A 5.5mm crochet hook is a US I-9, and UK size 5. I reach for this size most often when a worsted weight pattern feels a touch too dense at 5mm and I want a slightly more relaxed drape without jumping all the way to 6mm.
What is a 5.5mm crochet hook used for?
This size works well with worsted and light chunky yarn, and it’s a common choice for garments, hats, and blankets where a soft, drapey fabric matters more than a tight, dense one.
- Yarn weight: worsted to light chunky.
- Common projects: sweaters, hats, blankets, amigurumi in bulkier yarn.
How 5.5mm compares to nearby sizes
Compared to a 5mm (H-8) hook, 5.5mm produces a noticeably looser fabric in the same yarn, which I like for drapey garments but avoid for anything that needs structure, like a stuffed toy. A 6mm (J-10) goes a full step looser again, better suited to chunky yarn than worsted.
Frequently asked questions
What letter is a 5.5mm crochet hook?
A 5.5mm hook is a US I-9. In UK and Canadian sizing, the same hook is a 5.
Is 5.5mm the same as an I hook?
Yes, I and I-9 both refer to a 5.5mm hook in US sizing.
What yarn weight uses a 5.5mm hook?
Worsted and light chunky yarn most commonly call for this size, especially for garments and blankets with a soft drape.
See the full hook size chart
Compare 5.5mm against every other mm, US, and UK size in one table.