Does wool grow or shrink when blocked?

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.

Does cotton grow or shrink when blocked?

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.

What is the difference between wet, steam, and spray blocking?

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.

Should I always block my knitting?

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.

What is the most accurate way to predict blocking change?

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.

Does alpaca really grow a lot when blocked?

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.

Blocking Shrinkage Predictor — Size Change After Blocking

Predict how blocking will change the size of your knitted piece based on fiber content and method.

Fiber ContentAnimal fibres (wool, alpaca) tend to grow on wet blocking; cotton/linen can shrink, especially in length; acrylic barely moves. Always swatch and block your own sample to confirm.

Blocking MethodWet = full soak then pin (most change); steam = hover a steam iron above the fabric (moderate); spray = mist and pin (least change). Method multipliers here are approximate — swatch-test to confirm.

Current Stitch Gauge (per 10cm)

Stitches per 10cm from your unblocked swatch — used to predict your gauge after blocking. For the most reliable result, block a swatch and measure it directly.

Current Row Gauge (per 10cm)

Rows per 10cm from your unblocked swatch — used to predict your row gauge after blocking.

Piece Width

Flat (unstretched) width of the piece before blocking, e.g. half the finished chest circumference. Blocking change is widthways here; row/length gauge often changes by a different amount.

Piece Height

Length of the piece before blocking, measured up the rows. Length change can differ from width — cotton/linen in particular tend to lose length when washed.

How We Calculate This

This calculator estimates size change after blocking based on fiber properties and the blocking method used. Each fiber reacts differently to moisture and heat. The figures are typical ranges, not guarantees — controlled wet-block swatch tests show most yarns grow only about 2-7%, so the single most reliable predictor is to block your own swatch and measure it before and after.

Typical blocking changes

Wool: grows roughly 5-10% on a wet block, mostly widthways
Cotton/linen: little growth, can lose length when washed
Acrylic: minimal change (it holds its shape)
Alpaca: small dimensional growth (a few percent) on blocking — but the fabric tends to droop and lengthen over time when worn unsupported, which is a wear/drape effect rather than a blocking change

Note: the estimate applies the same percentage to width and length for simplicity, but blocking is directional — stitch (width) and row (length) gauge usually change by different amounts. Treat the figure as a widthways guide and swatch-test if length is critical.

Frequently Asked Questions

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Last updated: March 2026

All calculations are estimates based on standard yarn weights and gauge. Results may vary with individual tension, yarn brand, and needle size.