My yarn listings include WPI . . . and a lot of people aren't familiar with that! What Part Ice? When Purling Information?
WPI means WRAPS PER INCH. What it boils down to is how many times it takes the yarn to make one inch. So if I have a yarn labled 8 WPI, it means it takes 8 strands of that yarn to make an inch - that yarn averages about 1/8 of an inch wide.
Why do I include that information? Well, I noticed on Ravelry that a lot of pattern writers include the WPI desired for gauge. If you know the WPI of the yarn suggested, you can swap in another yarn of similar thickness without having to change your hook/needle size or number of stitches.
How do you figure out WPI? Well, the best way is actually NOT to wrap the yarn around a ruler. Because 1. That is tricky and 2. You'll have a tendency to pull the yarn tight, making it look skinnier than it is. So the best way is to put your yarn gently on the ruler to count. You will want to cover 2 - 3 inches with handspun so you can get a good average.
WPI - Knowledge is yarn swapping power!
This is the WPI tool I use. You do NOT need a special tool. Its just less cumbersome and easier to hide from my children!
You can just use any ruler (as pictured below.)