A worked answer
A 36 by 52 inch baby quilt has 176 inches of edge. Add 10 inches for joins, cut 2 1/2 inch strips from 42 inch fabric, and you are buying about 3/8 yard.
Perimeter is 36 + 52 + 36 + 52 = 176 inches. Quiltler's binding card adds 10 inches for joins and mitres, then divides by usable strip length on 42 inch fabric. Five 2 1/2 inch strips need 12 1/2 inches of length on the bolt, which shops sell as 3/8 yard. Change strip width if you prefer a narrower baby edge.
The sewing walkthrough is how to bind a quilt. Backing for this size is how much backing for a baby quilt. Design the top in the baby quilt designer.
For a 36 by 52 inch quilt, cut 2 1/2 inch strips. The perimeter is 176 inches, plus 10 inches for joins. That is 5 strips from 42 inch fabric, which comes to 3/8 yard.
A fat quarter is 18 by 22 inches. Crosswise 2 1/2 inch strips from the 22 inch side give only a handful of short strips. 3/8 yard of full-width cloth is the safer shop cut for straight-grain binding.
After. Binding cut for the paper size is often short of the quilt you actually have.