M20 tap drill size
The tap drill for M20×2.5 is 17.5 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% | 18.051 mm 0.7107" | 18 mm (62%) | 45/64" 0.7031" (66%) |
| 65% | 17.889 mm 0.7043" | 18 mm (62%) | 45/64" 0.7031" (66%) |
| 70% | 17.727 mm 0.6979" | 17.75 mm (69%) | 45/64" 0.7031" (66%) |
| 75% | 17.564 mm 0.6915" | 17.5 mm (77%) | 11/16" 0.6875" (78%) |
| 80% | 17.402 mm 0.6851" | 17.5 mm (77%) | 11/16" 0.6875" (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 18 mm drill gives 62%. For mild steel, brass and cast iron the 17.5 mm is fine.
- For a blind hole with 1.5 × D thread engagement (30 mm), drill about 40 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 M20
| Fit | Drill |
|---|---|
| Close | 21 mm (0.827") |
| Medium (normal) | 22 mm (0.866") |
| Free (loose) | 24 mm (0.945") |
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.
M20 thread data
| Designation | M20×2.5 |
|---|---|
| Standard | ISO 68-1 basic profile, ISO 261/262 series |
| Pitch | 2.5 mm pitch (10.2 TPI equivalent) |
| Thread angle | 60° |
| Major diameter | 20 mm (0.787") |
| Pitch (effective) diameter | 18.376 mm (0.723") |
| Minor diameter, external | 16.933 mm (0.667") |
| Minor diameter, internal | 17.294 mm (0.681") |
| Thread depth (internal) | 1.353 mm |
Basic sizes. Real screws are slightly under and real nuts slightly over these values because of tolerance classes.
Quick calculator
Nearest alternatives
If you don’t have the right drill, these give:
- 17.25 mm drill → about 85% thread
- 17.5 mm drill → about 77% thread
- 17.75 mm drill → about 69% thread
Other pitches of M20: M20 × 1.5.
Threads close in size (not interchangeable – the pitch and/or angle differ):
- M20 × 1.5 – 20 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 M20 tap?
The tap drill for M20×2.5 is 17.5 mm. That’s the standard chart size and gives about 77% of full thread.
What size clearance hole for M20?
For a M20 screw or bolt, drill 21 mm for a close fit, 22 mm for a medium (normal) fit or 24 mm for a free fit. These are the ISO 273 fine, medium and coarse sizes.
What is the pitch of M20?
M20×2.5 has a 2.5 mm pitch (10.2 TPI equivalent), a 60° thread angle and a 20 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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