I had an e-mail from someone who read my swatching article, asking about row gauge. It was a good question, so I'm answering it here, too.
For starters, you measure row gauge the same way you measure stitch gauge: Push in a pin between two rows, measure up or down a couple inches, then put in another pin. Count the rows between, divide by the inches (or cm) you measured over, and voila. Your rows per inch/cm.
Only problem is, the information's pretty much useless.
The problem? The same thing that drives physics students crazy: gravity. The world's largest swatch, pinned to a board and measured flat, is not going to give you the same row gauge as a sweater when it's worn, because gravity will pull it down. Obviously heavy fibers with no 'spring' (like cotton or silk) would be more affected than fluffy fibers with some springiness (wool or angora). Same goes for bulky vs. fingering weight; the heavier it is, the more it's going to hang lower and the further off the row gauge will be from your happy little swatch.
Sure, there are complex methods of tweaking the row gauge for measurement, like weighting the swatch and hanging it in the closet for a week, but those are inaccurate too, because the weight hanging on the yarn depends on where it is in the sweater. Down by the hem with no weight on it, the row gauge is fairly accurate, but up by the shoulder with the whole weight of the sweater hanging off it, it's pretty darn stretched out. THEN you get into stitch patterns; stranded color stretches less than stockinette because the two colors make it firmer, lace stretches like CRAZY, cables stretch more horizontally than vertically, stockinette stretches in all directions...
This is why I mostly ignore it.
HOWEVER. One of the reasons I can ignore it is because of the types of knitting I do; wraps and scarves and the like in which exact length is not an issue, or doilies in which there really is no length, or traditionally knit casual sweaters, where exact length is also not an issue. If you're knitting a sweater sideways, or diagonally (which is becoming popular with these new self-striping yarns), row gauge does come into play and it DOES matter.
Unfortunately you're still screwed, because the designer measures the stitch and row gauge on the finished sweater, after it's been blocked and the 'hang' is affecting it. Which means it doesn't even match the designer's little gauge swatch, let alone your own. Several people I've known have tried knitting these types of sweaters and run into major problems, even with their row gauge matching the pattern requirements. I suspect this is the major cause of it. What really needs to happen is, the designer needs to give swatch gauge and finished sweater gauge and let the knitter take it from there, but that would mean that gauge varies from swatch to sweater, something the publishing places will never admit. (Because then these screwed-up sweaters we sometimes knit might be THEIR FAULT. Horrors.)
So, how do I deal with row gauge? When I knit a sweater, I measure the length as I knit it. The way I measure it is a little different, though. I hang on to the needle, give the sweater a couple good shakes, and then HOLD THE SWEATER UP THE WAY IT WILL BE WORN and hold a ruler or tape measure against it. Even using this half-assed method, a sweater will lengthen by half an inch or more between what it was laying in my lap, and the shaken and held up version.
For anyone wanting to knit a sideways or diagonal project, I suggest getting a slightly smaller gauge than suggested on both stitch and row gauge, and then keep your fingers crossed. It's a lot more hit-and-miss than anyone will admit.