As a weaver, the only “true” way I understood the “weight” of the yarn, was by doing a WPI measurement. There really weren’t tons of tools out there to choose from to do this, (like there are today) I simply grabbed a yardstick, a pencil, or one of the kids’ rulers, and turned until I did 2 inches worth of wraps....counted, and divided by two.
I plan to do a better write-up of the information that is out there. For now, let me just say there are many guidelines and many charts and many different results, as far as the number of WPI for each weight of yarn. There are similar enough results to use as general guidelines, none of them should be taken as the absolute “truth”.
To complicate this further, WPI can be influenced heavily by the tension put on the yarn when wrapping, the humidity of the day, whether the yarn had been stored tightly wound, or loose....and yes.........even the mood the artist is in that day.
Until I get the document written, just know that everyone’s terminology differs, there are different classifications in different countries, and no one guideline is more correct than another. Using any of them will get you in the ballpark.
Different manufacturers will classify similar weight yarns under different categories. One company’s Fingering weight has the same WPI as another company’s Sport weight. Just don’t take any of it too seriously. Chances are the pattern designer was simply repeating what the yarn manufacturer listed for the yarn she/he was using. If you are using a different yarn, your results may vary. Use WPI as a guideline, and make a swatch to be sure.
Editor's Note: I have a little more thorough write up from my point of view -