Felting Shrinkage Calculator (Knit-to-Felt Size, Swatch Ratios, Wet-Felting Layout)

How it works

shrink ratio = after ÷ before (width and length separately) · pre-felt size = finished ÷ ratio · typical fulling: fine wool 20–25% width, 30–40% length; medium 15–20% / 25–35%; coarse 10–15% / 20–25% · wet felt shrinks 25–35% in both directions; fibre ≈ area × thickness × 90 g/m² per mm × 1.15

Wool felts because its scales lock together under heat, moisture and agitation, and the fabric contracts as they do — unevenly, because knitting shrinks more along the rows than across them. So a felted bag is knitted about a third longer and a quarter wider than its finished size, and a wet-felted piece is laid out a third bigger than the felt you want. The ratios differ between fibres and even between yarns of the same breed, which is why every felting pattern says "swatch": felt a marked square, measure it both ways, and let those two ratios size the real project — this calculator does the arithmetic from either your swatch or typical figures.

Worked example

A felted bag to finish at 30 × 25 cm in merino: with a swatch that went from 20 × 20 to 16 × 13 cm (20% width, 35% length shrinkage), knit the body 37.5 cm wide and 38.5 cm tall — width factor 1.25, length factor 1.54 — and make the handles 1.54× their finished length. A flat wet-felted mat of the same size at 3 mm needs a layout of about 43 × 36 cm and roughly 23 g of merino tops.

Frequently asked questions