The Craft Yarn Council's numbered weight system (0 through 7) is what most yarn labels, patterns, and shops use to describe yarn thickness. It's a rough category, not an exact measurement, since two yarns in the same numbered category can still knit up slightly differently depending on fiber and ply. Treat these ranges as a starting point for picking yarn and needles, then confirm with a gauge swatch.
| # | Category | Knit gauge (sts/4in) | US needle | Crochet hook |
|---|---|---|---|---|
| 0 | Lace | 33-40 | 000-1 (1.5-2.25mm) | B-1 and steel hooks |
| 1 | Super Fine (Sock, Fingering, Baby) | 27-32 | 1-3 (2.25-3.25mm) | B-1 to E-4 (2.25-3.5mm) |
| 2 | Fine (Sport, Baby) | 23-26 | 3-5 (3.25-3.75mm) | E-4 to 7 (3.5-4.5mm) |
| 3 | Light (DK, Light Worsted) | 21-24 | 5-7 (3.75-4.5mm) | 7 to I-9 (4.5-5.5mm) |
| 4 | Medium (Worsted, Afghan, Aran) | 16-20 | 7-9 (4.5-5.5mm) | I-9 to K-10.5 (5.5-6.5mm) |
| 5 | Bulky (Chunky, Craft, Rug) | 12-15 | 9-11 (5.5-8mm) | K-10.5 to M-13 (6.5-9mm) |
| 6 | Super Bulky (Super Chunky, Roving) | 7-11 | 11-17 (8-12.75mm) | M-13 to Q (9-15mm) |
| 7 | Jumbo | 6 or fewer | 17 and up (12.75mm+) | Q and up (15mm+) |
The categories exist to give patterns and yarn substitutions a common language, so a pattern written for "worsted weight" points you toward roughly the right yarn even if you're shopping a different brand than the designer used. Category 3 (light/DK) and category 4 (medium/worsted) sit closest together and get confused most often, since both are common sweater and blanket weights with overlapping gauge ranges.
Fiber content shifts things at the edges. A fuzzy mohair-blend lace yarn and a smooth cotton lace yarn can both sit in category 0 by gauge, but they'll behave very differently in a project. The number tells you roughly what needle to reach for. It doesn't tell you how the fabric will drape, felt, or hold a stitch pattern.