How do Japanese and Western nib sizes compare?

Japanese nibs (Pilot, Sailor, Platinum) run about one size finer than Western (Lamy, Pelikan, TWSBI): a Japanese M writes close to a Western F. The tradition traces to kanji, where dense strokes reward finer lines.

Practical rule: decide the line you want first, then translate per brand. Small handwriting on office paper points Japanese F/M; large handwriting on good paper enjoys Western M/B.