Fabric Shrinkage Calculator
Fabric shrinks the first time it is washed, and a project cut before that will not fit afterwards. Measure a test swatch to find your shrink rate, or work backwards to how much yardage to buy so it still measures right once prewashed.
Mark a long straight section before washing — the longer the marked length, the more accurate the percentage.
Formula & how it works
Shrinkage is (before − after) ÷ before × 100. To buy enough, divide by what remains: buy = needed ÷ (1 − shrinkage ÷ 100).
Dividing is not the same as adding the percentage — at 7% shrinkage you need about 7.5% more fabric, not 7%. Usable width after washing is bolt width × (1 − width shrinkage ÷ 100).
Worked example
A marked 36 inch length that measures 34.5 inches after washing has shrunk 4.17%. Going the other way: to have 3 yards left after a 7% length shrink you should buy 3.23 yards — about a quarter yard extra. And 44 inch fabric shrinking 3% in width finishes around 42.7 inches usable.
Prewashing with confidence
Length and width differ
Woven fabric usually shrinks more along the length than across the width, because the lengthwise yarns are held under tension during weaving and relax when washed. Measuring both directions separately matters most for wide pieces like quilt backs, where losing width can leave you short.
Test the way you will wash
Shrinkage is a function of heat and agitation, so a swatch tested on cold delicate tells you nothing about a garment that will go through a hot wash and tumble dry. Treat the test exactly as the finished item will be treated, including the dryer.
Buy the extra up front
Because you divide rather than add, the extra needed is slightly more than the shrink percentage suggests — and it compounds on larger projects. Buying the margin in the same cut avoids the dye-lot problem entirely, which is the real reason to work it out before leaving the shop.