How to knit a circular swatch flat

Sounds wrong doesn't it? But yes, you can knit a circular swatch flat. If you've ever knit I-cord, it's the same idea, only you don't pull the carried yarn tight. Knitting flat means you can work the swatch on fewer stitches than you would if you were trying to knit a tube to measure. But because you'll still be knitting every stitch, the result will be the same as if you had knit a tube.

You'll need double pointed needles or a circular needle (the ones you are using for your project, obviously). Cast on roughly 4 inches worth of stitches (look at the ball band and see how many stitches you should get in 4 inches and cast that many on). Now your stitches are all sitting on the right needle (in flat knitting you would turn and work back). I've knitted a few rows to make it easier to photograph, but this is approximately what you'll be looking at. Your last stitch and working yarn are at the tip of the right needle.Slide your knitting back across the cable so that the work is on your left needle. The last stitch worked and working yarn will be at the cable end of your needle.Now bring the yarn LOOSELY behind the work and knit the first stitch. Don't pull the stitch tight, you want a big loop of yarn on the back (this will ensure that your swatch will lay flat when it comes time to measure). Knit across the row as usual. The last stitch or 2 may be very loose because of the slack in the carried yarn. This is fine. Just knit the stitch as best you can. Continue in this manner until the swatch is big enough to measure. The back will look a mess...
And those end stitches will be sloppy, but you'll have a perfectly accurate gauge swatch for your circular project, without having to cast on enough stitches to knit a tube. These swatches are not usefully for anything else, as far as I can tell. Maybe some kind of punk rock bracelet. But I usually rip mine out anyway.