Crochet blocking growth calculator

Many yarns change size when they are washed. Superwash wool, bamboo and silk often grow, especially in length; untreated cotton and wool can shrink. This calculator works out how much your swatch changed in width and in length, and the size to crochet your piece to before blocking so it ends up the size you want.

How to use the calculator

1. Crochet a swatch at least 5 in (13 cm) square. Mark a square inside it with two contrasting threads or stitch markers and measure between them.
2. Wash and dry it exactly as you will the finished piece: same water temperature, same drying flat or hanging.
3. When it is completely dry, measure between the same markers and enter both sets of measurements.
4. Enter the finished width and length you want after blocking.
5. Crochet until your unblocked piece measures the sizes shown.

How the calculator works

Width and length are worked out separately, because crochet rarely changes by the same amount in both directions.

Change = (after − before) ÷ before

A positive change means the fabric grew; a negative one means it shrank. To end at your target size, the calculator divides by (1 + change) rather than taking the percentage off:

Crochet to = target ÷ (1 + change)

Dividing is what makes it exact. For a piece that grows 12% and should finish 24 in long, 24 ÷ 1.12 = 21.43 in, which grows back to exactly 24 in. Taking 12% off instead (24 × 0.88 = 21.12 in) would block to only 23.65 in.

Reading your results

• Width change and length change: how your yarn and stitch respond to washing. If either shrinks by more than 15%, the calculator warns you: that usually means felting.
• Crochet it to this width: the unblocked width to check against as you work. Your stitch count still comes from your blocked gauge; use the gauge calculator with the blocked swatch for that.
• Stop at this length: the number to use when a pattern says "work until the piece measures…". Stop when the unwashed piece reaches it.

Worked example

A superwash merino swatch measures 5 × 5 in before washing and 5.25 × 5.6 in after drying flat.

• Width change: (5.25 − 5) ÷ 5 = +5%. Length change: (5.6 − 5) ÷ 5 = +12%.
• For a cardigan body 21 in wide and 24 in long after blocking, crochet it to 21 ÷ 1.05 = 20 in wide and stop at 24 ÷ 1.12 = 21.43 in long.
• Crocheted straight to 21 × 24 in, it would block to about 22.05 × 26.88 in, almost 3 in too long.

Accuracy and limits

• The arithmetic is exact and covered by our automated tests. The result is only as good as your swatch measurements.
• It assumes the whole piece changes in the same proportion as the swatch, washed and dried the same way.
• A small swatch lying flat does not feel the weight of a full sweater hanging from your shoulders. Heavy garments, and those dried on a hanger, can stretch more than the swatch shows.
• Fibres behave differently: smooth, slippery fibres such as superwash wool, bamboo and silk tend to grow, while untreated cotton and wool can shrink, and wool felts with heat and rubbing. Your own swatch is the only reliable measure for your yarn.

Common mistakes

1. Measuring the outer edges. Curled edges flatten when blocked, which looks like growth even if the stitches did not change. Measure between markers inside the swatch.
2. Measuring before it is fully dry. Damp fibres stretch easily and can spring back as they dry.
3. Pinning a garment swatch hard. A sweater is not worn pinned out. Pat garment swatches flat to their natural shape; keep firm pinning for lace, doilies and granny squares.

Tips for more accurate results

• For a heavy garment, hang the dry blocked swatch for a day with a couple of clothes pegs clipped to its lower edge, then measure the height. That shows how tall stitches such as double crochet stretch when worn.
• Planning a felted bag or slippers? Felt the swatch first, then enter the before and after sizes; the calculator tells you how big to crochet so it shrinks to size.

Frequently asked questions

Related guides and tools

• Measure gauge before and after blocking