BenchReference / Drill sizes / 8 mm 8 mm Drill Decimal inches 0.3150″
Millimeters 8 mm
System Metric
Nearest equivalents in other systems System Nearest drill Size Difference Fractional inch 5/16″ 0.3125″ / 7.938 mm −0.063 mm (−0.0025″)
Number (wire gauge) #1 0.2280″ / 5.791 mm −2.209 mm (−0.0870″)
Letter O 0.3160″ / 8.026 mm +0.026 mm (+0.0010″)
−0.64 mm +0.64 mm 8 mm 7.8 mm −0.2 mm 7.9 mm −0.1 mm 8.1 mm +0.1 mm 8.2 mm +0.2 mm 5/16″ −0.063 mm O +0.026 mm ← #1 −2.209 mm tick position = true diameter · metric neighbors above, equivalents below · window ±0.64 mm
Positive difference = cuts a slightly larger hole than 8 mm. Sizes
beyond the window are listed with arrows — they are not close substitutes.
Neighboring metric sizes Drill Decimal in mm vs 8 mm 7.8 mm 0.3071 7.8 −0.2 mm 7.9 mm 0.3110 7.9 −0.1 mm 8 mm 0.3150 8 — 8.1 mm 0.3189 8.1 +0.1 mm 8.2 mm 0.3228 8.2 +0.2 mm
← 7.9 mm (smaller) 8.1 mm (larger) →
Compare 8 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 8 mm. See all data sources .