Work out drill bit size converter instantly with clear inputs, formula shown and shareable results.
Drill sizes come in three incompatible systems: metric millimetres, imperial fractions in sixty-fourths, and number gauges. A 6.5 mm bit is 0.2559 inches, closest to 16/64 or a quarter inch at 6.35 mm - a 0.15 mm difference that matters for a press fit but not for a clearance hole.
Metric to imperial
inches = mm / 25.4
Fractional size
sixty-fourths = inches x 64
It is 6.35 mm. The 0.15 mm gap is enough to turn an interference fit into a sloppy one, so mixing systems needs care.
The hole diameter for cutting a thread - roughly the bolt diameter minus the thread pitch. The clearance figure here is for a hole the bolt passes through.