US crochet hook letter sizes run from B-1, the smallest common yarn hook at 2.25mm, up through S at 19mm for jumbo projects. Treat the letters as a rough guide and confirm the mm size stamped on the hook itself, since letter assignments aren’t perfectly consistent between brands.
The full letter list
Not every mm size has a letter. Sizes like 2.5mm, 4.5mm, and 7mm fall between lettered sizes and are usually just called by their mm measurement instead.
- B-1: 2.25mm
- C-2: 2.75mm
- D-3: 3.25mm
- E-4: 3.5mm
- F-5: 3.75mm
- G-6: 4.0mm
- H-8: 5.0mm
- I-9: 5.5mm
- J-10: 6.0mm
- K-10.5: 6.5mm
- L-11: 8.0mm
- M/N-13: 9.0mm
- N/P-15: 10.0mm
Why the sequence skips numbers
The US system grew out of an older lettered sequence that later had numbers added alongside the letters, which is why H jumps straight to 8 instead of following D, E, F, G in order. The number jumps don’t follow a neat pattern, so it’s easier to memorize the letter-to-mm pairs you use most.
Frequently asked questions
What is the biggest lettered crochet hook size?
S, at 19mm, is generally the largest size that still commonly carries a letter. Anything bigger is usually just labeled by mm.
Why do some sizes have no letter at all?
The lettered sequence was never extended to cover every mm increment, so sizes like 2.5mm, 4.5mm, and 7mm sit in the gaps and are labeled by mm instead.
See the full mm, US, and UK chart
Get exact conversions for every hook size, lettered or not.