There is a particularly complete and easy-to-understand method for estimating how much fabric you will need for a slipcover.from Claudia Buchanan
A classic upholstery yardage chart is another good way to go. Since slips take more than upholstery you will want to add a couple of yards, or maybe three if you are matching a pattern. But this chart is a good one to copy or download for future reference.
I have always used the yardage estimates that I learned years ago from Mr. Bazer. That yardage chart, along with my labor prices is available on my primary website.
One-way stripes and prints are always going to require more fabric than solid color fabrics do. If I have a central pattern for each cushion and for inside and outside back, I add about 50% to any estimate for yardage. That is, if my cushions are wider than half a fabric width, (27") or if the pattern is a dropped pattern. To put it another way, I need more fabric if I have to cut each side of a cushion out of the middle of a pattern or stripe.
Another reason to add 50% to your yardage estimate is if the fabric is less than 48 inches wide. Some expensive prints have very wide margins on the side and so the fabric width doesn't matter. For instance, if there is a border printed on one or both sides of the print.