Add a waste allowance to the yardage a pattern calls for.
A 10\u201320% allowance covers cutting waste, squaring and shrinkage.
Quilting patterns list the yardage they need, but you often need extra. Mistakes in cutting, shrinkage when washing, directional prints, and the chance of a flaw in the fabric all use more than the pattern shows. This calculator adds a waste percentage to the yardage so you can see how much to buy.
A common allowance is about ten percent, and more is wise for a first quilt, for complicated cutting, or for prints that you cannot easily buy again. For a pattern that needs 5 yards, adding ten percent gives 5.5 yards.
Consider rounding up to the next quarter or half yard, since fabric is usually sold that way. Having a little extra also helps if you decide to change the size of the quilt, add a border, or need to replace a piece that did not turn out right.
Buy all the fabric for the project at the same time, because dye lots and print runs can vary. Any leftover fabric goes into your scrap collection, where it is useful for smaller projects, binding, and practice pieces.