Not sure if this will help... hard to know as this site measures the grapes in quarts before seeding and pulling the skins off... if your goal is to not use pectin then this may be of some help. Otherwise look up the pectin companies... kraft, sure gel, certo, etc where they use pure fruit juice, sugar and pectin... and if lemon juice is recommended they will list that too. https://nchfp.uga.edu/how/can_07/grape_jam_without_pectin.html

Thanks for the response. That's my problem. All the sites talk about weight or volume before preparing the fruit (and that's what I don't have.) There is no need to use pectin in grape jam. It jels up niccely if you use the right amount of sugar. I guess I'll just make an educated guess.

I'd go for equal weights prepared fruit to sugar. That's what is used for jellies.

Ditto. as floral Rx

I agree with floral and bragu, that's the accepted ratio for jellies and jams without commercial pectin. You might be able to cut that back a little, but if you cut it back too much, you won't get a gel, you'll just get a thickened "fruit spread".

Annie