Learning how to determine the gauge of your knitting is extremely important and will always save you time. Virtually every pattern you attempt has a knitting gauge that tells you how many stitches and rows per inch your knitting should have using the recommended yarn. But if you don't have access to or don't want to use the yarn the pattern recommends, knit a swatch with the yarn you've got and then match that to the ideal measurements given in the pattern to determine what adjustments you might need to make.
Step 1: Check Your Materials
One of the great benefits of swatching is that it enables you to make adjustments to materials- to try a different yarn type than the pattern indicates for example. For instance, if your pattern recommends bulky yarn and you choose a worsted weight instead, you'll probably want to start your first swatch with a bigger size needle.
Step 2: Cast on Stitches and Knit in Pattern
Cast on enough stitches to create a piece of fabric that is larger than the indicated gauge. For example if your pattern says 5 stitches per inch, cast on 25 stitches. Knit enough rows so that you have approximately a 5-inch square. Wash and block the swatch.
Step 3: Check the Gauge
The process of measuring your piece with a straight ruler is called 'checking the gauge.' Use straight pins to hold the swatch in place and set a straight ruler on the swatch. Mark off a 4-inch wide section and count the number of stitches in that section. Divide by 4 to find your stitch per inch count. If the stitch per inch count is less or more than the pattern recommends, you'll need to make some adjustments.
It can be very difficult to get exactly the right gauge, as every knitter works just a bit differently. If your gauge is within a half a stitch, that will suffice.
Step 4: Adjust the Gauge
If your piece is too small or if you have too few stitches per inch, you will want to try the next larger needle size and re-swatch. If the piece is too large or if there are too many stitches, you will want to try the next smaller needle size and also re-swatch. In both instances, you should re-swatch until you get it right.
Step 5: Begin Your Project
Once you've gotten the measurements as close as possible, you're ready to start working on your project!
Many knitters label and keep their swatches at this point so that they will know in the future the measurements for the combination of a particular type of yarn and needle.