Worsted weight yarn is the yarn most commonly used. It’s both fine enough to show basic details in your work, but thick enough so that projects knit up quickly and with enough body to hold in some warmth. It’s also the best weight of yarn to learn to knit or crochet with, as it’s neither too thick nor too thin.
Yarn weight refers to the thickness of yarn used by knitters, weavers, crocheters and other fiber artists. Changing yarn weight or needle size can have a significant impact on the finished project, so standardized systems have been spread about, as well as conversion systems for regional standards (especially needle sizes).Yarn weight is important in achieving the correct gauge or tension for a particular project and can help with yarn substitution. The Craft Yarn Council of America has developed a system that seeks to standardize the labeled weights of yarn. Most yarns state their weight on the ball band. Some brands use a standardized numbering system that uses 7 ranges of relative thickness of yarn.
Needle sizes US 7 to 9
Crochet hook size I to K.
Approximately 16-20 stitches per 4" stockinette stitch
800 to 1000 yards per pound