What yarn is recommended to use with the 5.5 mm (I-9) crochet hook?

It is recommended that "Weight 3 - Light" and "Weight 4 - Medium" yarn weights are used with the 5.5 mm (I-9) crochet hook size.

What is the comparable UK size of a I-9 crochet hook?

The United Kingdom size of 5 is the same as the US crochet hook size of I-9.

The metric size of 5.5 mm is the same as the US crochet hook size of I-9.

The US size of I-9 is the same as the metric crochet hook size of 5.5 mm.

US Name:	I-9
UK Name:	5
Metric Name:	5.5 mm
Yarn Weight:	This crochet hook size works well when paired with the following yarn weight(s). Light Weight #3: DK or Light Worsted Medium Weight #4: Worsted, Afghan or Aran

These free crochet patterns were all created using various yarn types and brands, but all use the 5.5 mm (I-9) sized crochet hook.