It's tedious but I calculate in square inches. Figure out how many square inches of each color per block. Multiple by number of blocks. There are 1584 sq. in. per yard of fabric (based on 44" wof) divide total from blocks by #/sq. yd. That's how many yards you need. I usually over buy up to the next 1/2 to full yard just for cutting/sewing booboos but otherwise it's usually pretty accurate.