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Crochet Hook Size Converter
Convert crochet hook sizes between millimetres, US letters and numbers, and old UK sizes. Steel thread hooks are included, and each hook shows the yarn weight and typical gauge it pairs with.
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I know the
mm
The mm size stamped on the hook's thumb rest
Enter your values to see the result.
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How to use the Crochet Hook Size Converter
- Choose what you know: the US letter, the mm size or an old UK number.
- Pick or type the size, for example H-8, 5.5 mm or Steel 7.
- Press Convert to see every equivalent.
- Read the yarn pairing and typical gauge for that hook.
- Go up or down a size if your swatch does not match the pattern.
Hook size by yarn weight
| Yarn weight | Also called | Hook (mm) | Hook (US) | Single crochet per 4 in |
|---|---|---|---|---|
| 0 Lace | Crochet thread, lace | Steel 1.4–1.6; regular 2.25 | Steel 6–8; B-1 | 32–42 dc |
| 1 Super Fine | Sock, fingering, baby | 2.25–3.5 | B-1 to E-4 | 21–32 |
| 2 Fine | Sport, baby | 3.5–4.5 | E-4 to 7 | 16–20 |
| 3 Light | DK, light worsted | 4.5–5.5 | 7 to I-9 | 12–17 |
| 4 Medium | Worsted, afghan, aran | 5.5–6.5 | I-9 to K-10½ | 11–14 |
| 5 Bulky | Chunky, craft, rug | 6.5–9 | K-10½ to M-13 | 8–11 |
| 6 Super Bulky | Super bulky, roving | 9–15 | M-13 to Q | 7–9 |
| 7 Jumbo | Jumbo, roving | 15 and up | Q and up | 6 or fewer |
Ranges follow the Craft Yarn Council Standard Yarn Weight System. Amigurumi is usually worked one or two hook sizes smaller than the label suggests so the fabric is tight and stuffing does not show.
Steel hooks and crochet thread
Steel hooks are made for cotton crochet thread used in doilies, lace and edgings. Thread is sized by number too, and finer thread needs a higher-numbered (thinner) hook. As a starting point, size 3 thread is often worked with a Steel 0–2 hook (2.25–3.25 mm), size 10 thread (the most common) with Steel 5–7 (1.65–1.9 mm), and size 20–30 thread with Steel 9–12 (1.0–1.4 mm). Always follow the pattern and test a small swatch.
- Check the shaft, not the head. The size refers to the diameter of the throat/shaft section that forms the loops.
- Inline vs tapered hooks of the same mm size can give slightly different gauge; stick with one style for a project.
- Old UK sizes appear in vintage British patterns, for example "No. 8 hook" = 4 mm and "No. 10" = 3.25 mm.
Frequently asked questions
What size is an H crochet hook in mm?
An H-8 hook is 5 mm. It is the most common hook for worsted-weight yarn alongside I-9 (5.5 mm).
What letter is a 4 mm crochet hook?
4 mm is a G-6 hook. Some older hooks labelled G are 4.25 mm, so check the mm stamp when it matters.
Why do steel hook numbers go backwards?
Steel hooks for crochet thread use a wire-gauge style numbering where higher numbers are thinner: Steel 00 is 3.5 mm and Steel 14 is 0.75 mm. Regular yarn hooks run the other way, from B-1 (2.25 mm) up to S (19 mm).
Are crochet hook letters the same in every brand?
Mostly, from B-1 to K-10½. Larger hooks vary: brands label 9 mm as M or N, 10 mm as N or P, and Q can be 15, 15.75 or 16 mm. The mm size is always the reliable one.
What hook size should I use for worsted yarn?
The Craft Yarn Council suggests 5.5–6.5 mm (I-9 to K-10½) for medium/worsted yarn, but many yarn labels and patterns use 5 mm (H-8). Make a swatch and change hook size until your gauge matches.
Is a 5 mm crochet hook the same as a 5 mm knitting needle?
The diameter is the same, but the US labels differ: a 5 mm crochet hook is H-8 and a 5 mm knitting needle is US 8. Crochet usually needs a hook one or two sizes larger than the knitting needle for the same yarn.
Are all the tools on FreeToolCabin tested for quality and accuracy?
Yes! Every tool on our site undergoes automated Q/A testing in real browser environments to ensure high precision, correct output, and zero errors. If you happen to spot a bug or have a suggestion, please contact us at [email protected] and our team will review it right away.
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