What is Wraps Per Inch (WPI)?

Measuring and universally communicating a yarn's thickness

WPI is a yarn thickness measurement that is familiar to spinners, but can be beneficial to non-spinners as well. WPI is an acronym for Wraps Per Inch.

This type of measurement provides a more universal way of describing the characteristics of a yarn's thickness, than sometimes happens with the confusing terms for worsted, sport, 8-ply, jumper, etc.

Have you ever had an occasion where the belly band of an unknown yarn was missing? Or an unspecified coned yarn? Also, you can expand beyond those yarns designated as "knitting yarns" to other perfectly good yarns known to weavers, hand spinners, etc., By understanding this concept, you can make more knowledgeable decisions about yarn substitution.

A simple way to measure WPI is the following –

1. Using an unsharpened pencil or small dowel, make 2 marks an inch apart. Also, cut a small notch in one end.
2. Catch the yarn to be measured in the notch, then wind the yarn onto the pencil/dowel between the 2 marks. I turn the pencil, rather than circling the yarn around the pencil, so as not to introduce more or less twist in the yarn. Wind so that the yarn strands just barely touch.
3. Count the number of strands between the 2 marks. This is your WPI.

Example of a yarn that has been wound onto the pencil and measures 13 WPI

If your yarn is bulky, you can get more accuracy by measuring over a longer distance. For example, make marks 2 inches apart. Then divide your number of wraps by 2 to get the WPI.

If your yarn is very fine, making marks a half inch apart may be sufficient. Remember to multiply by 2 to get your actual WPI.

The Pattern Index lists HeartStrings by yarn type. This will help you locate HeartStrings patterns suitable for the yarn thickness you want to use. For simplicity, the listing is categorized into four "weights" of yarn thicknesses: fine, medium, heavy, and bulky. You may find the correlation of WPI to popular knitting yarn types in these categories helpful for substituting yarns on HeartStrings and any other patterns.

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