Crochet hook size guide

5.5 mm crochet hook (I-9)

5.5 mm is the metric diameter. The Craft Yarn Council pairs it with i-9. Use the millimeter marking when labels disagree.

Immediate answer

5.5 mm → I-9

Diameter
5.5 mm
CYC US label
I-9

Relative diameter visualization—not a physical on-screen ruler.

CYC starting range

Often paired with category 3 (light) or category 4 (medium)

  • Check the pattern gauge first.
  • Make and measure a gauge swatch.
  • Change tool size if the swatch misses gauge.

How to use this size

Treat the label as a starting point.

  1. 1. Read the pattern. Note the stated tool, yarn, stitch pattern, and target gauge.
  2. 2. Match millimeters. If a letter or number conflicts, use 5.5 mm.
  3. 3. Swatch and wash. Measure the fabric the same way the finished project will be treated.
  4. 4. Adjust one step. Use a smaller tool for more stitches per inch or a larger one for fewer.

Source notes

Last reviewed September 1, 2026