31/64 Drill Bit: Size, Metric Equivalent and Chuck Fit

A 31/64-inch drill bit has a nominal cutting diameter of 0.484375 inch, or exactly 12.303125 mm. Catalogs typically round the inch dimension to 0.4844. It is 1/64 inch smaller than 1/2 inch, not another name for a half-inch bit. Dormer Pramet’s R10A31/64 specification uses the rounded 0.4844-inch size.

If a drawing or kit calls for 31/64 inch, buy that diameter unless its allowed tolerance permits a substitute. Then check the shank diameter, material application and usable drilling depth separately.

Is there a metric substitute?

12.3 mm is very close, but not exactly equivalent. These differences are calculated from the nominal dimensions using 25.4 mm per inch:

Bit size	Nominal diameter in mm	Difference from 31/64 inch
15/32 inch	11.90625	0.396875 mm smaller
12.3 mm	12.30000	0.003125 mm smaller
31/64 inch	12.303125	—
1/2 inch	12.70000	0.396875 mm larger

A 12.3 mm bit may be an acceptable substitute when the specified hole tolerance allows it. A 1/2-inch bit makes a meaningfully larger nominal hole; do not substitute it automatically for a fitted tube, bushing or tap-drilled hole. For a precision fit, measure the finished bore rather than relying only on the bit’s label.

Will it fit your drill chuck?

The cutting diameter does not tell you the shank diameter. Two configurations illustrate the difference:

• Full-size straight shank: MSC lists Michigan Drill’s 314 31/64 with a 31/64-inch shank. That requires a chuck with sufficient capacity—typically a 1/2-inch chuck—and will not fit a 3/8-inch-capacity chuck. Michigan Drill specifications at MSC
• Reduced shank: Penn State Industries lists its PK-3164 with a 3/8-inch shank, despite the 31/64-inch cutting diameter. PK-3164 specifications

A reduced shank solves the size-fit problem; it does not establish that the drill has enough capacity for the hole diameter and workpiece material. Check the drill manual as well as the bit listing.

What is a 31/64 drill bit used for?

One chart-listed application is preparing a hole for a 9/16-12 UNC cutting tap. Newman Tools lists 31/64 inch for that thread in its approximately 75%-thread chart. Guhring’s HSCO drill catalog also associates its 12.3 mm/31/64 entry with 9/16-12. Confirm the complete thread designation and the exact tap’s hole-size recommendation before drilling.

For 1/2-20, Newman’s chart lists 29/64 inch, not 31/64. If your tap is marked 1/2 inch, use our 1/2-inch tap-drill guide to distinguish the pitches.

The size also appears in pen-making: Penn State Industries specifies its 31/64-inch HSS bit for Majestic Jr. and Olympian styles. Follow the particular kit’s instructions rather than assuming all pen kits need this diameter. Compatible kit styles

Check depth and material before buying

For steel or cast iron, Dormer Pramet specifies its R10A31/64 HSS drill with a 5-7/8-inch overall length, 4-3/8-inch flute length and 4×D usable drilling-depth rating, describing the application as drilling without pecking. For that stated drilling condition, the depth ratio works out to about 1.94 inches at this diameter—not the full flute length. R10A31/64 manufacturer data

That distinction matters when drilling a deep blank or blind hole. Buy by the required diameter, compatible shank and manufacturer-rated application and depth—not by overall length or coating color alone.