BenchReference / Drill sizes / R R Drill Bit Decimal inches 0.3390″
Millimeters 8.611 mm
System Letter
Nearest equivalents in other systems System Nearest drill Size Difference Metric 8.6 mm 0.3386″ / 8.6 mm −0.011 mm (−0.0004″)
Fractional inch 11/32″ 0.3438″ / 8.731 mm +0.121 mm (+0.0047″)
Number (wire gauge) #1 0.2280″ / 5.791 mm −2.819 mm (−0.1110″)
−0.689 mm +0.689 mm R P −0.406 mm Q −0.178 mm S +0.229 mm T +0.483 mm 8.6 mm −0.011 mm 11/32″ +0.121 mm ← #1 −2.819 mm tick position = true diameter · letter neighbors above, equivalents below · window ±0.69 mm
Positive difference = cuts a slightly larger hole than R. Sizes
beyond the window are listed with arrows — they are not close substitutes.
Neighboring letter sizes Drill Decimal in mm vs R P 0.3230 8.204 −0.406 mm Q 0.3320 8.433 −0.178 mm R 0.3390 8.611 — S 0.3480 8.839 +0.229 mm T 0.3580 9.093 +0.483 mm
← Q (smaller) S (larger) →
Compare R 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 0.3390″. See all data sources .