We've all seen patterns that say things like this: "Gauge- 20 sts to 4" in stockinette stitch on a US 6, or needle required for gauge."
What does that really mean?
It means, that the person who wrote the pattern knitted their yarn (which may or may not be the same yarn you will use) at 5 stitches per inch. Swell. How do you replicate that gauge on the yarn you've got?
You need to make a swatch to see if your gauge, when you knit your yarn on a size 6, is going to give you 5 stitches to inch. There is more to gauge swatches, though. If you simply cast on 20 stitches, knit them on a size 6 needle in stockinette, and measure, you're going to measure a smaller gauge than you actually have. The edges of stockinette stitch curl inward, decreasing your measurement. To ensure proper gauge, you need to make a proper swatch.
Cast on 10 more stitches than your pattern tells you to have in 4 inches. For example, if I'm supposed to have 20 stitches in 4 inches of stockinette, I'm going to cast on 30 stitches for my swatch. I give myself 4 stitches on each side for a border, and two extra stitches inside for leeway.
Row 1: Knit 1, purl 1 across the row
Row 2: Purl 1, knit 1 across the row
(these two rows form seed stitch, where each knit stitch has a purl stitch above it and vice versa)
Repeat Rows 1 and 2 one time (you'll have 4 rows of seed stitch)
Row 5: Seed stitch first 4 stitches. Place marker. Knit to last 4 stitches. Place marker. Seed stitch last 4 stitches. To recap, Row 5 has 4 seed stitches, a marker, 22 knitted stitches, a marker, and 4 more seed stitches.
Row 6: Seed stitch first 4 stitches (to marker). Purl next 22 stitches (to second marker). Seed stitch last 4 stitches.
* You are doing seed stitch on the border of your swatch (the first 4 and last 4 stitches of any row) and stockinette in the middle. The middle in stockinette is the part you will eventually measure for your gauge.
Repeat rows 5 and 6 until the stockinette section of your swatch is about 4" long.
Next 4 Rows: Seed stitch all the way across your rows.
Cast off in seed stitch- DO NOT cut the yarn!
You now have a more-or-less square swatch with a seed stitch border to keep things flat. You are ready to measure.
Get out your tape measure and count the number of stitches in 2 inches of your swatch (the stockinette part)
If you've got 10 stitches in those 2 inches, you're golden. Start your project with confidence. If not, let's talk about what to do.
I just made my beautiful swatch and I have 9 stitches in 2 inches, where I'm supposed to have 10. I measured again, just to be sure. Still 9. So that means my knitting is more loose than it needs to be, or my needles are too big. It's much harder to change the way we knit, so let's talk about needles.
A good rule of thumb is that we get 1/2 stitch per inch of gauge with every needle size. So I'm supposed to have 5 stitches per inch, but I only have 4.5. That means I should go down a needle size, and I should get my 5 perfect stitches.
I could suppose that a size 5 needle is going to give me my gauge and start my project, or I could be SURE that a 5 is going to work and start another swatch. This is why we didn't cut the yarn. I can simply rip out my swatch (or keep it if you know you'll have enough yarn and want a souvenir) and knit a new one on size 5 needles.
This is the point at which people ask, "Isn't there an easier way?"
The answer is yes. If you don't particularly care about your gauge, don't worry about checking. If you want an accurate gauge for something that has to fit right, then you really are stuck making a gauge swatch for every piece you want to knit. If you try to hold your work flat without the seed stitch border, you're going to get a skewed gauge. The same goes for checking gauge when your swatch is still on knitting needles.
Think of it this way, you are going to spend lots of hours on your project and you probably spent good money on your yarn. Isn't it better that you won't have wasted your effort? You will be much happier with the results of a piece that fits just right. Take 45 minutes to an hour and knit your swatch. You might even decide to sew all of your swatches together later into something excellent.