What is worsted weight yarn?

Worsted weight yarn is a medium-weight yarn that is heavier than double knitting and sport weight yarn, but lighter than chunky or bulky yarn. It is a popular yarn weight used for both knitting and crocheting.

Worsted weight yarn is identified with a number four and the word "medium" printed on the label, in accordance with the Craft Yarn Council's standardized weight system. The best U.S. needle sizes to use with worsted weight yarn are between 5 and 7 for knitting, and the best hook sizes for crocheting are I-9 to K-10.5.

The term "worsted" primarily refers to yarn weight, but it also indicates the method that was used to process the wool.