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Fractional inch tools and metric fasteners meet in the same toolbox; this page converts the common sizes and flags the pairs that nearly collide.
Imperial socket labels are fractions of an inch, and one inch is exactly 25.4 mm. On that basis: 1/4 in is 6.35 mm, 5/16 in is 7.94 mm, 3/8 in is 9.53 mm and 1/2 in is 12.70 mm.
The closest pair in everyday use is 5/16 in against 8 mm: the inch socket is only 0.06 mm smaller, so it will often push onto an 8 mm head and appear to fit. It works in an emergency and wears the flats a little every time.
The pair that causes real trouble is 1/2 in against 13 mm. The 0.30 mm shortfall means a 1/2 in socket will not seat on an M8 hex head at all, while a loose 13 mm socket on a 1/2 in head rocks under load and rounds the corners.
On narrow screens, swipe or scroll the plate sideways.
Drive squares are inch-sized even on metric socket sets: 1/4, 3/8, 1/2, 3/4 and 1 in, with the 1/4 in square measuring 6.35 mm across the flats. The sockets clipped onto those squares, however, can be metric, imperial or both.
A practical habit: keep the two systems on separate rails, and test any doubtful socket by hand. A correct socket slides on without force and turns without rocking; anything tighter or looser belongs to the other system.
If a bolt has already been damaged by the wrong socket, the stripped-bolt procedure picks up from there — oil first, then impact, then a pilot hole and an extractor.
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