Vintage pattern yarn requirements

maybe these are questions that only have answers according to the circumstances … but I hope not.

can I actually figure out the yarn weight of the kind used in an ancient pattern from how many stitches per inch - is 5 stitches per inch almost always DK?
I mean, it could be on size 11? but wouldn’t be because it’s vintage?

and if the pattern says 7 balls, can I buy 8 and hope that’s good enough?

it’s elbow length sleeved, xs.

would you just try and figure it all out from the picture, or just figure they always used smaller needles and thinner yarn back in the day?



got a pic of the pattern?