WPI (wraps per inch) is one of the most useful ways to categorise your yarn. It tells you if your yarn is DK, ‘4-ply’, aran or laceweight, etc.
WPI is easy to measure, it literally means ‘how many times can you line strands of this yarn next to each other, within a one-inch span. You can buy a fancy WPI gauge (they’re pretty cheap, usually under five GB pounds. Here’s a lovely one from Doodlestop) or you can use a ruler.
The difficulty can come when you try to look up what your WPI value means.There are many WPI tables out there, but they don’t always agree. I researched a lot of different tables and tried to find a consensus on yarn gauges. My conclusions are shown in the very handmade table shown here: