The numbers -6 -8-10 etc are in 1/16's so 6/16 reduce to 3/8 and so on.
The numbers -6 -8-10 etc are in 1/16's so 6/16 reduce to 3/8 and so on.
That gets you the o.d. of a tube. A 3/8" hardline is a -6. The inside of a -6 braided hose is .34", -8 is 0.43", -10 is 0.56", -12 is 0.69". Now the nipple of the hose end is even smaller so if it really matters use the -#/16 method and bump it up one size.