M3 tap drill size
Use a 2.5 mm drill for an M3 x 0.5 coarse thread. That is 3 mm minus the 0.5 mm pitch and leaves about 77% thread, the usual balance of strength and tapping torque.
| Drill | mm | inch | Thread % | Typical use |
|---|---|---|---|---|
| 2.4 mm | 2.4 | 0.0945 | 92.4% | Tight: high tapping torque, tap breakage risk |
| 2.45 mm | 2.45 | 0.0965 | 84.7% | General purpose |
| #40 (inch drill) | 2.49 | 0.0980 | 78.6% | General purpose |
| 2.5 mm (ISO 2306) | 2.5 | 0.0984 | 77.0% | General purpose |
| 2.55 mm | 2.55 | 0.1004 | 69.3% | Hard or tough material, deep or blind holes |
| 2.6 mm | 2.6 | 0.1024 | 61.6% | Hard or tough material, deep or blind holes |
| Thread | Tap drill | Tap drill (in) | Clearance (ISO 273 medium) |
|---|---|---|---|
| M2 x 0.4 | 1.6 mm | 0.0630 in | 2.4 mm |
| M2.5 x 0.45 | 2.05 mm | 0.0807 in | 2.9 mm |
| M3 x 0.5 (this page) | 2.5 mm | 0.0984 in | 3.4 mm |
| M3.5 x 0.6 | 2.9 mm | 0.1142 in | 3.9 mm |
| M4 x 0.7 | 3.3 mm | 0.1299 in | 4.5 mm |
Practical notes
For through holes, a spiral-point (gun) tap pushes chips ahead and suits machine tapping; for blind holes use a spiral-flute tap and drill a few pitches deeper than the full thread you need.
Typical engagement lengths: about 1 × diameter (3 mm) in steel, 1.5 × in cast iron or brass and 2 × in aluminium. Going from about 75% to full thread adds little strength but much more tapping torque.
Thread-forming (roll) taps need a larger hole, about 2.75 mm here (D − 0.5 × P). Treat that as a starting point and check the tap maker's chart; forming suits ductile materials such as aluminium, mild steel and brass, not cast iron.
Clearance holes come from ISO 273: fine (close) 3.2 mm, medium (normal) 3.4 mm, coarse (loose) 3.6 mm. Use fine for location, medium as the default and coarse when hole positions vary.
FAQ
What size drill for an M3 tap?
Use a 2.5 mm drill (0.0984 in) for a cut M3 x 0.5 tap, per ISO 2306 (major diameter minus pitch). It gives about 77% thread.
What clearance hole for an M3 bolt?
ISO 273 gives 3.2 mm (fine/close), 3.4 mm (medium/normal) and 3.6 mm (coarse/loose) for M3, whatever the pitch. Medium is the usual default.
What inch drill can I use for an M3 tap?
The closest inch drill is #40 (0.0980 in, 2.49 mm), which gives about 79% thread on M3 x 0.5.
Sources: ISO 261 — ISO general purpose metric screw threads, general plan (pitches) · ISO 724 — basic dimensions (minor diameter D1 = D − 1.0825 P) · ISO 2306 — drills for use prior to tapping (tap drill = D − P) · ISO 273 — clearance holes for bolts and screws · Machinery's Handbook — percentage-of-thread formula
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