Crochet hooks use three different sizing systems — US letters, US numbers, and UK numbers — and most patterns assume you know all of them. The calculator above converts between every system in one click, plus tells you which yarn weights pair with each hook size. If you have an unmarked hook, the Print & Identify tab gives you a life-size reference to lay it against.
This page covers the full crochet hook size chart, what each size is used for, the yarn weights each hook works with, and answers the common “what is a size H hook?” / “what is a 4 mm hook in US?” type questions.
How to use the crochet hook converter
Open the calculator and pick what you know — metric (mm), US letter (B, C, D, E, F, G, H, I, J, K, L…), US number (1, 2, 3, 4, 5, 6, 7, 8…), or UK number (14 down to 000) — and the tool returns every equivalent. Switch tabs at the top for yarn pairing, hook-to-yarn reverse lookups, steel hooks for thread crochet, and the printable life-size hook reference.
Crochet hook size chart (full conversion table)
This is the standard crochet hook size chart used by the Craft Yarn Council and most major yarn brands — Lion Brand, Bernat, Red Heart, Lily Sugar’n Cream, Caron, and Patons all follow it. Conversions are exact where possible; some values (especially in the larger sizes) vary slightly between manufacturers.
| Metric (mm) | US Letter | US Number | UK | Common name |
|---|---|---|---|---|
| 2.25 mm | B | 1 | 13 | B/1 |
| 2.5 mm | – | – | 12 | 2.5 mm |
| 2.75 mm | C | 2 | – | C/2 |
| 3.0 mm | – | – | 11 | 3 mm |
| 3.25 mm | D | 3 | 10 | D/3 |
| 3.5 mm | E | 4 | 9 | E/4 |
| 3.75 mm | F | 5 | – | F/5 |
| 4.0 mm | G | 6 | 8 | G/6 |
| 4.5 mm | – | 7 | 7 | 7 |
| 5.0 mm | H | 8 | 6 | H/8 |
| 5.5 mm | I | 9 | 5 | I/9 |
| 6.0 mm | J | 10 | 4 | J/10 |
| 6.5 mm | K | 10½ | 3 | K/10½ |
| 7.0 mm | – | – | 2 | 7 mm |
| 8.0 mm | L | 11 | 0 | L/11 |
| 9.0 mm | M/N | 13 | 00 | M/N-13 |
| 10.0 mm | N/P | 15 | 000 | N/P-15 |
| 11.5 mm | P | 16 | – | P-16 |
| 12.0 mm | – | 17 | – | 12 mm |
| 15.0 mm | P/Q | 19 | – | P/Q-19 |
| 16.0 mm | Q | – | – | Q |
| 19.0 mm | S | – | – | S |
| 25.0 mm | T/U/X | 50 | – | T/U/X |
Crochet hook sizes explained
The metric (mm) size is the diameter of the hook’s shaft at its thickest point. This is the universal reference — every modern crochet hook has its mm size printed on the thumb-rest or shaft. If your hook only has a letter or a number, look up the equivalent in the chart above or use the calculator.
What is a size H crochet hook?
A size H crochet hook is 5.0 mm (also written as H/8 in the US system, or UK size 6). It’s the most popular hook in the US — the default size for worsted weight yarn (the most common yarn weight). If a pattern doesn’t specify a hook, an H is usually a safe starting point.
What is a size G crochet hook?
A size G crochet hook is 4.0 mm (G/6 in full US notation, UK size 8). It’s used with sport and light DK yarns, and is a good all-rounder for amigurumi at the smaller end and light garments at the larger.
What is a size I crochet hook?
A size I crochet hook is 5.5 mm (I/9, UK size 5). It sits between H (5 mm) and J (6 mm) — used for worsted-to-aran yarn, blankets, and looser garments.
What is a size J crochet hook?
A size J crochet hook is 6.0 mm (J/10, UK size 4). It’s the go-to for aran weight yarn, blanket squares, and scarves where you want a slightly drapier fabric than worsted gives at an H hook.
What is a size F crochet hook?
A size F crochet hook is 3.75 mm (F/5, no direct UK equivalent — between UK 9 and 8). Used for sport weight yarn and tighter amigurumi work.
What is a size K crochet hook?
A size K crochet hook is 6.5 mm (K/10½, UK size 3). It bridges aran and chunky weights — common for fast scarves, hats, and blanket projects.
What size is a 4 mm crochet hook?
A 4 mm crochet hook is US letter G (G/6) and UK size 8. It’s a popular size for sport and DK yarns and a common starting hook for European patterns.
What size is a 5 mm crochet hook?
A 5 mm crochet hook is US letter H (H/8) and UK size 6. It’s the single most-used hook size in the US, paired with standard worsted weight yarn.
What size is a 6 mm crochet hook?
A 6 mm crochet hook is US letter J (J/10) and UK size 4. Used for aran-weight yarn and roomy fabric.
What size is a 3.5 mm crochet hook?
A 3.5 mm crochet hook is US letter E (E/4) and UK size 9. Used for baby weight and lightweight yarns, plus fine amigurumi.
What size is an 8 mm crochet hook?
An 8 mm crochet hook is US letter L (L/11) and UK size 0. Used for chunky/bulky weight yarn and fast projects.
Yarn weight to crochet hook size chart
The Craft Yarn Council classifies yarn into eight weight categories (0–7). Each category recommends a hook range — but within that range, smaller hooks give denser fabric and larger hooks give drapier fabric. Always crochet a gauge swatch before starting a real project.
| CYC # | Yarn weight | Also called | Recommended hook (mm) | US Letter |
|---|---|---|---|---|
| 0 | Lace | 10-count thread, fingering | 1.6–2.25 mm | Steel 6–14 / B |
| 1 | Super Fine | Fingering, sock, baby | 2.25–3.5 mm | B–E |
| 2 | Fine | Sport, baby | 3.5–4.5 mm | E–7 |
| 3 | Light | DK, light worsted | 4.5–5.5 mm | 7–I |
| 4 | Medium | Worsted, afghan, aran | 5.5–6.5 mm | I–K |
| 5 | Bulky | Chunky, craft, rug | 6.5–9 mm | K–M/N |
| 6 | Super Bulky | Super bulky, roving | 9–15 mm | M/N–P/Q |
| 7 | Jumbo | Jumbo, roving | 15+ mm | P/Q and larger |
UK crochet hook sizes (the reverse system)
UK hook sizes use the opposite convention to US: higher numbers mean smaller hooks. This is a leftover from a 19th-century gauge-wire numbering system. The UK system covers sizes 14 (smallest, 2 mm) through 000 (largest, 10 mm). Modern UK patterns often list metric and US sizes alongside the UK number, but vintage and traditional patterns may use UK only.
The most common UK sizes you’ll encounter:
| UK size | Metric | US equivalent |
|---|---|---|
| 14 | 2 mm | – (between B and a fine steel) |
| 13 | 2.25 mm | B/1 |
| 12 | 2.5 mm | – |
| 11 | 3 mm | – |
| 10 | 3.25 mm | D/3 |
| 9 | 3.5 mm | E/4 |
| 8 | 4 mm | G/6 |
| 7 | 4.5 mm | 7 |
| 6 | 5 mm | H/8 |
| 5 | 5.5 mm | I/9 |
| 4 | 6 mm | J/10 |
| 3 | 6.5 mm | K/10½ |
| 2 | 7 mm | – |
| 0 | 8 mm | L/11 |
| 00 | 9 mm | M/N-13 |
| 000 | 10 mm | N/P-15 |
Steel crochet hook sizes (for thread crochet)
Steel crochet hooks are a separate category — much smaller than regular hooks, used for thread crochet: doilies, fine lace, edgings, and Irish-crochet work. Sizes range from 00 (largest, 3.5 mm) down to 14 (smallest, 0.6–0.75 mm depending on brand).
Steel hook numbering is also reverse: higher number = smaller hook. The naming overlaps with regular hooks but the sizes are completely different — a regular “size 0” hook (8 mm) is nothing like a steel “size 0” (3.25 mm). Always check whether a pattern means regular or steel before buying.
Common steel hook sizes:
| US size | Metric (mm) | Used with |
|---|---|---|
| 00 | 3.5 mm | Heaviest thread crochet |
| 0 | 3.25 mm | Heavy thread |
| 1 | 2.75 mm | Size 3 thread |
| 2 | 2.25 mm | Size 5 thread |
| 4 | 2.0 mm | Size 10 thread (most popular) |
| 6 | 1.8 mm | Size 10 thread, doilies |
| 7 | 1.65 mm | Size 20 thread |
| 8 | 1.5 mm | Size 20 thread |
| 10 | 1.3 mm | Size 30 thread, hairpin lace |
| 12 | 1.0 mm | Size 50 thread |
| 14 | 0.75 mm | Size 80 thread, extra fine lace |
How to identify an unmarked crochet hook
If you’ve inherited a vintage hook or the markings have worn off, the Print & Identify tab in the calculator at the top of the page gives you a life-size reference. Print it at 100% scale (no “Fit to page”), then lay your hook’s shaft against each black circle until you find the match. That match is your hook’s mm size — cross-reference with the chart to find the US letter and UK number.
A few extra clues for vintage US hooks: brand and material can hint at the size. Older Boye and Susan Bates aluminum hooks usually have the size stamped near the thumb-rest; bone, ivory, or wooden hooks are usually larger sizes (6 mm and up); steel hooks are nearly always for thread crochet.
Crochet hook brand differences (Boye, Susan Bates, Clover, Prym)
Different brands publish slightly different conversion charts. The metric (mm) measurement is consistent, but the US letter assigned to a given mm can shift between brands. The most common differences:
- Boye vs. Susan Bates — Boye tends to label slightly larger hooks with the same US letter. A Boye “H” can be 5.25 mm where a Susan Bates “H” is exactly 5.0 mm.
- Clover (Japanese brand, popular for ergonomic Amour and Soft Touch hooks) — usually publishes both US and metric, with metric as the primary reference.
- Prym (German brand) — uses metric primarily, with US/UK shown as cross-reference. Their conversion chart is widely cited in European patterns.
- Susan Bates Silvalume — strictly follows the Craft Yarn Council chart (the values shown above).
When a pattern matters and precision matters, work to the millimeter, not the letter.
What hook should I use for amigurumi?
Amigurumi (small crocheted toys) uses a smaller hook than the yarn label suggests so that stitches are tight and the stuffing doesn’t show through. The common rule: go 1 to 2 hook sizes smaller than the yarn’s recommended range.
- Worsted weight yarn (CYC 4, normally 5.5–6.5 mm hook) → use a 3.5–4 mm hook for amigurumi
- Sport weight yarn (CYC 2, normally 3.5–4.5 mm) → use a 2.25–3 mm hook
- Most amigurumi patterns published since 2015 specify the hook directly — follow the pattern.
What hook should I use for blankets?
For blankets, the goal is drape and speed, so you go larger than the yarn’s recommended sweet spot:
- Worsted weight blanket → 5.5–6.5 mm (use the upper end of the range)
- Chunky/bulky weight blanket → 8–10 mm
- Super bulky throw blanket → 10–15 mm
- Jumbo/arm-crochet blanket → 15 mm+ or hands directly
Common hook size by project type
| Project | Yarn weight | Hook size |
|---|---|---|
| Doily, fine lace | Crochet thread | Steel 4–10 (1.3–2.0 mm) |
| Socks | Fingering (CYC 1) | 2.25–3.5 mm (B–E) |
| Baby blanket | Sport/DK (CYC 2–3) | 4–5 mm (G–H) |
| Amigurumi | Worsted (CYC 4) | 3.5–4 mm (E–G) |
| Standard scarf | Worsted (CYC 4) | 5–5.5 mm (H–I) |
| Sweater/garment | Worsted (CYC 4) | 5–6 mm (H–J) |
| Afghan/throw | Worsted to bulky | 5.5–8 mm (I–L) |
| Chunky blanket | Bulky/super bulky | 8–10 mm (L–N) |
| Arm-crochet blanket | Jumbo | 15 mm+ or arms |
FAQ section (single box)
What’s the difference between a US and UK crochet hook size?
US sizes use letters (B through Q) paired with numbers (1 through 19). UK sizes use only numbers, in reverse — higher numbers mean smaller hooks. The metric (mm) measurement is the same worldwide and is the safest way to translate between systems. A 5 mm hook = US H/8 = UK 6.
Are all crochet hook sizes the same between brands?
Not exactly. The metric (mm) measurement is consistent, but the US letter assigned to a given mm can vary by 0.25 mm or more between Boye, Susan Bates, Clover, Prym, and Tulip hooks. If a pattern matters, work to the mm rather than the letter.
What is the most common crochet hook size?
The 5 mm hook (US H/8) is the most common size in the US, paired with standard worsted-weight yarn. If you’re buying a single starter hook, an H is the safest pick.
What size crochet hook do beginners need?
Most beginners do well with a 5.5 mm (US I/9) hook and worsted weight yarn — slightly larger than the “standard” 5 mm pairing gives a looser, easier-to-see fabric. As you build tension control, drop to a 5 mm hook for the typical worsted-weight pattern.
What does 4 mm mean on a crochet hook?
The number on a crochet hook (like 4 mm or 5 mm) is the diameter of the shaft — the part the yarn loops over to form each stitch. A 4 mm hook makes loops 4 mm wide, which translates to a specific stitch height and gauge.
What hook for chunky yarn?
For chunky / bulky yarn (CYC 5), use a 6.5–9 mm hook (US K/10½ to M/N-13). The most common chunky-yarn hook is 8 mm (US L/11).
What’s a size 7 crochet hook?
A size 7 crochet hook is 4.5 mm (no US letter — sits between G and H). The US number “7” and the UK number “7” happen to map to the same hook here, which is unusual.
What crochet hook for worsted weight yarn?
Worsted weight yarn pairs best with a 5–5.5 mm hook (US H/8 or I/9). The exact choice depends on whether you want denser fabric (H) or drapier fabric (I).
What’s the smallest crochet hook?
The smallest commonly available is size 14 steel (about 0.6–0.75 mm) for fine thread crochet. Below that, sizes 16+ exist for collectors but are rarely used. For regular yarn hooks, the smallest is around 2 mm (steel size 4 / regular size 13 UK).
What’s the biggest crochet hook?
Mass-produced hooks go up to about 25 mm (size T/U/X, sometimes called “Q”). Above that, crocheters generally switch to arm crochet (using their forearms as hooks) for jumbo-yarn blankets.
Is crochet hook size the same as knitting needle size?
The mm system is the same across both — a 5 mm crochet hook and a 5 mm knitting needle have the same shaft diameter. But for the same yarn weight, crocheters typically use a slightly larger tool than knitters. Worsted weight pairs with a 5 mm crochet hook (US H/8) but a 4.5 mm knitting needle (US 7).