Spark plug wrench sizing?

[Roymondo]

Got a new set of plugs to go in my TC. Got a plug spanner that is marked "10mm" but it's too big! A 14mm spanner fits the flats on the plug perfectly. So what gives? I always understood metric spanner sizes were measured across the flats? I'm going to take one of the plugs into local Halfords and see what fits.

 

[Roymondo]

To answer my own query - it is an 8mm plug. Found a deep 3/8" drive 14mm socket in Halfrauds that has internal clearance for the plug insulator. So that's got that sorted

 

[Pukmeister]

Sizing is by thread size, not across flats.