When I am figuring up yardage needed, I start with how many of each block I'm going to need. Due to salvages, I usually only figure 40" of usable width of fabric. If I'm cutting 5" blocks, I know that one strip of 40" fabric will allow me to cut it into 8 pieces that are 5" wide. Then I figure out how many 5" strips I will need. If one strip gives me 8 squares, then 2 strips will yield 10 blocks. Just figure out how many 5" strips you will need. If I need 80 blocks, that's 10 strips. Then take the width of your strip (5") and multiply by how many strips you need (10). That means you need 50" of yardage. I always tend to round up another 18" (1/2 yard) on what I need. Sometimes I'll add a full yard. Take the total number of inches you need and divide by 36 to get the number of yards to buy.