BenchReference

5/16″ Drill Bit

Decimal inches 0.3125″
Millimeters 7.938 mm
System Fractional inch
In 64ths 20/64″

Nearest equivalents in other systems

System Nearest drill Size Difference
Metric 7.9 mm 0.3110″ / 7.9 mm −0.037 mm (−0.0015″)
Number (wire gauge) #1 0.2280″ / 5.791 mm −2.146 mm (−0.0845″)
Letter O 0.3160″ / 8.026 mm +0.089 mm (+0.0035″)
−0.635 mm+0.635 mm5/16″19/64″−0.397 mm21/64″+0.397 mm7.9 mm−0.037 mmO+0.089 mm← 9/32″ −0.794 mm← #1 −2.146 mm11/32″ +0.794 mm →tick position = true diameter · fractional neighbors above, equivalents below · window ±0.64 mm
Positive difference = cuts a slightly larger hole than 5/16″. Sizes beyond the window are listed with arrows — they are not close substitutes.

Tap drill for

Thread Major diameter Pitch
3/8″-16 UNC 0.3750″ / 9.525 mm 16 TPI

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

Neighboring fractional sizes

Drill Decimal in mm vs 5/16″
9/32″ 0.2813 7.144 −0.794 mm
19/64″ 0.2969 7.541 −0.397 mm
5/16″ 0.3125 7.938 —
21/64″ 0.3281 8.334 +0.397 mm
11/32″ 0.3438 8.731 +0.794 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 fractional size is 0.3125″. See all data sources.