By Janel Laidman

WPI stands for "wraps per inch," and is a way to measure the grist, or thickness, of a yarn. This is a particularly useful measurement if you are substituting one brand of yarn for another that is called for in a pattern, or if you are substituting handspun yarn, of course!

To measure WPI, all you need is a ruler, although it's fun to use fancy turned gauges. Start by wrapping the yarn around the ruler at one of the inch markings and continue wrapping around until you get to the next inch marking, then count how many times you wrapped around in that inch space.

It is important to have a uniform way of wrapping the yarn. You don't want to pull the yarn tight as you wrap, because that will stretch the yarn and make it thinner. You should only use as much tension as is necessary to keep it in place while wrapping. When winding the yarn on, wrap each successive loop so that it is kissing the one next to it, but not so that it is crowding the one next to it. On the other hand, you don't want to see space in between the loops either.

If your yarn is hairy or thick and thin or bumpy, you might want to measure your WPI over three inches then divide by 3, rather than over one, to minimize error.

It is useful to check your WPI periodically as you are spinning, particularly when you leave your spinning and come back later, because your mood can affect how you are spinning. Sometimes you may be spinning slow and dreamy, other times you might be nervous and fast. If you check your WPI periodically, you can adjust to keep the grist of your yarn even.

A nice tool for the "quick and dirty" method of checking WPI is the "Spinner's Control Card" made by VIP Fibers, and available at many online spinning retailers. The card is clear plastic with black lines in the diameter of various WPI, but it goes in increments of two, so some estimating is needed. It does, however, provide a quick check without having to wrap.