Responding to an Instagram prompt of "Gauge," I was starting to type a little more than could conveniently fit as a photo caption, so I thought I'd better move some of my many thoughts on the subject to somewhere more suited to long-form responses, namely, here.
These are just a few tips on how to ensure a good level of accuracy when taking gauge measurements; if you have more tips, please share them in the comments! I have plans for a follow-up post as well for "things that swatches are good for besides measuring gauge, " so drop by again sometime this summer for that one!
To get as accurate a gauge reading as possible, make a large swatch. No, larger than that. Really.
If your project is to be worked flat, work your swatch flat. If your project is to be worked in the round, work your swatch in the round.
Wash your swatch and block it in the same manner required for the finished object. Some swatches will change more than others when washed.
Take multiple gauge measurements for both stitch gauge and row gauge from several spots on your swatch, towards the central areas where the borders are less likely to distort the fabric.
Some stitch patterns are more difficult to measure consistently than others. If gauges are listed in the pattern for more than one stitch pattern, swatch them all.
Adjust needle sizes as needed to match gauge as closely as possible. Sometimes, two sets of needles with the same sizing label will produce different results, sometimes because of what material they are made with, but sometimes because of minute differences in the actual size of the needles.