A F crochet hook, also written F-5, is 3.75mm. 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.
F hook in mm, and what it’s used for
At 3.75mm, a F hook matches DK weight yarn. In UK and Canadian sizing, the same hook is a 8, so always convert by mm rather than assume a UK number lines up with a US letter.
How F compares to nearby letters
A E-4 hook sits one step smaller, producing a tighter fabric in the same yarn. A G-6 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 F-5 the same as 3.75mm?
Yes, F and F-5 both refer to the same 3.75mm hook in US sizing. The two names are used interchangeably.
What yarn weight is a F hook for?
Dk weight yarn is the standard match for a F hook, and it’s the pairing to try first.
See the full hook size chart
Compare F against every other letter, mm, and UK size in one table.