13.49375 mm
Close metric substitute	13.5 mm	0.53150 in	13.5 mm
Lower-engagement chart option	9/16 in	0.56250 in	14.2875 mm
Example forming-tap range	Manufacturer range	0.582–0.587 in	14.78–14.91 mm

Why some charts show a different decimal size

The 17/32-inch bit is a standardized fractional choice, not the exact result of every percentage-of-thread calculation. LittleMachineShop’s chart lists 17/32 inch for its 75% 5/8-11 selection and 9/16 inch for a lower, 50% selection in steel, stainless steel and iron (LittleMachineShop).

By comparison, Morse’s cutting-tool chart gives 0.5364 inch as its calculated hole for a 75% 5/8-11 cut thread (Morse Cutting Tools). A 17/32-inch bit is about 0.0052 inch smaller, so it leaves slightly more material for the tap than that calculated value.

For routine work, 17/32 inch remains the straightforward fractional answer. For a controlled production process, specified thread percentage or difficult material, use the hole size supplied by the tap maker and verify the drilled diameter rather than relying only on the bit marking.

Confirm the pitch and tap type

5/8-11 means a nominal 5/8-inch thread with 11 threads per inch. It is the coarse UNC pitch. Do not use this drill-size answer for 5/8-18 UNF; its finer pitch requires a different starting hole.

Tap type matters just as much:

• Cutting tap: Use 17/32 inch as the common chart size unless the tooling data specifies otherwise.
• Forming or roll tap: Do not use the cutting-tap drill size. A forming tap displaces material rather than producing chips and requires a larger, more tightly controlled hole.

For example, Guhring specifies a 0.582- to 0.587-inch drill range for one particular 5/8-11 general-purpose form tap (Guhring). That range applies to the identified tool, not every 5/8-11 forming tap. Check the data for the exact tap and work material.

Can you use 9/16 inch instead?

A 9/16-inch hole is 0.03125 inch larger than 17/32 inch. The larger hole leaves less material for the tap, which can reduce tapping load but also produces less thread engagement.

Use 9/16 inch only when the drawing, required thread engagement or tooling guidance permits it. It is not a close substitute for 17/32 inch in the way a 13.5 mm bit is.

Drill and tap the hole

1. Verify the callout. Confirm 5/8-11 UNC and identify whether the tap cuts or forms the thread.
2. Secure and locate the work. Clamp the part and spot the hole so the drill starts on location.
3. Drill the specified hole. For an ordinary cutting tap, use 17/32 inch or the close 13.5 mm equivalent. Match the drill material and geometry to the workpiece.
4. Check the finished diameter when it matters. Runout, drill wear and the work material can make the hole differ from the marked bit size.
5. Add a light entrance chamfer. Remove the sharp edge and give the tap a clean start without excessively enlarging the functional thread.
6. Start the tap square. Use suitable cutting fluid and a tap style appropriate to a through or blind hole. In a blind hole, account for the tap’s lead and the space needed to manage chips.
7. Inspect the thread. Use the specified thread plug gage when the thread must meet defined limits; trying a convenient bolt does not verify them.

For another coarse-thread example, a 3/8-16 cutting tap commonly uses a 5/16-inch drill.