You need to provide a little more information. What is the repeat of the fabric and will you be using welting? Also, home dec fabric is really not designed to be washed. It won't look the same. If you really plan on washing it, you need to do it before the cushion cover is made.

The fabric is thin striped and there will be no welting. It is machine washable fabric. Even though it says its machine washable, do you still think it won't look the same after being washed? I plan on washing it before the cushions are made. Can you suggest a specific type of fabric that can be washed? Thank you very much for all of your help and suggestions!

I figured that you'll need 4 yards. I'm curious as to what 54" wide fabric you found that is machine washable. Most of those fabrics are home dec and not considered washable. What is the fabric made of? If it is something that would shrink, like cotton, I'd buy extra. I don't have a way of figuring that, no telling how much it could shrink. I would probably buy a little extra anyway, in case you miscut something. 4 yards allows for no waste.