What is US crochet hook size J in mm?

A US J-10 crochet hook is 6.0mm.

What crochet hook size do I use for worsted yarn?

Worsted weight yarn (#4) typically calls for a US H-8 to J-10 hook (5.0–6.0mm). The yarn label will usually give a specific recommendation.

Are UK crochet hook sizes the same as UK knitting needle sizes?

The old UK crochet hook sizing and old UK knitting needle sizing both derived from the British Standard Wire Gauge, so they share the same numbering logic — higher numbers mean thinner tools. However, the specific mm values at each number differ between hooks and needles, so you can't directly cross-reference them.

Conversion Guide

Crochet Hook Size Converter

Q: What size is a 5mm crochet hook in US sizes?

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

US crochet hooks use letters and numbers. Metric hooks use millimeters. Old UK hooks use a descending number scale. Here's the full chart for all three systems, plus steel hooks for thread crochet.

Most looked-up conversions

H-8 = 5.0mm · I-9 = 5.5mm · J-10 = 6.0mm · K-10.5 = 6.5mm · G-6 = 4.0mm

Looking for Knitting Needle Sizes?

Open Knitting Needle Converter →

The full knitting needle converter covers US, UK, metric, and Japanese sizes in one tool.

Standard Aluminum & Plastic Hook Sizes

These are the hooks used for yarn crochet — the sizes you'll see in most published patterns. US sizes use letter/number combinations; metric sizes are the actual hook shaft diameter.

Most commonly used sizes

Metric (mm)	US Size	Old UK Size	Yarn Weight
2.25 mm	B-1	13	Lace / Fingering
2.75 mm	C-2	12	Fingering / Sock
3.25 mm	D-3	10	Sport
3.5 mm	E-4	9	Sport / DK
3.75 mm	F-5	9	DK
4.0 mm	G-6	8	DK
4.5 mm	7	7	DK / Worsted
5.0 mm	H-8	6	Worsted
5.5 mm	I-9	5	Worsted / Aran
6.0 mm	J-10	4	Bulky
6.5 mm	K-10.5	3	Bulky
8.0 mm	L-11	0	Super Bulky
9.0 mm	M/N-13	00	Super Bulky
10.0 mm	N/P-15	000	Super Bulky
12.0 mm	O-16	—	Jumbo
15.0 mm	P-16 / Q	—	Jumbo
19.0 mm	S	—	Jumbo

Highlighted rows are the most commonly used sizes for standard yarn projects.

Steel Hook Sizes (Thread Crochet)

Steel hooks are used with crochet thread — very fine yarn for doilies, lace, and thread amigurumi. The US numbering for steel hooks runs backwards from aluminum hooks: higher US numbers mean smaller steel hooks, not larger.

Metric (mm)	US Steel Size	Notes
0.6 mm	14	Finest — thread lace
0.75 mm	13
0.85 mm	12
1.0 mm	11
1.1 mm	10
1.3 mm	9
1.4 mm	8
1.5 mm	7
1.65 mm	6
1.8 mm	5
2.0 mm	4
2.1 mm	3
2.25 mm	2
2.75 mm	00	Largest steel hook

How the US Crochet Hook System Works

Standard US crochet hooks use a letter-number combination: B-1, C-2, D-3, and so on up through the alphabet. The letter and number refer to the same size — H-8 and just "8" mean the same hook. Some brands label their hooks with only the letter, others with only the number, and some with both. Metric is the most reliable label since it measures the actual shaft diameter.

At the larger end of the scale, the letter/number correlation gets a bit inconsistent between brands. K-10.5 is one example — the 10.5 echoes the US knitting needle half-size at the same diameter. Above size L the pattern breaks down and you'll see labels like M/N-13 where different manufacturers disagree on the exact letter.

Where US crochet and knitting sizes overlap

There's a useful overlap between crochet hook mm sizes and knitting needle mm sizes in the common range. A 5.0mm crochet hook (US H-8) is the same diameter as a 5.0mm knitting needle (US 8). They're different tools — hooks have a throat and head, needles are straight — but if you need to check a measurement or cross-reference a pattern, the mm value is directly comparable.

Old UK crochet hook sizes

Old UK crochet hook sizes follow the same logic as old UK knitting needle sizes — higher numbers mean thinner tools. Modern UK and European patterns now use metric (mm) sizing, but you'll still see the old UK numbers in vintage patterns. The mm column in the chart above is the most reliable way to find the right hook for any old UK size.

Which Hook Size for Which Yarn?

The yarn label is always your starting point. Most yarn labels include a recommended hook size alongside a recommended needle size. The hook size printed on the label assumes standard crochet tension — you may need to go up or down a size depending on how tightly or loosely you crochet.

As a rough guide: lace and thread — steel hooks 0.6–2.75mm; fingering/sock — B-1 to C-2 (2.25–2.75mm); sport and DK — D-3 to G-6 (3.25–4.0mm); worsted — H-8 to J-10 (5.0–6.0mm); bulky — K-10.5 to L-11 (6.5–8.0mm); super bulky and jumbo — M-13 and above.

For a full breakdown by yarn weight, see the needle and hook sizes by yarn weight guide.

Frequently Asked Questions

What size is a 5mm crochet hook in US sizes? +

H-8. The 5mm hook is the standard worsted-weight hook and probably the most commonly owned size.

What is a US J-10 hook in mm? +

6.0mm — used for bulky weight yarn.

Why does US J go up to 10.5 then skip to 11? +

The half-size at 10.5 (K-10.5, 6.5mm) mirrors the knitting needle convention at the same mm range. It was added to fill the gap between J-10 (6.0mm) and L-11 (8.0mm), which would otherwise be a very large jump.

Are crochet hook mm sizes the same as knitting needle mm sizes? +

Yes — the mm measurement refers to the shaft diameter in both cases. A 5mm hook and a 5mm knitting needle are the same diameter. The tools are shaped differently, but the mm value is directly comparable.

What's the most common crochet hook size for beginners? +

US H-8 (5.0mm) paired with worsted weight yarn is the most common beginner recommendation — the same logic as knitting on US 8. Large enough to see clearly, with yarn that's easy to handle.

What size crochet hook for amigurumi? +

US D-3 or E-4 (3.25–3.5mm) with worsted or DK yarn is the most common starting point. Going one or two sizes smaller than the yarn label suggests creates a tighter fabric that holds stuffing in place — you don't want gaps showing through. Swatch until there are no holes.