Blocking Shrinkage Predictor — Size Change After Blocking

Wool typically grows 5-10% when wet blocked, especially in width. The fibres relax and the fabric opens up. This is why many wool garments are knitted slightly smaller than the finished measurements. Superwash wool may grow even more as it lacks the scales that hold the fibres together.

Cotton tends to shrink 2-5% when washed and blocked, particularly in length. Unlike wool, cotton does not have elastic memory, so it does not bounce back. Knit cotton garments slightly larger than target to account for shrinkage.

Wet blocking (soaking the piece) has the most dramatic effect on size and shape. Steam blocking gives a moderate effect and works well for wool. Spray blocking (misting and pinning) has the mildest effect and is best for acrylic and items that need minimal reshaping.

Blocking evens out stitches, opens up lace, relaxes cables, and gives a professional finish. Most garments benefit from blocking. The exception is ribbing and textured stitches where you want the fabric to remain elastic and springy.

Block your own swatch exactly the way you will block the finished piece, then measure it before and after. Shrinkage or growth as a percentage is (before minus after) divided by before, times 100. This calculator gives a fibre-and-method estimate to plan with, but a measured swatch beats any lookup table, because actual change depends on the specific yarn, your tension, and how hard you stretch the pins.

On the blocking table alpaca changes very little dimensionally — controlled wet-block tests show it among the least-changing fibres, typically a few percent. What alpaca is famous for is drooping and lengthening over time when worn unsupported, because the fibres are smooth and heavy. That is a wearing and draping effect, not a one-off blocking change, so plan structure (firm gauge, supportive stitch patterns) rather than expecting 10 to 15 percent growth from blocking alone.