Making a tension swatch is just as important in crochet as it is in knitting. The tension is also specified as a number of stitches and a number of rows to a given measurement, for example: “20 sts and 22 rows = 10 cms square measured over double crochet on 3.50mm hook”. The hook size suggested is the one the designer will have used to work out the instructions on the basis of the given tension and the one most knitters will use to achieve this tension. However, it is the tension that is important and not the hook size.
Work a tension swatch of 20 sts plus 2 (the 2 end stitches are slightly distorted and should not be included when checking a tension), and 22 rows in length. Fasten off. Lay this swatch down on a flat surface and measure it - first horizontally within first and last stitch for stitch tension (Fig.128 - Note: diagram shows only 5 cms) and then vertically for row tension (Fig.129). If your square has too few stitches or rows to the measurement, your tension is too loose, and you should try again with a size smaller hook. If it has too many stitches try a size larger hook.