BenchReference / Drill sizes / 9.9 mm 9.9 mm Drill Decimal inches 0.3898″
Millimeters 9.9 mm
System Metric
Nearest equivalents in other systems System Nearest drill Size Difference Fractional inch 25/64″ 0.3906″ / 9.922 mm +0.022 mm (+0.0009″)
Number (wire gauge) #1 0.2280″ / 5.791 mm −4.109 mm (−0.1618″)
Letter W 0.3860″ / 9.804 mm −0.096 mm (−0.0038″)
−0.792 mm +0.792 mm 9.9 mm 9.7 mm −0.2 mm 9.8 mm −0.1 mm 10 mm +0.1 mm 10.1 mm +0.2 mm W −0.096 mm 25/64″ +0.022 mm ← #1 −4.109 mm tick position = true diameter · metric neighbors above, equivalents below · window ±0.79 mm
Positive difference = cuts a slightly larger hole than 9.9 mm. Sizes
beyond the window are listed with arrows — they are not close substitutes.
Neighboring metric sizes Drill Decimal in mm vs 9.9 mm 9.7 mm 0.3819 9.7 −0.2 mm 9.8 mm 0.3858 9.8 −0.1 mm 9.9 mm 0.3898 9.9 — 10 mm 0.3937 10 +0.1 mm 10.1 mm 0.3976 10.1 +0.2 mm
← 9.8 mm (smaller) 10 mm (larger) →
Compare 9.9 mm 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 metric size is 9.9 mm. See all data sources .