After years of knitting, I made the very useful discovery that I am a pretty loose purler and a kind of tight knitter. I know there are other imperfectly balanced knitters out there because I encounter your frustration and confusion all the time at Purl. You knitted your swatch, you carefully counted your stitches, you did everything you were supposed to do, and yet somehow your gauge took on a life of its own and now your sweater would fit a small hippopotamus!
The problem sometimes lies in having knit a flat gauge swatch even though you plan to knit in the round. In a circular knitting situation, the normal knit one row, purl one row swatch can be very misleading. Instead, to accurately replicate stockinette stitch in the round, you should make a swatch that uses only knit stitches.
Of course Elizabeth Zimmerman has figured out how to do that for us. It's perhaps a little unsatisfying in its untidiness, but well worth the affront to our sense of order!
Knitting a Circular Gauge
The basic idea here is to knit a big messy I-cord. (If you don't know how to make an I-cord, you may find our I-cord Tutorial a helpful companion to this tutorial.)
Begin by casting on to a circular needle. (I like to first knit a few rows of just normal garter stitch to help prevent the bottom edge from getting in my way when I measure the gauge.) Then, instead of turning the work around so you can purl the next row, keep the front of the work facing you and SLIDE it down the needle to the other end. The working yarn is now coming from the left side of the knitting.
Bringing the yarn from the left end to the right end, leave a very loose loop of yarn behind the work and knit the first stitch of the row.
Finish knitting the row. Then slide the work to the right end of the needle, bring the yarn from the left, leaving a big loop of slack, and knit the next row. Continue in this way until you have good size gauge swatch with lots of messy loops.
Don't worry about the side stitches being loose and uncooperative. Just ignore them!
I know some peoples' eyes start to glaze over at the mention of gauge swatches, but I really hope this simple tip will save people from some major sizing woes. In my experience, everything miraculously fits a lot better since I started knitting circular gauges! -Whitney