I’ve been spinning a lot recently and I’m designing a sock pattern right, so gauge and WPI have been in my mind a great deal.
WPI, or wraps per inch, is the measuring standard used by spinners to determine the thickness of a yarn. The higher the WPI, the finer the yarn.
Gauge (tension in the UK) is a measure of the number of stitches in 4 inches of stockinette. To my mind, an ideal gauge swatch is 6 x 6 inches, with a garter stitch border. I generally block swatches before measuring them.
Gauge is the trickier measure, and thus decreed by fate to be the one most commonly used in knitting patterns. Gauge varies from yarn to yarn, even within the same weight category — the gauge of a worsted weight yarn may range from 16 to 20. And this before we’ve even started knitting.
Individual knitting styles add further variation – a tight knitter will get more stitches per inch on the same yarn and needles then a looser knitter will, making the manufacturers declared gauge nothing but a very rough guideline. Anna Zilboorg observes, in ‘Knitting for Anarchists’ that
“…three or four needle sizes will give me the same number of stitches per inch.”
This agrees well with my personal experience — trying to get gauge by adjusting needle size is a bitch.
It is in Ms. Zilboorg’s elegant explanation of this difficulty that gauge meets WPI. She writes:
“…changing needle size is far more apt to change the number of rows per inch then the number of stitches per inch. This makes sense. Basically, in each stitch two strands of yarn are lying side by side. The yarn continues to be the same width no matter how thick or thin the needle is. However the yarn must go up over the needle and down the other side. If it must go up twice as far and down twice as far, it stands to reason that the resulting stitch will be longer, giving you fewer rows per inch.”
“The yarn continues to be the same width…” There in lies WPI, and why you can measure wraps per inch accurately around a ruler, a spare knitting needle, or any of the lovely and variable WPI tools available to the spinner.
Practically speaking, I think that this means you’d be better off matching the WPI of the yarns when you want to substitute one for another, and that it’s probably easier to get gauge through appropriate yarn selection then through needle changes.
‘Knitting for Anarchists’, Anna Zilboorg
Measuring WPI, Spindlicity, Spring 2006
Standard Yarn Weight System, Yarnstandards.com