8 mm to inches

Formula: inches = millimeters ÷ 25.4 (exact). Or multiply by 0.0393701 for a quick estimate of the same definition.

1. Start with 8 mm.
2. Divide by 25.4: 8 ÷ 25.4 = 0.3150 in.
3. Reverse check: 0.3150 × 25.4 = 8 — the conversion round-trips cleanly.

Dividing by 25.4 is exact: 8 ÷ 25.4 = 0.31496063 in, shown as 0.3150 in. The 5/16 in reading is the nearest 1/64 (5/16 = 20/64), 0.0625 mm smaller than 8 mm, not an equal size.

The decimal column is exact (shown to four places). The fraction is the nearest 1/64 approximation used on rulers and bit charts — so the two can disagree. Here, 8 mm = 0.3150 in exactly, while 5/16 in differs by -0.062 mm. The converter disclaimer (“rounded to the nearest 1/64 fraction”) exists so that difference is never mistaken for a data error.

No. 5/16 in equals 7.9375 mm, so 8 mm is 0.0625 mm larger. The exact decimal is 0.3150 in. An 8 mm drill bit, an M8 bolt shank, and an 8 mm socket are all that metric size; 5/16 in is the nearest common imperial size and it is slightly undersize.

Compared with 5/16 in (7.94 mm): 8 mm is +0.063 mm (metric size is larger).

This page is for specification checks — drill bits, sockets, tubing, and stock — not body measurements. For shop picks across many metric diameters, see the drill bit & socket mm to inches chart.

An M8 bolt has an 8 mm nominal shank, which is 0.3150 in, not a 5/16 in bolt (7.9375 mm). A 5/16 in drill is 0.0625 mm too small to pass that shank; use an 8 mm bit when the hole itself must be 8 mm. An 8 mm socket is the tool for an 8 mm hex — hex-head M5 bolts use an 8 mm wrench, while hex-head M8 bolts use 13 mm. A 5/16 in socket is 0.0625 mm smaller than 8 mm and generally will not go on.