I have a hard time estimating how much yarn I will need to make a project when I am just fiddling around and not following a pattern that clearly tells me. The first time I made a scarf, I bought one skein, and was rally surprised at how quickly I went through it. When I made my first afghan, one skein got me through about 5 rows, and I had to run out and buy a lot more yarn than I had originally planned.
I’ve decided the see how large of a square a full skein of this yarn will make. I’ve just finished 8 rounds, and while there is still quite a bit left, each round uses more than the last. I have a feeling this project will also require a lot more yarn than I imagine.
alookatthelittlethings 
I’ve learned my lesson over the last few years and now I over buy on yarn. The problem with that is that I end up using it forever.
I try to over buy too since it is never a hardship to have more yarn for my stash, and then I end up googling “One skein projects.”
If you don’t mind a bit of arithmetic it isn’t too hard to get an idea of how huch yarn is needed. One way (if it is all the same stitch eg trebles, crochet 10 or twenty of them and undo. Measure how much yarn was used then calculate how many trebles there will be in the finished item divide by the ten or twenty then multiply by the length you used. Note length on side of yarn ball and divide into answer above.
Second way if there is a repeat of one design. Make one of design, measure size decide how many for final item. Undo sample design and multiply length by how many you will make then do same sum as above re length of yarn in a ball.
If not mixing dye lots matters, over rather than under estimate.
I’m making a 4ft by 6ft blanket and I calculate it will take about 16 x 100g balls of acrylic.
Great advice! I will give this a try even if it does include math. 🙂