Fabric shrinkage and pre-washing

Many fabrics, especially cotton, linen, rayon and wool, get smaller in the first wash. They can also shrink by different amounts lengthwise and crosswise. Pre-wash the fabric the way you'll wash the finished item, before you cut.

Measure, don't guess

Shrinkage depends on fibre, weave, finishing (for example pre-shrunk or "sanforized" cotton) and how you wash and dry. Cotton Incorporated's technical guide treats it as something to control and test for, not a fixed number (Cotton Incorporated, "A Guide to Improve Shrinkage Performance of Cotton Fabrics"). Laboratories use standard test methods, for example ISO 675, which measures dimensional change of woven fabrics (principally cotton) after commercial laundering near the boiling point (ISO 675:2014). At home:

1. Cut a swatch, for example 25 × 25 cm or 10 × 10 in, and mark a square on it with permanent marker.
2. Wash and dry it as you will the finished item.
3. Measure again. Shrinkage % = (before − after) ÷ before × 100, worked out separately for length and width.

The buying formula

If you need a length L after washing and the fabric shrinks by s %, buy L ÷ (1 − s/100). That's slightly more than adding s % on top.

Worked example: you need 100 cm after washing and the fabric shrinks 5 % lengthwise. Buy 100 ÷ 0.95 = 105.3 cm, which rounds up to 110 cm. Adding 5 % (105 cm) would leave you short. Width works the same way in reverse: 112 cm fabric that shrinks 3 % ends up 112 × 0.97 = 108.6 cm wide. The planner subtracts the selvages after that.