Size converter

Knitting needle and crochet hook size converter

Metric, US and old UK sizes for needles and hooks, from the Craft Yarn Council tables, with the places where charts and makers disagree spelled out.

A 4 mm needle is US 6 and old UK 8.

4 mm

US: 6
Old UK: 8
Yarn: 3 Light

One of two sizes CYC lists as US 6.

Knitting needle sizes

Metric and US sizes as the Craft Yarn Council lists them; old UK numbers where a metric size matches one.

Metric	US	Old UK / Canada	Notes
1.5 mm	000	—
1.75 mm	00	—
2 mm	0	14
2.25 mm	1	13
2.5 mm	—	—	Not in the CYC table. Wikipedia's chart and some makers call it US 1½.
2.75 mm	2	12
3 mm	—	11	CYC lists 3 mm with no US number. Wikipedia's chart and some makers call it US 2½.
3.125 mm	3	—	One of two sizes CYC lists as US 3.
3.25 mm	3	10	One of two sizes CYC lists as US 3.
3.5 mm	4	—	Falls between old UK 10 (3.25 mm) and 9 (3.75 mm).
3.75 mm	5	9
4 mm	6	8	One of two sizes CYC lists as US 6.
4.25 mm	6	—	One of two sizes CYC lists as US 6.
4.5 mm	7	7
5 mm	8	6
5.25 mm	9	—	One of two sizes CYC lists as US 9.
5.5 mm	9	5	One of two sizes CYC lists as US 9.
5.75 mm	10	—	One of two sizes CYC lists as US 10.
6 mm	10	4	One of two sizes CYC lists as US 10.
6.5 mm	10½	3
7 mm	—	2	CYC lists 7 mm with no US number.
7.5 mm	—	1	Not in the CYC table; old UK 1.
8 mm	11	0
9 mm	13	00
10 mm	15	000
12.5 mm	17	—	One of two sizes CYC lists as US 17.
12.75 mm	17	—	One of two sizes CYC lists as US 17.
15 mm	19	—	CYC lists US 19 as 15 mm; Wikipedia's chart gives 16 mm.
19 mm	35	—
25 mm	50	—
35 mm	70	—

Crochet hook sizes

Regular hooks for yarn, as the Craft Yarn Council lists them; old UK numbers on the same wire-gauge scale as needles.

Metric	US	Old UK / Canada	Notes
2.25 mm	B-1	13
2.5 mm	—	—	CYC lists 2.5 mm with no US letter.
2.75 mm	C-2	12
3.125 mm	D	—	One of two sizes CYC lists as D.
3.25 mm	D-3	10	One of two sizes CYC lists as D.
3.5 mm	E-4	—
3.75 mm	F-5	9
4 mm	G-6	8	One of two sizes CYC lists as G; brands differ.
4.25 mm	G	—	One of two sizes CYC lists as G; brands differ.
4.5 mm	7	7	No letter: just US 7.
5 mm	H-8	6
5.25 mm	I	—	One of two sizes CYC lists as I.
5.5 mm	I-9	5	One of two sizes CYC lists as I.
5.75 mm	J	—	One of two sizes CYC lists as J.
6 mm	J-10	4	One of two sizes CYC lists as J.
6.5 mm	K-10½	3
7 mm	—	2	CYC lists 7 mm with no US letter.
8 mm	L-11	0
9 mm	M/N-13	00
10 mm	N/P-15	000
11.5 mm	P-16	—
12 mm	—	—	CYC lists 12 mm with no US letter.
15 mm	P/Q	—
15.75 mm	Q	—	One of three sizes CYC lists as Q.
16 mm	Q	—	One of three sizes CYC lists as Q.
19 mm	S	—
25 mm	T/U/X	—
30 mm	T/X	—

Steel hooks for thread and lace

Steel hooks are numbered the other way round: the higher the number, the smaller the hook. Makers number them differently, so the same number appears at several sizes. Buy by millimetres.

Metric	Steel hook number(s) as CYC lists them
3.5 mm	00
3.25 mm	0
2.75 mm	1
2.7 mm	00
2.55 mm	0
2.35 mm	1
2.25 mm	2
2.2 mm	2
2.1 mm	3
2 mm	4
1.9 mm	5
1.8 mm	6
1.75 mm	4/0
1.7 mm	5
1.65 mm	7
1.6 mm	6
1.5 mm	8/7/2
1.4 mm	9/8
1.3 mm	10
1.25 mm	9/4
1.15 mm	10
1.1 mm	11
1.05 mm	11
1 mm	12/6
0.95 mm	13
0.9 mm	14/8
0.85 mm	13
0.75 mm	14/10
0.6 mm	12

Where old UK numbers come from

The old UK numbers match the Imperial Standard Wire Gauge, fixed by Order in Council in 1883: the higher the number, the thinner the wire.

Old UK number	Wire gauge (inches)	Wire gauge (mm)	Metric needle sold as
14	0.08 in	2.03 mm	2 mm
13	0.092 in	2.34 mm	2.25 mm
12	0.104 in	2.64 mm	2.75 mm
11	0.116 in	2.95 mm	3 mm
10	0.128 in	3.25 mm	3.25 mm
9	0.144 in	3.66 mm	3.75 mm
8	0.16 in	4.06 mm	4 mm
7	0.176 in	4.47 mm	4.5 mm
6	0.192 in	4.88 mm	5 mm
5	0.212 in	5.39 mm	5.5 mm
4	0.232 in	5.89 mm	6 mm
3	0.252 in	6.4 mm	6.5 mm
2	0.276 in	7.01 mm	7 mm
1	0.3 in	7.62 mm	7.5 mm
0	0.324 in	8.23 mm	8 mm
00	0.348 in	8.84 mm	9 mm
000	0.372 in	9.45 mm	10 mm

How to use the converter

Choose needle or hook, then the system your size is written in, and type it: 4.5 or 4,5 for millimetres; 10½, 10 1/2 or 10.5 for a US needle; G, 6 or G-6 for a US hook; 8 or 000 for an old UK number. The matching rows light up in the tables.

Three systems

Metric is the diameter in millimetres, printed on most needles and hooks sold today. It is the only system that measures something directly, which is why the Craft Yarn Council advises relying on it.
US needles are numbered from 000 up to 70, growing with the needle. US hooks use a letter, a number or both, from B-1 (2.25 mm) upwards.
Old UK and Canadian numbers run the other way, from 14 (2 mm) to 000 (10 mm). They match the Imperial Standard Wire Gauge, where a higher number is a thinner wire: UK 8 is 0.160 in, or 4.06 mm, sold today as 4 mm.

Where the systems disagree

The CYC table itself lists two millimetre sizes under several US labels: US 3 needles are 3.125 or 3.25 mm, and a G hook can be 4 or 4.25 mm. Some metric sizes, such as 3 mm and 7 mm, have no US number at all. Other charts differ again: Wikipedia's lists 3 mm as US 2½ and US 19 as 16 mm, where CYC says 15 mm. When a pattern gives both, follow the millimetres.

Worked example

An old British pattern asks for No. 10 and No. 8 needles. No. 10 is 3.25 mm (US 3) and No. 8 is 4 mm (US 6). Buy 3.25 mm and 4 mm, then swatch to check the gauge.

Sources

Craft Yarn Council, Hooks & Needles: knitting needle and crochet hook conversion tables and the advice to rely on millimetres.
Craft Yarn Council, Steel Crochet Hook & Crochet Thread Sizes: the steel hook table.
Wikipedia, Knitting needle, needle size conversion table: old UK numbers and the alternative US sizes noted above.
Wikipedia, Standard wire gauge: wire diameters for each number and the 1883 Order in Council.

Questions about needle and hook sizes

Why are US 6 needles sold as both 4 mm and 4.25 mm?

US numbers are labels, not measurements, and makers have not always agreed on them. The Craft Yarn Council's own table lists both 4 mm and 4.25 mm as US 6. The millimetre size on the needle is the one to trust.

What size is a G crochet hook?

Usually 4 mm, labelled G-6. The Craft Yarn Council also lists a 4.25 mm G, so check the millimetres stamped on the hook, especially on older ones.

What does old UK needle size 10 mean?

Old UK size 10 is 3.25 mm, the same needle as US 3. Old UK numbers get smaller as the needle gets bigger, so size 8 is 4 mm and size 000 is 10 mm.

How do I find the size of an unlabelled needle?

Push it through the holes of a needle gauge: the smallest hole it slides through gives its size. Measure the shaft rather than the tapered tip, and for circular needles measure the tips, not the cable.

What are steel crochet hooks?

Small hooks for crochet thread and lace weight yarn. They are numbered so that a higher number is a smaller hook, and makers number them differently, so the same number can mean different sizes. Go by millimetres.