Crochet Hook Size Converter

Convert crochet hook sizes between millimetres, US letters, old UK numbers, steel hooks and Japanese sizes, and see which yarn weight each hook suits.

My pattern or hook says

Usually used with yarn. See the yarn weight chart

≈ means the closest size in that system; brands vary by a fraction of a millimetre.

Crochet hook size chart

Crochet hook sizes in millimetres, US and old UK sizes, with the yarn weights they suit
Metric	US	Old UK	Yarn weight
2 mm	–	14	Lace
2.25 mm	B-1	13	Lace, Super Fine
2.5 mm	–	12	Super Fine
2.75 mm	C-2	–	Super Fine
3 mm	–	11	Super Fine
3.25 mm	D-3	10	Super Fine
3.5 mm	E-4	9	Super Fine, Fine
3.75 mm	F-5	–	Fine
4 mm	G-6	8	Fine
4.5 mm	7	7	Fine, Light
5 mm	H-8	6	Light
5.5 mm	I-9	5	Light, Medium
6 mm	J-10	4	Medium
6.5 mm	K-10½	3	Medium, Bulky
7 mm	–	2	Bulky
8 mm	L-11	0	Bulky
9 mm	M/N-13	00	Bulky, Super Bulky
10 mm	N/P-15	000	Super Bulky
11.5 mm	P-16	–	Super Bulky
15 mm	P/Q	–	Super Bulky, Jumbo
16 mm	Q	–	Jumbo
19 mm	S	–	Jumbo
25 mm	T/U/X	–	Jumbo

Steel hooks (thread)

US steel	Metric
00	3.5 mm
0	3.25 mm
1	2.75 mm
2	2.25 mm
3	2.1 mm
4	2 mm
5	1.9 mm
6	1.8 mm
7	1.65 mm
8	1.5 mm
9	1.4 mm
10	1.3 mm
11	1.1 mm
12	1 mm
13	0.85 mm
14	0.75 mm

Japanese hooks

Japanese	Metric
2/0	2 mm
3/0	2.3 mm
4/0	2.5 mm
5/0	3 mm
6/0	3.5 mm
7/0	4 mm
7.5/0	4.5 mm
8/0	5 mm
10/0	6 mm

Larger Japanese hooks are sold in millimetres (7 mm, 8 mm, 10 mm…).

Why sizes differ

Hooks are measured by the diameter of the shaft. Metric sizes are exact; US letters and the old UK numbers are names for a size, and a few have no match in the other system. Steel hooks for thread run the other way: the bigger the number, the finer the hook.

Sizes follow the Craft Yarn Council's hook chart. If your gauge is off, change the hook, not the yarn.