Reference chart

Crochet hook size conversion chart: mm, US and UK

A crochet hook size conversion chart matches metric millimeter sizes to US letter-number sizes and old UK numbers. A 5 mm hook is US H-8 and UK 6, a 5.5 mm hook is US I-9 and UK 5, and a 6 mm hook is US J-10 and UK 4. The millimeter size is the most reliable measure.

By the crochetpatterns.io editorial team · Reviewed September 13, 2026 · Editorial policy

Crochet hook sizes: millimeters, US and old UK designations with matching yarn weights
Metric (mm)	US size	Old UK size	Yarn weight category
2.25	B-1	13	1
2.75	C-2	12	1
3.25	D-3	10	1
3.5	E-4	9	1, 2
3.75	F-5	—	2
4	G-6	8	2
4.5	7	7	2, 3
5	H-8	6	3
5.5	I-9	5	3, 4
6	J-10	4	4
6.5	K-10½	3	4, 5
7	—	2	5
8	L-11	0	5
9	M/N-13	00	5, 6
10	N/P-15	000	6
11.5	P-16	—	6
12	—	—	6
15	P/Q	—	6, 7
16	Q	—	7
19	S	—	7

Which crochet hook size measurement is most accurate?

The metric millimeter size is the most accurate crochet hook measurement because it describes the shaft diameter directly, while US letters and UK numbers vary between brands.

Why do old UK hook sizes count backwards?

Old UK hook sizes count backwards because they were based on wire gauge numbers, where a higher number means a thinner wire. A UK 14 hook is smaller than a UK 8 hook.

What are steel crochet hooks?

Steel crochet hooks are small hooks, usually 0.6–2.25 mm, used with crochet thread. Steel hooks use their own numbering, where a larger number means a smaller hook.

Source and method: Conversions follow the Craft Yarn Council hook size conventions. Manufacturers occasionally label letters differently; the metric millimeter size is the authoritative measurement. Yarn weight categories use the 0–7 Standard Yarn Weight System.

Frequently asked questions

What is a 5 mm crochet hook in US sizes?

A 5 mm crochet hook is US size H-8.

What is a J hook in mm?

A US J-10 crochet hook is 6 mm.

Is a 4 mm hook a G hook?

Yes. A 4 mm hook is US G-6 and old UK size 8.