Why does blocking change the size?

Wet fibres relax and the stitches open up, so the fabric grows when pinned out. The amount depends on the fibre: untreated wool and especially lace grow a lot, superwash grows and can keep growing, while cotton and linen barely move or even shrink.

What growth percentages are typical?

Untreated wool grows roughly 5 to 20 percent, with open lace at the high end. Superwash wool grows about 10 to 15 percent and is prone to over-stretching. Cotton, linen, and silk sit around minus 5 to 0 percent. Always swatch to confirm.

Should I block a swatch first?

Yes. Knit a generous swatch, measure it dry, block it the way you will block the finished piece, then measure again. The percent change is your most accurate growth figure to enter here, far better than the fibre averages.

Does negative growth make sense?

Yes. Cotton and some plant fibres can relax tighter or shrink slightly after a wash, giving a negative percentage. Entering a negative number tells the calculator to compute a smaller blocked dimension than the dry measurement.

Will the piece stay at the blocked size?

Untreated wool holds its blocked shape well. Superwash and slippery fibres tend to relax and grow further with wear and washing, so block them a little smaller than the target to leave room for creep.

Knitting Blocking Dimensions Calculator

Blocking transforms a knit, especially lace, but it is easy to over- or under-stretch a piece. This calculator takes your dry measurements and an expected growth percentage for the fibre and tells you exactly how large to pin the piece out when wet blocking.

How it works

Blocking growth is a simple percentage applied to each dimension:

blocked = dry × (1 + growth% / 100)

A positive growth percent enlarges the piece; a negative percent (typical of cotton and linen) shrinks it. The width and length are scaled independently so you get a target pin-out rectangle.

Example and tips

A lace shawl measuring 90 cm wide by 40 cm deep in untreated wool with an expected 15 percent growth blocks out to about 103.5 cm by 46 cm. The single most reliable input is a blocked swatch: knit it, block it, measure the before and after, and enter that exact percentage rather than the fibre averages. Remember superwash keeps relaxing with wear, so pin it slightly under target.