Your knitting gauge is not the same when knitting flat compared to knitting in the round. That's because a knitter's tension is often quite different when purling than when knitting, and it takes a different amount of yarn to make a knit stitch versus a purl.
Since knitting in the round most often involves Stockinette Stitch, which is made entirely with knit stitches in the round, that changes the way your yarn measures up.
And if you're knitting a garment that needs to fit -- or you don't want to run out of yarn because your project is coming out much bigger than the pattern suggested -- you need to know your gauge on the yarn you are using knit in the round rather than flat.
One Way to Work a Circular Swatch
Of course the most direct way to knit a circular gauge swatch is to cast on a bunch of stitches (for ease of working you will probably want more than you typically would cast on for a swatch), join for working in the round and work a sample using double-pointed needles, two circulars or the magic loop method.
While this is direct it isn't always easy, and the best way to measure such a swatch is to cut it open, lay it flat and measure the gauge that way (unless you make a swatch that's twice as big as you need) and cutting into knitting gives a lot of knitters pause.
How to Fake a Circular Gauge Swatch
Alternatively, you can knit up a fake circular gauge swatch with two double-pointed needles (or a circular needle).
Here's how it's done.
- Cast on enough stitches to give you a piece of knitting that is about 5 inches/13 cm wide.
- Knit the first row.
- Slide the stitches to the opposite end of the needle, without turning the work.
- Knit the next row, making sure you keep the strand of yarn loose across the back of the work.
- Continue in this manner until the swatch is about 5 inches/13 cm long.
- Bind off, or slide the stitches onto a stitch holder to make measuring easier. Then you can rip out the swatch later if you need the yarn.
You wouldn't be able to reuse the yarn in the more traditional method of knitting a circular gauge swatch because you will have cut the strands. So this method is a better choice if you're worried that you're going to need that yarn later, or if you just hate having swatches lying around and want to reuse the yarn.
When working a swatch in this fashion, you really need to make sure you leave the strands very loose in the back so that the edges don't curl in too much and distort your measurement.
You might have noticed this method is basically the procedure you would use to make an I-cord, except you don't pull the stitches tight when working a swatch like this. Instead you want it as loose as possible.
This sort of circular swatch will give you a good idea how your yarn will behave in the round, and it may feel like it is faster and easier to work this way.