I'm planning on making a bunch of stepping stones as a Christmas present for my grandma, with the names of all the grandchildren (all 15 of them!) stamped in the stones. Now here's a math problem for you - I have a 12 inch round mold, and I have to make 5 stones. How much concrete do I need?
I can't remember for sure how big the bag of vinyl patch compound I used for my 12" stones, but I know that one bag did 2 stones that were about 2" thick.
Fifteen names seems like a lot to fit on a 12" stone. I did family stones and I used photos mounted to glass marbles then sealed the backs before setting them into the concrete. I left mine outside over winter and the photos are fading. My sisters all put theirs inside and they're not fading.