BenchReference

7/8″ Drill Bit

Decimal inches 0.8750″
Millimeters 22.225 mm
System Fractional inch
In 64ths 56/64″

Nearest equivalents in other systems

System Nearest drill Size Difference
Metric 21 mm 0.8268″ / 21 mm −1.225 mm (−0.0482″)
Number (wire gauge) #1 0.2280″ / 5.791 mm −16.434 mm (−0.6470″)
Letter Z 0.4130″ / 10.49 mm −11.735 mm (−0.4620″)
−1.778 mm+1.778 mm7/8″27/32″−0.794 mm55/64″−0.397 mm57/64″+0.397 mm29/32″+0.794 mm21 mm−1.225 mm← #1 −16.434 mm← Z −11.735 mmtick position = true diameter · fractional neighbors above, equivalents below · window ±1.78 mm
Positive difference = cuts a slightly larger hole than 7/8″. Sizes beyond the window are listed with arrows — they are not close substitutes.

Tap drill for

Thread Major diameter Pitch
1″-8 UNC 1.0000″ / 25.4 mm 8 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 7/8″
27/32″ 0.8438 21.431 −0.794 mm
55/64″ 0.8594 21.828 −0.397 mm
7/8″ 0.8750 22.225 —
57/64″ 0.8906 22.622 +0.397 mm
29/32″ 0.9063 23.019 +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 0.8750″. See all data sources.