Waste amount and percentage from what you bought versus used.
Works in any unit \u2014 yards, grams, metres \u2014 as long as both match.
Waste is the amount you bought minus the amount you used, and the waste percentage is the waste divided by the amount bought, multiplied by 100. If you buy 4 yards of fabric and use 3.25 yards, you have 0.75 yard of waste, which is about 19 percent. Tracking this over several projects shows how much extra you really need to buy.
Waste is hard to avoid completely, because cutting pieces from a flat width always leaves offcuts, and patterns with a direction or a repeat need extra. You can reduce it by planning the layout on paper before cutting, nesting pattern pieces closely, and checking the grain line on each piece.
Keep useful scraps for small projects such as appliqué, patchwork, and stuffing, and for testing stitch settings. It is still sensible to buy a little extra for shrinkage and mistakes, especially if you cannot easily buy the same dye lot again. Note the amount you use each time so your next estimate is better.
To work out how much to buy, divide the amount you need by one minus the waste percentage. If a project needs 3 yards and your usual waste is 15 percent, buy about 3.5 yards, since 3 divided by 0.85 is 3.53.