BenchReference

#21 Drill Bit

Decimal inches 0.1590″
Millimeters 4.039 mm
System Number (wire gauge)

Nearest equivalents in other systems

System Nearest drill Size Difference
Metric 4 mm 0.1575″ / 4 mm −0.039 mm (−0.0015″)
Fractional inch 5/32″ 0.1563″ / 3.969 mm −0.07 mm (−0.0027″)
Letter A 0.2340″ / 5.944 mm +1.905 mm (+0.0750″)
−0.323 mm+0.323 mm#21#23−0.127 mm#22−0.051 mm#20+0.051 mm#19+0.178 mm5/32″−0.07 mm4 mm−0.039 mmA +1.905 mm →tick position = true diameter · number neighbors above, equivalents below · window ±0.32 mm
Positive difference = cuts a slightly larger hole than #21. Sizes beyond the window are listed with arrows — they are not close substitutes.

Tap drill for

Thread Major diameter Pitch
#10-32 UNF 0.1900″ / 4.826 mm 32 TPI

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

Neighboring number sizes

Drill Decimal in mm vs #21
#23 0.1540 3.912 −0.127 mm
#22 0.1570 3.988 −0.051 mm
#21 0.1590 4.039 —
#20 0.1610 4.089 +0.051 mm
#19 0.1660 4.216 +0.178 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 number size is 0.1590″. See all data sources.