Yarn weight describes how thick the strand is, not how heavy it feels. The Craft Yarn Council sorts yarn into eight standard categories: 0 Lace, 1 Super Fine (fingering or sock), 2 Fine (sport), 3 Light (DK), 4 Medium (worsted and aran), 5 Bulky, 6 Super Bulky and 7 Jumbo. The higher the number, the thicker the yarn and the larger the needle or hook you need.

Worsted weight is category 4, also called medium weight. It is the most popular yarn weight because it works for almost any project, from garments to accessories and home decor. It usually knits at 16 to 20 stitches over 4 inches on US 7 to 9 needles (4.5 to 5.5 mm), and crochets with a 5.5 to 6.5 mm hook.

Lace is 0, fingering or sock is 1, sport (also called baby weight) is 2 and DK (double knit) is 3. Fingering usually uses US 1 to 3 needles (2.25 to 3.25 mm) and is used for socks, shawls and baby garments. Sport works well for colorwork, cables and lightweight sweaters.

Chunky is commonly used for bulky (category 5); super bulky is category 6. Bulky is about twice as thick as worsted and works on US 9 to 11 needles (5.5 to 8 mm), for cozy sweaters, rugs and throws. Super bulky knits on US 11 to 17 needles (8 to 12.75 mm), for cowls, scarves and hats. Jumbo (category 7) is thicker still, for big stitch projects.

Yarns in the same weight category are generally interchangeable, but you should still knit a gauge swatch with the yarn you plan to use and compare it to your pattern's gauge. Two yarns with the same weight number can knit up differently, so the swatch is what tells you whether the size will come out right. If the yarn is in a different weight category, the pattern's stitch counts will not work as written.