Wraps Per Inch
WPI: If it sounds like so much crochet alphabet soup to you, don’t worry. I didn’t know what wraps per inch (wpi) meant until a couple of years ago. Now I figure wpi for all of the yarns used in the Interweave Crochet patterns. Along with the characteristics of a specific fiber, wpi is an invaluable tool when you find you need to substitute a yarn. WPI is simply the number of times you can wrap the yarn around an object in an inch.
You can either use a wpi tool or a ruler. You want to measure the number of times you wrap the yarn around your wpi tool or an inch of the ruler. If your yarn strand variegates between chunky sections and thin sections be aware that this may not be an entirely accurate way to measure the yarn. Remember when you are wrapping the yarn to wrap it firmly, but don’t pull too tightly. Also don’t squish as many wraps as you can possibly squeeze and pry into that inch.
You are measuring the width of the yarn strands. Now take your wpi information to your LYS or personal stash. Now pick a yarn that you would like to use. Using the same technique, measure your chosen yarn’s wpi. If both wpi measurements closely match, the substitute yarn should give you a correct gauge.