Reply by crank49

What did you measure it with?
Calipers or a micrometer, or with a ruler?

Even if it were metric approximation of English units 5/16” is 7.94mm so an 8mm would be the metric “equivalent”.

But 3/8” is over 9.53mm and is often equated to a 10mm chisel. That’s a whole size range off.

-- Michael :-{| Diapers and politicians both need to be changed often; and for the same reason.