First I figure out if the backing fabric is sufficiently wider than my quilt. For example, if my quilt is 90" wide and the backing fabric is 108" wide, I know I don't have to worry about width.
Hopefully this is the case with your quilt. If so, all you have to do is measure the length of your quilt. If you don't intend to pre-wash your backing, then you can order the length of your quilt plus 10 inches or 15 inches (depending on how much extra you want). If you are going to pre-wash, I would order at least 1/2yd extra in case the fabric shrinks.
You can do the reverse also. That is, see if the wide backing fabric is sufficient for the length of your quilt and, if so, simply measure the width and add some extra.
If the backing fabric isn't wide enough, that is a whole 'nother story because you will need to piece the backing.