A J crochet hook, also written J-10, is 6mm. 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.
J hook in mm, and what it’s used for
At 6mm, a J hook matches chunky weight yarn. In UK and Canadian sizing, the same hook is a 4, so always convert by mm rather than assume a UK number lines up with a US letter.
How J compares to nearby letters
A I-9 hook sits one step smaller, producing a tighter fabric in the same yarn. A K-10.5 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 J-10 the same as 6mm?
Yes, J and J-10 both refer to the same 6mm hook in US sizing. The two names are used interchangeably.
What yarn weight is a J hook for?
Chunky weight yarn is the standard match for a J hook, and it’s the pairing to try first.
See the full hook size chart
Compare J against every other letter, mm, and UK size in one table.