Wraps per inch is commonly used by weavers, but it can be helpful for all yarn users to identify the weight of yarn if its label is missing. To measure yarn in wraps per inch (or WPI) you need a ruler, and something to wrap the yarn around that has a consistent circumference, like a pencil.
Begin by wrapping your yarn around the pencil for a few inches. The yarn should be snug, and the wraps should lay side by side without any over lapping, or large gaps between strands. Don’t pull the yarn too tightly to avoid skewing the measurements. If you’re unsure of your tension, practice with a yarn that you know the weight of.
Once the yarn is wrapped, use the ruler to measure how many wraps are in an inch. Measure a few places as you would a gauge swatch. Use the chart provided to compare your WPI number to the weight category to determine the weight of your yarn, and what hook or needle to use to reach a standard size gauge.
Keep in mind that WPI is subjective, and results will vary depending on how tightly the yarn is wrapped. Always work up a gauge swatch before starting a project.
Wraps Per Inch (WPI) by yarn weight
|1 Super Fine||14-30|
|6 Super Bulky||5-6|
Source - Craft Yarn Council
Here are some examples of how I count my WPI's