How much to buy so it still fits after pre-washing.
Typical cottons shrink 3\u20138%. Pre-wash before cutting.
Many fabrics shrink the first time they are washed, so you need to buy more than the finished length you need. Divide the finished length by one minus the shrinkage percentage. If you need 100 inches and the fabric shrinks 5 percent, buy about 105 inches, since 100 divided by 0.95 is 105.3.
To find the shrinkage of a particular fabric, cut a square, mark a 10 inch line in each direction, and wash and dry it the way you will treat the finished item. Measure again and compare. Fabrics can shrink by different amounts in length and in width.
Cotton, linen, and rayon are the most likely to shrink, and knits often shrink more than wovens. Synthetics such as polyester shrink very little. Some fabrics are sold preshrunk, but it is still safest to wash a test piece.
Wash and dry the fabric before you cut, using the same method you will use for the finished piece, since washing also removes the sizing that makes fabric stiff and can change how it feels.
Make a note of the fabric content and the care instructions on the bolt, since they often hint at how much the fabric will shrink. Buying a small sample and washing it first is a cheap way to avoid a costly surprise, especially for large projects such as curtains and quilts.