I know, I know, you’ve just bought some lovely yarn and a new pattern, and you want to cast on right away. Why bother with a gauge swatch?
Every knitter is different. Even though a pattern says that the stitch gauge is 18 sts/4” with size 8 needles, you may need size 9 needles to get that gauge, or size 7. Or size 6. The only way to know is to knit a sample and measure it. It matters because this determines the fit of your garment. If you are, say, casting on 200 sts for a sweater, and the stitch gauge is 18 sts/4”, the garment will measure 44.4” around. What if, with that same yarn and the indicated needle size, you get 20 sts/4” (which is 5 sts/inch instead of 4.5 sts/inch)? Your sweater will be 40” around instead of 44.4.” Your comfortable sweater just got tight.
To make a gauge swatch, knit a square using the yarn you plan to use and the needle size you think you’ll need. The larger the better, though 6x6” is a good size. The pattern will tell you what stitch pattern was used to measure gauge; if it says “gauge in st st” (stockinette stitch), then knit your gauge swatch in stockinette stitch. If it uses a different stitch pattern to measure gauge, it will tell you which one to use. Gauge is expressed in sts (stitch gauge) and rows (row gauge) per 4.” When the swatch is big enough, pull the needles out. Place a pin at each edge next to the selvedge stitches (don’t include them, as they are often distorted) and measure the number of inches between the pins (include fractions). You’ll get the most accurate measurement using a ruler rather than a flexible tape measure. Then count the stitches between; knit stitches appear as Vs, so count the Vs. Convert this number to sts per 4”. This number is your stitch gauge; it is the more important of the two measurements because it determines how big around your garment will be. Next, measure row gauge: place pins top and bottom, not including the cast on edge nor the last row (whether bound off or live stitches). Measure inches and count rows, and convert to rows per 4”. Row gauge is less important, as length in patterns is usually measured in inches rather than number of rows. You will need to wash the swatch and then re-measure to get your final gauge, but this will be a good approximation.
If your stitch gauge is a higher number than that given, then your stitches are too small (more stitches in an inch), so you need to use a larger needle, and vice versa. If your stitch gauge is off, decide whether you need to up or down in needle size, and try needles one size different. Rather than starting another swatch, you can put the new needle into the stitches at the top of your swatch. Purl a row on the front to create a dividing line, and keep knitting stockinette (or whatever stitch pattern was used to determine gauge) with the new needles for a few inches more. Pull the needles out, and measure again; change needle sizes again if you need to. Once you have the correct stitch gauge, you are not done yet; you need to make sure you will still have gauge once it’s washed. It usually doesn’t change, but it’s possible that a yarn will shrink or grow after being wet, and if that’s going to happen, you need to know. Wash your swatch the way you will wash the garment (I like to use a wool wash that doesn’t need rinsing), then lay it out or pin it out, let dry, and re-measure.
Save this gauge swatch. If you need to pick up stitches on your garment for a button band or neck band, or have to work an edging you’ve never done before, you can practice on the gauge swatch. If you run short of yarn, you can always unravel it later.
Must you always do a gauge swatch? No, only when the size of the project matters. If you are making a bag and don’t really care how big it is, then no, you don’t need to, but if your gauge is way off and you make it a lot bigger than the pattern calls for, you may run out of yarn.
Don’t think of a gauge swatch as a chore; think of it as a chance to play with your new yarn and ensure that your garment will fit well. A one-day delay to do this saves you wasting months of knitting, and a significant chunk of your yarn budget, on an ill-fitting garment.