How does the shrink factor work?

Here’s the easy way to tackle the maths involved in felting…

After felting, the fabric will have shrunk, usually more widthways than lengthways because there’s more space between stitches than rows. Calculating the ‘shrink factor’, or how much a fabric will shrink during felting, is vital to being able to make an item the size you want. This starts with swatching.

(If you’re making an item where the finished size is not important then you don’t need to worry about any of this!) The shrink factor is different for stitches than rows, so you need to make two sets of calculations. We’ll explain what to do for stitches, then just repeat the process for rows.

To calculate the shrink factor for stitches, make a 10tr x 6-row swatch and note: Measurement after felting (e.g. 10cm) divided by measurement before felting (e.g. 13cm) = shrink factor (0.77).

You can use the shrink factor to work out the size of the item after felting. For example, with a rug that’s 145cm across before felting, work 145 x 0.77 = 112cm across after felting. You can also use the shrink factor to work out how big to crochet an item to achieve a specific size after felting. For example, if you want a rug that’s 120cm across after felting, work 120 ÷ 0.77 = 156cm before felting. Use this figure to calculate how many stitches you need to achieve 156cm, by working: Desired size (156) ÷ swatch size before felting (13) = Making ratio (12). Then work: Making ratio (12) x stitches in swatch (10) = Stitches needed (120). So, you’d need 120 stitches for the rug. Follow the same process to work out the number of rows for your desired size.