In-Depth Yarn Size Characteristics
There are many many factors that go into determining the size AKA "weight" of a yarn. I went into the basics in a previous post here: https://www.themoerae.com/post/yarn-weights
However, there are more complex details!
The weight of the yarn is scaled numerically from 0 to 8, with smaller numbers representing smaller yarns. They also have standardized names for each.
One of the most measurable characteristics is "wraps per inch", abbreviated "WIP". The way this is measured is by wrapping the yarn around a ruler without overlapping the strands. When the yarn reaches 1 inch, the number of rotations around the ruler are counted. Therefore, more wraps around the ruler means that the size of the yarn is smaller.
Another effect of yarn weight is the suggested size of crochet hook and knitting needle. In this case, the smaller the yarn, the smaller the designated tool.
Since the yarn and tool are both smaller for smaller weight yarns, the crochet or knit stitches are smaller. Therefore the number of stitches and rows that can fit into a 4-inch by 4-inch gauge square is greater in a small-weight yarn than a larger yarn.
See more about gauge in my infographic and post about yarn gauge and tension:
Here is a table with the details for each weight of yarn:
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