BenchReference / Drill sizes / 1/4″ 1/4″ Drill Bit Decimal inches 0.2500″
Millimeters 6.35 mm
System Fractional inch
In 64ths 16/64″
Nearest equivalents in other systems System Nearest drill Size Difference Metric 6.3 mm 0.2480″ / 6.3 mm −0.05 mm (−0.0020″)
Number (wire gauge) #1 0.2280″ / 5.791 mm −0.559 mm (−0.0220″)
Letter E 0.2500″ / 6.35 mm exact
−0.508 mm +0.508 mm 1/4″ = E 15/64″ −0.397 mm 17/64″ +0.397 mm 6.3 mm −0.05 mm ← 7/32″ −0.794 mm ← #1 −0.559 mm 9/32″ +0.794 mm → tick position = true diameter · fractional neighbors above, equivalents below · window ±0.51 mm
Positive difference = cuts a slightly larger hole than 1/4″. Sizes
beyond the window are listed with arrows — they are not close substitutes.
Neighboring fractional sizes Drill Decimal in mm vs 1/4″ 7/32″ 0.2188 5.556 −0.794 mm 15/64″ 0.2344 5.953 −0.397 mm 1/4″ 0.2500 6.35 — 17/64″ 0.2656 6.747 +0.397 mm 9/32″ 0.2813 7.144 +0.794 mm
← 15/64″ (smaller) 17/64″ (larger) →
Compare 1/4″ 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 fractional size is 0.2500″. See all data sources .