M18 tap drill size
The tap drill for M18×2.5 is 15.5 mm. That’s the standard chart size and gives about 77% of full thread.
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Tap drill by thread percentage
75% is the usual workshop figure. Less thread is easier to tap; more adds little strength.
| Thread % | Ideal drill | Nearest metric drill | Nearest imperial drill |
|---|---|---|---|
| 60% | 16.051 mm 0.6319" | 16 mm (62%) | 5/8" 0.6250" (65%) |
| 65% | 15.889 mm 0.6256" | 16 mm (62%) | 5/8" 0.6250" (65%) |
| 70% | 15.727 mm 0.6192" | 15.75 mm (69%) | 5/8" 0.6250" (65%) |
| 75% | 15.564 mm 0.6128" | 15.5 mm (77%) | 39/64" 0.6094" (78%) |
| 80% | 15.402 mm 0.6064" | 15.5 mm (77%) | 39/64" 0.6094" (78%) |
About this size
ISO metric coarse is the default thread for general engineering in the UK and Europe. If a drawing just says “M10”, it means the coarse pitch.
- For stainless steel, duplex and other tough materials, go to about 65% thread: a 16 mm drill gives 62%. For mild steel, brass and cast iron the 15.5 mm is fine.
- For a blind hole with 1.5 × D thread engagement (27 mm), drill about 37 mm deep. That leaves room for the 3–4 tapered lead threads on a second or plug tap. In steel, 1 × D engagement is usually enough; in aluminium or cast iron allow 1.5–2 × D.
Clearance holes for M18
| Fit | Drill |
|---|---|
| Close | 19 mm (0.748") |
| Medium (normal) | 20 mm (0.787") |
| Free (loose) | 21 mm (0.827") |
Fine, medium and coarse series from ISO 273. Medium suits most jobs; use fine where parts must locate closely and coarse for quick assembly or coated bolts.
M18 thread data
| Designation | M18×2.5 |
|---|---|
| Standard | ISO 68-1 basic profile, ISO 261/262 series |
| Pitch | 2.5 mm pitch (10.2 TPI equivalent) |
| Thread angle | 60° |
| Major diameter | 18 mm (0.709") |
| Pitch (effective) diameter | 16.376 mm (0.645") |
| Minor diameter, external | 14.933 mm (0.588") |
| Minor diameter, internal | 15.294 mm (0.602") |
| Thread depth (internal) | 1.353 mm |
Basic sizes. Real screws are slightly under and real nuts slightly over these values because of tolerance classes.
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Nearest alternatives
If you don’t have the right drill, these give:
- 15.25 mm drill → about 85% thread
- 15.5 mm drill → about 77% thread
- 15.75 mm drill → about 69% thread
Other pitches of M18: M18 × 1.5.
Threads close in size (not interchangeable – the pitch and/or angle differ):
- M18 × 1.5 – 18 mm major, 1.5 mm pitch (16.9 TPI equivalent)
- 3/4" UNC – 19.05 mm major, 10 TPI (2.54 mm pitch)
- 3/4" UNF – 19.05 mm major, 16 TPI (1.587 mm pitch)
- 3/4" BSW – 19.05 mm major, 10 TPI (2.54 mm pitch)
- 3/4" BSF – 19.05 mm major, 12 TPI (2.117 mm pitch)
Questions
What size drill for a M18 tap?
The tap drill for M18×2.5 is 15.5 mm. That’s the standard chart size and gives about 77% of full thread.
What size clearance hole for M18?
For a M18 screw or bolt, drill 19 mm for a close fit, 20 mm for a medium (normal) fit or 21 mm for a free fit. These are the ISO 273 fine, medium and coarse sizes.
What is the pitch of M18?
M18×2.5 has a 2.5 mm pitch (10.2 TPI equivalent), a 60° thread angle and a 18 mm major diameter.
Sources: Chart drill: the common metric coarse tapping chart. Percent values worked out from the ISO 68-1 basic profile. Clearance holes: ISO 273 (checked 25/10/2026). Values are for reference only – check your tap maker’s chart. See the disclaimer.
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