Most spinners spin yarn and use it without measuring it—and there is nothing wrong with that. But you may one day find yourself in a situation when you need to describe your yarn to someone else. Or maybe you want to make a similar yarn. It is handy to know the grist of the yarn. Wraps per inch is a good place to start—but it won’t tell you everything you need to know about your yarn.
Yards per pound (ypp) will give you a better idea of how much yarn you need to make for a given project, or how much fiber you need to start with if you want to spin yarn for a particular pattern.
Measure yards per pound by weighing your yarn and counting the yards after you’ve skeined it on a niddy-noddy. Or you can measure a length of yarn on a McMorran balance. Actually, it is a good idea to do both.