Most knitting patterns that require the finished project to be a particular size will advise you to check your gauge before beginning. This is accomplished by knitting a gauge swatch, which will tell you how many stitches and rows per inch you get when you knit with the yarn you want to use in the pattern stitch that is called for.
Getting Started with a Swatch
Start with the yarn you want to use on your project and the size needles that are called for in the pattern. Knit a swatch that you think will be big enough to give you about six inches square. It doesn't have to be perfect, but most gauge measurements are based on four inches, so you'll want at least that much to work with.
When you've finished your swatch, lay it out flat on a table or the floor. Pick a point to measure from and use a straight pin to mark that spot. Use a tape measure to mark out four inches and count the number of stitches. Use the same process to count the number of rows.
Compare these numbers to the numbers listed in your pattern. If you're spot on, congratulations and get to knitting. If you have more stitches per inch than the pattern calls for, that means your stitches are too small. Try again with the next largest sized needle.
If you have fewer stitches per inch than the pattern states, your stitches are too big. Make another gauge swatch using the next smaller needle.
If your gauge swatch is more than a stitch or two per inch off, it may be that the yarn you've chosen is not a good match for the pattern. Not every yarn can be used for every pattern, so you might need to look for a different pattern that will suit your yarn better.
The length measurement is not as critical as the number of stitches per inch on most projects, because you can always adjust that by knitting more or fewer rows to suit your needs. It is really the number of stitches per inch that plays a key role in sizing.
When Can You Skip the Gauge Swatch?
If you're not concerned about the finished size of your project, go ahead and skip the gauge swatch. Projects like scarves, baby blankets, shawls and throws don't need to be an exact size, so you can probably get away with skipping this step.
If you're making anything that is fitted, from a hat to a baby bootie to a sweater, it pays to take the time to make a gauge swatch so your know your sizing is perfect.
Fun With Yarn
As knitters, most of us are pretty passionate about fiber. Consider making gauge swatches as just another way to get to explore yarn and have fun with it. Making a gauge swatch will teach you about the yarn you are working with as well as teaching you more about knitting.
If you don't believe me, here's a quick gauge game you can play. Take any worsted weight yarn and any three sizes of needles that are each a couple of sizes different from each other. (In the pictured sample, I used sizes 7, 10.5 and 15.) Knit small swatches of, say, 12 stitches and 12 rows of garter stitch. Measure each swatch. Study them. Consider the different characteristics of the stitches and how they might look in a finished project. Playing with yarn in this way will help you learn about gauge, about yarn and about knitting.