A I crochet hook, also written I-9, is 5.5mm. This is one of the letters crocheters ask about often, since patterns list the letter but hooks are usually easiest to double check by mm.
I hook in mm, and what it’s used for
At 5.5mm, a I hook matches worsted to light chunky weight yarn. In UK and Canadian sizing, the same hook is a 5, so always convert by mm rather than assume a UK number lines up with a US letter.
How I compares to nearby letters
A H-8 hook sits one step smaller, producing a tighter fabric in the same yarn. A J-10 hook goes one step larger, giving a looser drape. Choose between them based on whether the project needs structure or drape.
Frequently asked questions
Is I-9 the same as 5.5mm?
Yes, I and I-9 both refer to the same 5.5mm hook in US sizing. The two names are used interchangeably.
What yarn weight is a I hook for?
Worsted to light chunky weight yarn is the standard match for a I hook, and it’s the pairing to try first.
See the full hook size chart
Compare I against every other letter, mm, and UK size in one table.