21 mm Drill

Decimal inches 0.8268″

Millimeters 21 mm

System Metric

Nearest equivalents in other systems

System	Nearest drill	Size	Difference
Fractional inch	53/64″	0.8281″ / 21.034 mm	+0.034 mm (+0.0014″)
Number (wire gauge)	#1	0.2280″ / 5.791 mm	−15.209 mm (−0.5988″)
Letter	Z	0.4130″ / 10.49 mm	−10.51 mm (−0.4138″)

Positive difference = cuts a slightly larger hole than 21 mm. Sizes beyond the window are listed with arrows — they are not close substitutes.

Tap drill for

Thread	Major diameter	Pitch
M24 × 3	0.9449″ / 24 mm	3 mm

The standard chart recommendation (≈75% thread engagement). Application-specific factors — material, hole depth, tap type — can justify a different drill.

Clearance drill for

Thread	Fit
M18 × 2.5	loose
M20 × 2.5	close
M18 × 1.5	loose
M20 × 1.5	close

Neighboring metric sizes

Drill	Decimal in	mm	vs 21 mm
20 mm	0.7874	20	−1 mm
20.5 mm	0.8071	20.5	−0.5 mm
21 mm	0.8268	21	—

Data provenance. ASME B94.11M, Twist Drills (standard drill size series) (ASME). Machinery’s Handbook — tap drill charts (Industrial Press). Millimeter and inch values are converted with the exact definition 1 in = 25.4 mm; the canonical dimension for this metric size is 21 mm. See all data sources.