Spinners are all very familiar with WPI. It is a way to measure yarn which tells you what the yarn's weight is. If you are spinning your own yarn by hand, you want to measure its WPI so that you know what general thickness you are spinning (fingering, DK, bulky, etc). This in turn helps you estimate how much of it you have, what sort of needle size you may want to start with, and so forth.
I recently busted out my WPI knowledge in order to tackle a skein of mystery yarn that I unearthed when I was shifting some boxes around. The ball band was long gone, and although I clearly remembered the manufacturer's make and model, they are no longer making this particular kind of yarn. And it is old enough, and obscure enough, that I wasn't able to find any historical information for it online.
What thickness was it, exactly? DK weight? Worsted? How many yards did I have? And what needle size should I start swatching with?
Measuring the WPI can help answer these questions.
To actually measure the number of wraps per inch, all you need is a ruler and something to wrap the yarn around. This is similar to the process of measuring your gauge. Wrap the yarn around something (a pencil works nicely) and then measure the number of wraps per inch. It is often best to wrap several inches of the pencil, and then measure the wraps at several different places, to make sure you're getting a good number. (Just as with gauge!)
My general rule of thumb is to place each wrap beside its neighbor, sliding it into place just once. I find that if I fidget the yarn around too much, it throws off the measurements.
Obviously, the thinner the yarn, the higher the WPI. Imagine how many times you would have to wrap sewing thread around a ruler to make an inch. (Dozens!) Then imagine how many times you would have to wrap a super-ultra-mega-bulky yarn. (Maybe twice!)
This yarn fell in at around 12 WPI. I looked up a WPI measurement chart online and found that this made it a worsted yarn. Once I had that information I could look up a similar yarn and get the yardage, figuring that I was somewhere in the right neighborhood there.
If you need to measure your yardage more precisely, you need a small accurate digital scale and a measuring tape or yardstick. Measure out ten yards of yarn, and weigh it on the scale (in grams). Then weigh the entire skein.
Divide the skein's weight by the number of grams in 10 yards of yarn. Then multiply that number by ten, and that's the number of yards that you have.
For example, let's say that 10 yards weighed 5 grams. You have 100 grams of yarn altogether. Divide 100 by 5 (20). Then multiply that by 10. In this example, you have 200 yards of yarn.
Photo credit: Flickr/jek in the box