Metric screw & bolt size chart

Reading M8 × 1.25

M8 is the nominal diameter. 1.25 is the pitch in mm. When the pitch is left off, the coarse pitch is implied, so "M8" means M8 × 1.25. Fine pitches such as M8 × 1.0 are always written out.

The tap drill for a coarse metric thread is close to diameter minus pitch: an M8 × 1.25 takes a 6.8 mm drill and an M6 × 1 a 5.0 mm drill.

tap drill ≈ D − P (M8 × 1.25 → 6.75, use 6.8 mm)

ISO and DIN wrench sizes

Four sizes changed when DIN 934 gave way to ISO 4032: M10 went from 17 to 16 mm, M12 from 19 to 18 mm, M14 from 22 to 21 mm and M22 from 32 to 34 mm. Older machinery and many hardware-store nuts still use the DIN sizes, so keep both wrenches.

Clearance holes: fine or medium

ISO 273 gives a fine (close) and a medium clearance for every size. Use fine for precise location and medium for general assembly where parts must line up across several holes. For M6 that is 6.4 mm and 6.6 mm.

Metric screws in an inch workshop

Metric hardware often turns up on imported machines, bikes and electronics. The closest inch sizes are close but never interchangeable: M5 sits between #10 and #12, M6 just under 1/4", M8 a hair over 5/16", M10 between 3/8" and 7/16". Always check pitch before you force a fastener. Where you have to tap a metric thread with an inch drill set, the chart lists the closest fractional bit and the thread percentage it actually gives.