What WPI measures
Wraps per inch measures how many adjacent wraps of a yarn occupy one inch. It is an approximate thickness reference, not a measure of fiber content, length per gram, or finished project size.
How to measure
Wrap the yarn around an object with a consistent circumference, such as a pencil. Lay the wraps beside each other without overlap or large gaps, and do not stretch the yarn. Count the wraps within an inch at several places. Repeat the measurement when wrapping tension is uncertain.
Read overlapping ranges
The cited CYC guide gives Lace 30–40+, Super Fine 14–30, Fine 12–18, Light 11–15, Medium 9–12, Bulky 6–9, Super Bulky 5–6, and Jumbo 1–4 WPI. Compare them in the yarn weight chart, but treat them as subjective guidance ranges, not mutually exclusive bins. A fractional value in a gap should remain unresolved rather than being forced into a category.
Worked comparison: 12 WPI
A reading of 12 WPI falls in Fine, Light, and Medium ranges. The result therefore suggests several candidates. It does not prove that a yarn is DK or worsted. Compare the actual label, pattern gauge, and representative swatch.
Use the right gauge comparison
CYC distinguishes stockinette knitting gauge from single-crochet gauge. Do not compare a crochet swatch to knitting stitch ranges. Follow the pattern stitch and row gauge and check the resulting fabric.
What to do next
Use the WPI calculator to compare ranges, then swatch the actual yarn. If you need remaining yardage, use its label or measured length-to-weight ratio; WPI alone cannot supply that ratio.