A gauge (tension) is given with each pattern to help you make your hat the same size as the sample. The gauge is given as the number of stitches and rows you need to work to produce a 4-in (10-cm) square of knitting.
Using the recommended yarn and needles, cast on 8 stitches more than the gauge (tension) instruction asks for—so if you need to have 10 stitches to 4in (10cm), cast on 18 stitches. Working in pattern as instructed, work eight rows more than is needed. Bind (cast) off loosely.
Lay the swatch flat without stretching it. Lay a ruler across the stitches as shown, with the 2in (5cm) mark centered on the knitting, then put a pin in the knitting at the start of the ruler and at the 4in (10cm) mark: the pins should be well away from the edges of the swatch. Count the number of stitches between the pins. Repeat the process across the rows to count the number of rows to 4in (10cm).
If the number of stitches and rows you’ve counted is the same as the number asked for in the instructions, you have the correct gauge (tension). If you do not have the same number then you will need to change your gauge (tension).
To change gauge (tension) you need to change the size of your knitting needles. A good rule of thumb to follow is that one difference in needle size will create a difference of one stitch in the gauge (tension). You will need to use larger needles to achieve fewer stitches and smaller ones to achieve more stitches.