WPI (wraps per inch) is less reliable method for determining the yarn weight than the amount of yards/meters per 100 gr, described earlier. But it’s handy enough to understand the weight of the mystery yarn (without any label). It might help to save some time without crocheting gauge with this unknown yarn of yours.
Take a ruler graduated in inches or a simple pencil. Hold the yarn firmly and wrap it evenly around the chosen subject. The yarn should not be overlapping and your wraps should lay flat side by side. Push the yarn together so there are no gaps between wraps. Smooth it out so it is neither too loose nor too tight. If you are measuring thicker yarn, yarn with some funky texture, or just for better accuracy wind more than 1 inch and measure the wraps at the center of your yarn sample.
Here we wrapped yarns of different weight around a pencil until the width of wrapped pieces reached 1 inch and counted the number of wraps per inch (WPI). It is possible to check yarn with the pencil and ruler without unwinding the skein, simply slipping the pencil under approximately an inch of the strands and holding a inch-ruler against them.
Important notes: if all yarn strands are tightly woven your yarn will belong to a thinner yarn category. If all strands are loosely woven, or not woven at all, fluffy, have a ribbon structure, or a cable structure (2 ply yarn is twisted once, then 2 plies of 2-ply yarn are twisted again), the yarn will belong to a thicker yarn category. Yarn weight depends on the fiber content: acrylic with the same yardage per 100 gr is fluffier than 100 % wool, wool with the same yardage per 100 gr is thicker than cotton (because wool is loosely woven or not woven). Thinner yarn is more suitable for crocheting than thicker 2-9 WPI yarn, images of very thick yarn are not shown.
Super bulky or 6 (USA), 16 ply (UK), 2-5 WPI, 70-80 yards per 100 gr (no image available)
Chunky weight or 5 (USA), 14 ply (UK), 5-6 WPI, 80-95 yards per 100 gr (no image available)
Bulky weight or 5 (USA), 12 ply (UK), 7-8 WPI, 100-130 yards per 100 gr
7 WRI(7 wraps per inch), unknown acrylic (very fluffy!)
Aran weight, Medium or 4 (USA), 10 ply (UK), 8-9 WPI, 140-190 yards per 100 gr
9 WRI(9 wraps per inch), unknown wool (not woven!)
Worsted weight, Medium or 4 (USA), 10 ply (UK), 10-11 WPI, 200-225 yards per 100 gr
11 WRI (11 wraps per inch), unknown acrylic (fluffy!)
DK weight, Light or 3 (USA), 8 ply (UK), 12-16 WPI, 230-300 yards per 100 gr
12 WRI(12 wraps per inch), 100 % cotton 262 yards in 100 gr (loosely woven)
13 WRI(13 wraps per inch), unknown acrylic (fluffy!)
14 WRI (14 wraps per inch), 100 % acrylic, 330 yards in 100 gr (fluffy!)
16 WRI(16 wraps per inch), 100 % wool, 195 yards in 100 gr (fluffy!)
Sport weight, Fine or 2 (USA), 5 ply (UK), 16-21 WPI, 300-380 yards per 100 gr
Examples: perle cotton 3/2, Catania.
19 WRI (19 wraps per inch), 100 % cotton, 275 yards in 100 gr (loosely woven!)