Why it changes your gauge
Washing and blocking can change the size of knitted or crocheted fabric. Prepare your gauge swatch as your pattern directs, using a method suited to the yarn’s care instructions. Compare measurements taken in the same condition as the pattern’s gauge.
Wet blocking is one method
For a yarn that can be soaked, wet blocking means gently wetting the sample, removing excess water without wringing or stretching, and laying it flat to dry. Smooth it into shape; use pins if the project calls for them. Lace may need deliberate stretching to open the stitch pattern.
Steam blocking is another method, but heat affects fibers differently. Follow the yarn and pattern instructions and try the method on a swatch first.
For example, if an area measures 12 cm before washing and 13 cm after drying, its stitch count is unchanged but its gauge has changed. Let the sample dry and relax before using the stitch-gauge calculator.