BenchReference / Drill sizes / 21/64″ 21/64″ Drill Bit Decimal inches 0.3281″
Millimeters 8.334 mm
System Fractional inch
Nearest equivalents in other systems System Nearest drill Size Difference Metric 8.3 mm 0.3268″ / 8.3 mm −0.034 mm (−0.0014″)
Number (wire gauge) #1 0.2280″ / 5.791 mm −2.543 mm (−0.1001″)
Letter Q 0.3320″ / 8.433 mm +0.098 mm (+0.0039″)
−0.667 mm +0.667 mm 21/64″ 5/16″ −0.397 mm 11/32″ +0.397 mm 8.3 mm −0.034 mm Q +0.098 mm ← 19/64″ −0.794 mm ← #1 −2.543 mm 23/64″ +0.794 mm → tick position = true diameter · fractional neighbors above, equivalents below · window ±0.67 mm
Positive difference = cuts a slightly larger hole than 21/64″. Sizes
beyond the window are listed with arrows — they are not close substitutes.
Clearance drill for Neighboring fractional sizes Drill Decimal in mm vs 21/64″ 19/64″ 0.2969 7.541 −0.794 mm 5/16″ 0.3125 7.938 −0.397 mm 21/64″ 0.3281 8.334 — 11/32″ 0.3438 8.731 +0.397 mm 23/64″ 0.3594 9.128 +0.794 mm
← 5/16″ (smaller) 11/32″ (larger) →
Compare 21/64″ with another size
Data provenance. ASME B94.11M, Twist Drills (standard drill size series) (ASME). Millimeter and inch values are converted with the exact definition 1 in = 25.4 mm; the canonical dimension for this fractional size is 0.3281″. See all data sources .