BenchReference / Drill sizes / X X Drill Bit Decimal inches 0.3970″
Millimeters 10.084 mm
System Letter
Nearest equivalents in other systems System Nearest drill Size Difference Metric 10.1 mm 0.3976″ / 10.1 mm +0.016 mm (+0.0006″)
Fractional inch 25/64″ 0.3906″ / 9.922 mm −0.162 mm (−0.0064″)
Number (wire gauge) #1 0.2280″ / 5.791 mm −4.293 mm (−0.1690″)
−0.807 mm +0.807 mm X V −0.508 mm W −0.279 mm Y +0.178 mm Z +0.406 mm 25/64″ −0.162 mm 10.1 mm +0.016 mm ← #1 −4.293 mm tick position = true diameter · letter neighbors above, equivalents below · window ±0.81 mm
Positive difference = cuts a slightly larger hole than X. Sizes
beyond the window are listed with arrows — they are not close substitutes.
Neighboring letter sizes Drill Decimal in mm vs X V 0.3770 9.576 −0.508 mm W 0.3860 9.804 −0.279 mm X 0.3970 10.084 — Y 0.4040 10.262 +0.178 mm Z 0.4130 10.49 +0.406 mm
← W (smaller) Y (larger) →
Compare X 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 letter size is 0.3970″. See all data sources .