Wraps per inch is a nifty way of figuring out what size yarn you have without needing to make a gauge swatch. It’s pretty simple, but I’ve never thought to keep one handy for my spinning.
For those who also spin, the Knitting Review ( specifically this post ) has a handy dandy chart and instructions for finding out what size yarn you’ve spun. For those of you who have yarn that you’ve lost the size for, then might I recommend you also take a gander?
I put the chart up as an image, so you can reference it here quickly (and so can I!).
Have fun and happy knitting!