Every knitter and crocheter has heard of it. Most ignore it. The smart ones know better. What is it? Gauge, of course.
You'll see gauge (also sometimes referred to as tension) mentioned in your pattern and on the yarn label. Assuming you are knitting with the same yarn as the pattern used, the gauge on the label may or may not be the same.
The gauge on the label is only a suggestion…a starting point for the gauge of the yarn the manufacturer felt was best. You'll see a needle size noted too; this is also just a suggestion. All yarn works to a variety of gauges with various needles sizes; in fact, some yarn labels will give you a range of suggested gauges and needle sizes.
If the pattern gauge is different than the label, this is gauge you need to achieve. Ignore the label. Remember, the gauge and needle size of a pattern is only the gauge that particular designer achieved with that size needle. Your mileage may vary. This is why you need to check your gauge before beginning to knit the project. If you fail to do this, you may end up very disappointed at the outcome.
Don't believe it's important? Let's say you are knitting a sweater and the back should measure 20" across. The gauge in your pattern is 16 sts = 4", in other words 4 sts = 1", so the number of sts you'll work over will be 80 (20 x 4). Suppose you are getting 3.75 sts to the inch instead of 4. Your piece will measure 21.4” (80 divided by 3.75). If you were knitting at 4.25 sts to the inch instead of 4, your piece would measure 18.8" instead of 20. So, as the math shows you that even a quarter of a stitch in your gauge indeed makes a huge difference! The more stitches you are working over, the larger this difference will be.
The only exception where gauge isn't vital is in a project that doesn't need to be a particular size, such as a scarf, afghan, or hot pad. But be aware that if you don't check it, your project may not be the indicated size.
To check your gauge, cast on enough stitches to work at least 4 inches, preferably more as this will make your gauge swatch easier to measure and more accurate. If the gauge is 16 sts = 4" then cast on 18 sts minimum. Work in the stitch pattern indicated by your pattern (it isn't always stockinette so pay particular attention to how you need to knit your gauge swatch) for at least 4 inches. The goal is to have a swatch at least 4 inches wide by 4 inches long.
You can surround your swatch by a few rows of seed stitch, especially if your gauge is measured over stockinette. This will help it lay flat as you measure.
Bind off your stitches and let the swatch rest for a few hours.
Now count off the number of stitches that should equal four inches. You might want to place a straight pin on each side. They should measure exactly four inches. This is imperative. Remember how the example sweater I discussed turned out to be too big or too small?
Now measure the number of rows in the same way. Sometimes you will be able to achieve the stitch gauge but not the row gauge. Unless the pattern is worked from side to side, the stitch gauge is more important. Go with that.
If you have more stitches per inch than your pattern calls for, your stitches are too small and you need to work the swatch again with a larger needle. If you have fewer stitches per inch than your pattern calls for, your stitches are too large and you need to work the swatch again with a smaller needle.
Finally, keep in mind that the correct gauge will help to address a multitude of ills, from running out of yarn too soon to patterns that don't lay flat. For more on gauge, see the following resources: