M12 Tap Drill Bit Size: 10.2 mm for the Coarse Thread

Use a 10.2 mm drill bit before tapping M12 × 1.75 with a conventional cutting tap. That is the standard coarse-pitch M12 thread, and the size listed in Sutton Tools’ tapping chart. An OSG M12 × 1.75 cutting-tap listing also specifies 10.2 mm.

Check the full thread marking before drilling: M12 × 1.5 and M12 × 1.25 need different holes, and forming taps need larger holes than cutting taps.

M12 cutting-tap drill sizes by pitch

Thread marking	Pitch	Common cutting-tap drill size
M12 × 1.75 — coarse	1.75 mm	10.2 mm
M12 × 1.5 — fine	1.5 mm	10.5 mm
M12 × 1.25 — fine	1.25 mm	10.8 mm

These are suggested sizes from Sutton Tools’ chart. Haas also specifies 10.5 mm for its M12 × 1.5 spiral-point cutting tap. If your exact tap’s instructions specify a different hole for the material or thread tolerance, follow those instructions.

In the marking M12 × 1.75, 12 is the nominal thread diameter in millimetres; 1.75 is the distance between adjacent threads. The drill must leave material for the tap to cut, so 12 mm is not the tapping-drill size.

A useful cutting-tap estimate is:

Drill diameter ≈ nominal thread diameter − pitch

For M12 × 1.75, that gives 12 − 1.75 = 10.25 mm. This is a sizing estimate, not a reason to disregard the published 10.2 mm recommendation. Sutton’s technical guide provides the formula and explains that material and permissible hole tolerance also affect the choice.

Can you use a 13/32-inch bit?

13/32 inch is a documented inch alternative for an M12 × 1.75 cutting tap, appearing alongside 10.2 mm in Carbide Depot’s metric tap chart.

It is not an exact conversion:

• 10.2 mm is approximately 0.4016 inch.
• 13/32 inch is 0.40625 inch, or 10.31875 mm.
• The fractional bit is therefore about 0.119 mm larger in nominal diameter.

For a new purchase, choose 10.2 mm unless your tap’s instructions specify otherwise. Use the fractional alternative only where the permitted hole size allows it, especially on a dimensioned part. Do not substitute 3/8 inch simply because it is nearby in the drill index: it measures only 9.525 mm. An undersized hole increases tapping load; Sutton warns that an excessively small tapping hole can overload and break the tap.

Forming taps are a separate lookup

Do not use the 10.2 mm cutting-tap hole for an M12 × 1.75 forming or roll tap.

Haas’s roll-tap chart lists 11.2 mm for M12 × 1.75, but explicitly calls its recommendations starting points. It recommends drilling and reaming, with adjustments for material, hardness and tolerance. Use the hole specification for your particular forming tap rather than treating 11.2 mm as universal.

Before you drill

Confirm the pitch and tap type, then check the bit’s cutting diameter—not just its shank size. Clamp the work securely and keep the drill and tap aligned. For material-specific bit selection, see our guide to drill bits for metal.

For a blind hole, allow depth beyond the required full threads for the tap’s chamfer and bottom clearance. A bottoming tap still has a chamfer: Sutton’s tap geometry guide lists short chamfers for bottoming taps intended for blind holes. Check the actual tap geometry before setting the drilling depth.