Which socket size will suitable for M12 bolt?

Answer 1

Well presuming the bolt has a hexagonal head then the German standard is 19mm across the flats of the head and will require a 19mm socket but you may well find that the ANSI/ISO standard applies which will mean the across flats dimension will be 18mm and will require an 18mm socket.

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Whilst I accept the American car industry might have its own version, be aware that almost all commercial ISO-M12 bolts and nuts, and the reference charts, have 19mm A/F hexagons, NOT 18mm. By co-incidence a 3/4" A/F spanner will fit a 19mm A/F hexagon but this the ONLY such coincidence between the inch-based and ISO-Metric fastenings.