M18 tap drill size
Drill 15.5 mm for an M18 x 2.5 tap (about 77% thread). At this diameter you would normally pilot-drill first and power-tap or thread-mill rather than tap by hand.
| Drill | mm | inch | Thread % | Typical use |
|---|---|---|---|---|
| 15.0 mm | 15.0 | 0.5906 | 92.4% | Tight: high tapping torque, tap breakage risk |
| 39/64" (inch drill) | 15.48 | 0.6094 | 77.7% | General purpose |
| 15.5 mm (ISO 2306) | 15.5 | 0.6102 | 77.0% | General purpose |
| 16.0 mm | 16.0 | 0.6299 | 61.6% | Hard or tough material, deep or blind holes |
| Thread | Tap drill | Tap drill (in) | Clearance (ISO 273 medium) |
|---|---|---|---|
| M14 x 2.0 | 12.0 mm | 0.4724 in | 15.5 mm |
| M16 x 2.0 | 14.0 mm | 0.5512 in | 17.5 mm |
| M18 x 2.5 (this page) | 15.5 mm | 0.6102 in | 20.0 mm |
| M20 x 2.5 | 17.5 mm | 0.6890 in | 22.0 mm |
| M22 x 2.5 | 19.5 mm | 0.7677 in | 24.0 mm |
Practical notes
Drilling 15.5 mm in one pass puts a lot of load on the spindle and workholding. Pilot-drill first, then open up to size; an undersize hole is the most common cause of a seized or broken tap at this size.
Tapping torque rises steeply with diameter. On a machine use a tapping head or rigid tapping; for one-off or high-value parts, thread milling avoids the risk of a broken tap and lets you adjust the fit. Threads in tubes or bosses this size are often single-point cut on a lathe instead.
Typical engagement lengths: about 1 × diameter in steel, 1.5 × in cast iron or brass and 2 × in aluminium. In softer materials a longer engagement does more for strength than a tighter tap drill.
Thread-forming taps need about 16.75 mm (D − 0.5 × P); this is approximate, so check the tap maker's chart before drilling.
Clearance holes come from ISO 273: fine (close) 19.0 mm, medium (normal) 20.0 mm, coarse (loose) 21.0 mm. Use fine for location, medium as the default and coarse when hole positions vary.
FAQ
What size drill for an M18 tap?
Use a 15.5 mm drill (0.6102 in) for a cut M18 x 2.5 tap, per ISO 2306 (major diameter minus pitch). It gives about 77% thread.
What clearance hole for an M18 bolt?
ISO 273 gives 19.0 mm (fine/close), 20.0 mm (medium/normal) and 21.0 mm (coarse/loose) for M18, whatever the pitch. Medium is the usual default.
What inch drill can I use for an M18 tap?
The closest inch drill is 39/64" (0.6094 in, 15.48 mm), which gives about 78% thread on M18 x 2.5.
Is there a fine-thread M18?
Yes. A common fine pitch for M18 is M18 x 1.5 (16.5 mm drill). The coarse M18 x 2.5 (15.5 mm drill) is the default; fine pitches are chosen for adjustment, thin walls or vibration.
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
Need it made?
Post the job free — verified makers quote you.
Post a Metal fabrication job Open the Tap & Drill Size Chart calculator