BenchReference

F Drill Bit

Decimal inches 0.2570″
Millimeters 6.528 mm
System Letter

Nearest equivalents in other systems

System Nearest drill Size Difference
Metric 6.5 mm 0.2559″ / 6.5 mm −0.028 mm (−0.0011″)
Fractional inch 1/4″ 0.2500″ / 6.35 mm −0.178 mm (−0.0070″)
Number (wire gauge) #1 0.2280″ / 5.791 mm −0.737 mm (−0.0290″)
−0.522 mm+0.522 mmFD−0.279 mmE−0.178 mmG+0.102 mmH+0.229 mm1/4″−0.178 mm6.5 mm−0.028 mm← #1 −0.737 mmtick position = true diameter · letter neighbors above, equivalents below · window ±0.52 mm
Positive difference = cuts a slightly larger hole than F. Sizes beyond the window are listed with arrows — they are not close substitutes.

Tap drill for

Thread Major diameter Pitch
5/16″-18 UNC 0.3125″ / 7.938 mm 18 TPI

The standard chart recommendation (≈75% thread engagement). Application-specific factors — material, hole depth, tap type — can justify a different drill.

Neighboring letter sizes

Drill Decimal in mm vs F
D 0.2460 6.248 −0.279 mm
E 0.2500 6.35 −0.178 mm
F 0.2570 6.528 —
G 0.2610 6.629 +0.102 mm
H 0.2660 6.756 +0.229 mm
Data provenance. ASME B94.11M, Twist Drills (standard drill size series) (ASME). Machinery’s Handbook — tap drill charts (Industrial Press). Millimeter and inch values are converted with the exact definition 1 in = 25.4 mm; the canonical dimension for this letter size is 0.2570″. See all data sources.