The pattern should tell you, otherwise it's math 101.

Seems like when I use fabric that I already have I'm short in end and have to compromise or I feel obgligated to use fabrics from my stash when I'd really rather buy fabrics for the pattern I want to make. I've decided to stop the random fabric purchases and stash only left overs that I use for scrap quilts.

I usually add at least an extra 1/2 yard to what is required in a pattern. or ask for help at you LQS

I generally try to figure out how much yardage you will need for the back - say 6 yards. Then estimate the % of each color, or if they are evenly divided. so if you have 25%, 25% and 50%, you can guesstimate the color divide as 1 1/2 yds of 2 colors and 3 of another. Then I add another yard to be safe. Or as much more as you want. Generally comes up OK.

I thought I had bought enough material and when I found out I needed more, the store was out of one color. Had to buy a darker color. Will use it for the outside blocks. I tried to figure out how much I needed but guess my calculations were a TAD off. It will be fine. Just glad the store had the other colors.
Rita

I make lots of scrap quilts, seldom make quilts with less than 10 fabrics.

I may really go overboard with figuring out the details, but I use graph paper, draw a the layout of the quilt I'm making, determine the number of blocks, width of borders, etc. and how many different colors. Then I use the graph paper to lay out the best way to cut the fabrics based on a 40-42 inch width of fabric and then keep laying out until I have the number of pieces I need from each different fabric and see what yardage length I've used. Then I add at least enough fabric to get another row of pieces (plus a few extra inches) in case I make mistakes. I do this with each color. I also do this for the borders and binding. And don't forget the backing.

I sometimes end up with extra fabric, but it goes into the stash for scrappies.

You took the words right out of my iphone! I am making more and more scrappy quilts, just decide on a color combo and start cutting!!!

Me too!!! Extra is always best!

This is even better! Especially if it is your favorite or most common fabric you use.

I had a 50% coupon and bought 10 yards of my favorite fabric which i use on most of my backs. Now I don't have to shop for it for a good long while.