Use a 2.5 mm Drill for a Standard M3 Cutting Tap

Use a 2.5 mm drill bit before tapping M3 × 0.5 threads with a conventional cutting tap. That is the standard coarse-pitch M3 combination listed in Optimas’s tapping chart and Carbide Depot’s metric tap chart.

Check the tap’s pitch and whether it cuts or forms threads before drilling. A fine-pitch M3 tap needs a different hole, and a forming tap needs a substantially larger one.

M3 drill sizes by thread and tap type

Thread and operation	Drill size	When to use it
M3 × 0.5, cutting tap	2.5 mm	Standard coarse-pitch M3 thread
M3 × 0.35, cutting tap	2.65 mm	Fine-pitch M3 thread
M3 × 0.5, forming tap	Check exact tap data; around 2.8 mm	Not interchangeable with the cutting-tap hole

The two cutting-tap sizes come from Optimas’s coarse- and fine-thread tables. For forming taps, Guhring’s chart lists 2.80 mm for M3 × 0.5 at 57.5% thread and 2.78 mm at 65% thread. Those values correspond to different thread percentages—not one universal forming-tap recommendation.

For a quick cutting-tap estimate, subtract pitch from nominal diameter:

• M3 × 0.5: 3.0 − 0.5 = 2.5 mm
• M3 × 0.35: 3.0 − 0.35 = 2.65 mm

Use that as a lookup check, not a substitute for the exact tool’s specifications. For example, OSG’s M3 × 0.5 CC-SUS cutting tap lists a tap-drill range of 2.459–2.599 mm rather than a single diameter.

What if you only have inch drill bits?

A 2.5 mm bit is approximately 0.09843 inch. Numbered bits offer much closer alternatives than common fractional bits.

Bit	Nominal diameter in mm*	Difference from 2.5 mm
No. 40 — 0.0980 in	2.4892	0.0108 mm smaller
No. 39 — 0.0995 in	2.5273	0.0273 mm larger
3/32 in	2.3813	0.1188 mm smaller
7/64 in	2.7781	0.2781 mm larger

Metric values and differences are calculated from nominal inch dimensions and rounded to four decimal places.

Imperial Supplies’ chart lists No. 40 for M3 × 0.5, while Carbide Depot lists No. 39. Either is a documented chart alternative; buy the 2.5 mm bit if you want the straightforward metric choice.

Do not treat 3/32 or 7/64 inch as equivalent. The smaller hole increases the material the tap must cut; the larger hole leaves shallower threads. Guhring explains that drill diameter affects thread depth, tapping torque and tool life.

A tapped hole is not a screw-clearance hole

If the M3 screw should pass through the part and engage a nut or another threaded component, use a clearance hole instead. Optimas lists:

• 3.2 mm: close fit
• 3.4 mm: medium fit
• 3.6 mm: free fit

These are not tap-drill sizes. Likewise, drilling 3.0 mm simply because the tap says “M3” removes too much material for the intended cutting-tap thread.

Before hand-tapping the 2.5 mm hole

Clamp the work firmly, drill square to the surface, and use a tap wrench that holds the tap securely. Apply a lubricant suitable for the tap and workpiece, start square, and stop rather than forcing a tap that becomes difficult to turn. These practices follow Sutton Tools’ instructions for hand-tapping a through hole.

Follow the tool’s instructions for chip clearing rather than applying a universal “forward and back” routine. Sutton’s guide gives different reversal advice for carbon and HSS taps.

Working with the next size up? See the M4 tap-drill guide.