I only buy for projects - not for stash. For pre-purchased patterns I'll by anywhere from 1/4 to 1 yard of extra...depends on how much of that particular fabric is called for in the pattern. If I'm making up my own pattern, I usually do so in EQ and it calculates yardage for you. I think their calculations are a bit off so I just do a quick double check simply multipling out square inches/yards as a double check. But then still purchase extra to cover my own cutting errors.