Crochet hook sizes: US, UK and mm

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Hook sizes are labeled three different ways depending on where the pattern came from, and the letters don’t line up neatly with the numbers. The millimeter measurement is the one that never lies — it’s stamped on the hook itself, so when a pattern and a label disagree, trust the mm.

A letter is a brand’s label, not a measurement. The Craft Yarn Council says so itself: “Because letter and number sizing vary from company to company, rely on the package millimeter (mm) sizing.” A hook stamped G is 4 mm from Susan Bates or Clover, 4.25 mm from Boye and 4.5 mm from addi; P is 10 mm, 11.5 mm or 15 mm depending on who made it. The letters below are the common reading, and the mm column is the hook.

The UK numbers are history. British hooks have been sold in millimeters since the 1970s; these turn up in vintage patterns and on inherited hooks. And UK crochet hooks were numbered differently from UK knitting needles below 4 mm — a 2.5 mm hook was a 12 while a 2.5 mm needle was not, and 3.5 against 3.75 swaps the same way. A great many charts online print the needle ladder under a crochet heading. This one is the hook lineage.

Regular hooks

The ones you use for normal yarn. A 5.5 mm (US I-9) with medium (4) yarn is the usual place to start learning. Tap any size for what that hook is called everywhere else and which yarn weights it suits.

31 sizes, smallest first.
Metric	US	UK
2 mm	—	14
2.25 mm	B-1	13
2.5 mm	—	12
2.75 mm	C-2	—
3 mm	—	11
3.25 mm	D-3	10
3.5 mm	E-4	9
3.75 mm	F-5	—
4 mm	G-6	8
4.25 mm	—	—
4.5 mm	7	7
4.75 mm	—	—
5 mm	H-8	6
5.5 mm	I-9	5
6 mm	J-10	4
6.5 mm	K-10.5	3
7 mm	—	2
7.5 mm	—	—
8 mm	L-11	0
9 mm	M/N-13	00
10 mm	N/P-15	000
11.5 mm	P-16	—
12 mm	O	—
15 mm	P/Q	—
15.75 mm	Q	—
16 mm	Q	—
19 mm	S	—
20 mm	—	—
25 mm	T/U/X	—
30 mm	T/X	—
35 mm	Y	—

Every letter size, in millimeters

American hooks carry a letter and a number — a “G-6” is one hook, not two. Older hooks and older patterns often print only one of the pair, which is half the confusion. The millimeter is the measurement underneath both.

B hook

A US size B-1 crochet hook is 2.25 mm at most brands — so if your hook says 2.25 mm, it is usually a B. In the old UK numbering it was a 13.

C hook

A US size C-2 crochet hook is 2.75 mm at most brands — so if your hook says 2.75 mm, it is usually a C.

D hook

A US size D-3 crochet hook is 3.25 mm at most brands — so if your hook says 3.25 mm, it is usually a D. In the old UK numbering it was a 10.

E hook

A US size E-4 crochet hook is 3.5 mm at most brands — so if your hook says 3.5 mm, it is usually an E. In the old UK numbering it was a 9.

F hook

A US size F-5 crochet hook is 3.75 mm at most brands — so if your hook says 3.75 mm, it is usually an F.

G hook

A US size G-6 crochet hook is 4 mm at most brands — so if your hook says 4 mm, it is usually a G. In the old UK numbering it was an 8.

7 hook

A US size 7 crochet hook is 4.5 mm at most brands — so if your hook says 4.5 mm, it is usually a 7. In the old UK numbering it was a 7.

H hook

A US size H-8 crochet hook is 5 mm at most brands — so if your hook says 5 mm, it is usually an H. In the old UK numbering it was a 6.

I hook

A US size I-9 crochet hook is 5.5 mm at most brands — so if your hook says 5.5 mm, it is usually an I. In the old UK numbering it was a 5.

J hook

A US size J-10 crochet hook is 6 mm at most brands — so if your hook says 6 mm, it is usually a J. In the old UK numbering it was a 4.

K hook

A US size K-10.5 crochet hook is 6.5 mm at most brands — so if your hook says 6.5 mm, it is usually a K. In the old UK numbering it was a 3.

L hook

A US size L-11 crochet hook is 8 mm at most brands — so if your hook says 8 mm, it is usually an L. In the old UK numbering it was a 0.

M/N hook

A US size M/N-13 crochet hook is 9 mm at most brands — so if your hook says 9 mm, it is usually an M/N. In the old UK numbering it was a 00.

N/P hook

A US size N/P-15 crochet hook is 10 mm at most brands — so if your hook says 10 mm, it is usually an N/P. In the old UK numbering it was a 000.

P hook

A US size P-16 crochet hook is 11.5 mm at most brands — so if your hook says 11.5 mm, it is usually a P.

O hook

A US size O crochet hook is 12 mm at most brands — so if your hook says 12 mm, it is usually an O.

P/Q hook

A US size P/Q crochet hook is 15 mm at most brands — so if your hook says 15 mm, it is usually a P/Q.

Q 15.75mm hook

A US size Q crochet hook is 15.75 mm at most brands — so if your hook says 15.75 mm, it is usually a Q.

Q 16mm hook

A US size Q crochet hook is 16 mm at most brands — so if your hook says 16 mm, it is usually a Q.

S hook

A US size S crochet hook is 19 mm at most brands — so if your hook says 19 mm, it is usually an S.

T/U/X hook

A US size T/U/X crochet hook is 25 mm at most brands — so if your hook says 25 mm, it is usually a T/U/X.

T/X hook

A US size T/X crochet hook is 30 mm at most brands — so if your hook says 30 mm, it is usually a T/X.

Y hook

A US size Y crochet hook is 35 mm at most brands — so if your hook says 35 mm, it is usually a Y.

Steel hooks (for thread)

Much finer, and numbered the opposite way round — a bigger steel number means a smaller hook. These are for thread crochet and fine lace, not for yarn.

19 sizes. Note the numbering runs backwards.
Metric	US	UK
0.75 mm	14	—
0.85 mm	13	7
1 mm	12	6.5
1.1 mm	11	6
1.25 mm	—	—
1.3 mm	10	5.5
1.4 mm	9	5
1.5 mm	8	4.5
1.65 mm	7	4
1.8 mm	6	3.5
1.9 mm	5	3
2 mm	4	2.5
2.1 mm	3	2
2.25 mm	2	1.5
2.5 mm	—	—
2.75 mm	1	1
3 mm	—	—
3.25 mm	0	0
3.5 mm	00	—

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