BenchReference / Drill sizes / O O Drill Bit Decimal inches 0.3160″
Millimeters 8.026 mm
System Letter
Nearest equivalents in other systems System Nearest drill Size Difference Metric 8 mm 0.3150″ / 8 mm −0.026 mm (−0.0010″)
Fractional inch 5/16″ 0.3125″ / 7.938 mm −0.089 mm (−0.0035″)
Number (wire gauge) #1 0.2280″ / 5.791 mm −2.235 mm (−0.0880″)
−0.642 mm +0.642 mm O M −0.533 mm N −0.356 mm P +0.178 mm Q +0.406 mm 5/16″ −0.089 mm 8 mm −0.026 mm ← #1 −2.235 mm tick position = true diameter · letter neighbors above, equivalents below · window ±0.64 mm
Positive difference = cuts a slightly larger hole than O. Sizes
beyond the window are listed with arrows — they are not close substitutes.
Neighboring letter sizes Drill Decimal in mm vs O M 0.2950 7.493 −0.533 mm N 0.3020 7.671 −0.356 mm O 0.3160 8.026 — P 0.3230 8.204 +0.178 mm Q 0.3320 8.433 +0.406 mm
← N (smaller) P (larger) →
Compare O 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 0.3160″. See all data sources .