Crochet Hook Size Converter: Metric, US & UK Conversion Tool
Convert between metric (mm), US letter/number, and UK/Canadian hook sizes. Enter any size in any system and see the equivalents — plus the recommended yarn weight pairing.
Crochet hook sizes use three independent systems: metric (millimetres, universal), US letter or number (e.g. H/8 or 8), and UK/Canadian (numbers, where smaller numbers mean larger hooks). The metric measurement is the diameter of the hook shaft. Use the converter below for any size.
The complete crochet hook size chart
This table shows every standard size across all three systems. Sizes marked "—" don't have a direct equivalent in that system. The most commonly used sizes (H/8 to K/10.5) are highlighted.
| Metric (mm) | US | UK / Canadian | Typical yarn weight |
|---|---|---|---|
| 2.0 | — | 14 | Lace (#0) |
| 2.25 | B/1 | 13 | Lace / Fingering |
| 2.75 | C/2 | 12 | Fingering (#1) |
| 3.25 | D/3 | 10 | Sport (#2) |
| 3.5 | E/4 | 9 | Sport / DK |
| 3.75 | F/5 | — | DK (#3) |
| 4.0 | G/6 | 8 | DK / Worsted |
| 4.5 | 7 | 7 | Worsted (#4) |
| 5.0 | H/8 | 6 | Worsted (#4) |
| 5.5 | I/9 | 5 | Worsted / Bulky |
| 6.0 | J/10 | 4 | Bulky (#5) |
| 6.5 | K/10.5 | 3 | Bulky (#5) |
| 8.0 | L/11 | 0 | Super Bulky (#6) |
| 9.0 | M/13 | 00 | Super Bulky (#6) |
| 10.0 | N/15 | 000 | Super Bulky / Jumbo |
| 15.0 | P/Q | — | Jumbo (#7) |
| 19.0 | S | — | Jumbo (#7) |
Why three systems exist
The metric system is the modern universal standard — it just measures hook diameter in millimetres. The US system uses letters that increase as hooks get larger (B → S), with numbers attached for redundancy (B/1, H/8, M/13). The UK/Canadian system uses numbers derived from the old Steelmaker's Wire Gauge, where smaller numbers mean larger hooks — the opposite of US logic, which is the source of most confusion.
What every system is ultimately describing is the diameter of the hook's shaft, the section behind the throat that each loop is drawn over. Seeing which part of a crochet hook the size refers to explains why two hooks stamped with the same letter can still produce different gauge: the labels match, the measured shaft does not.
If you're between hook sizes and your yarn label says "5.0–6.0 mm," start with the middle (5.5 mm) and adjust based on your gauge swatch. Going up creates looser, drapier fabric; going down creates tighter, denser fabric. Neither is wrong — choose based on the fabric feel you want.
Steel hooks (for thread crochet)
Steel crochet hooks are a separate category used exclusively for thread crochet — doilies, lace edgings, and delicate work. Steel hooks are much smaller than standard aluminium or bamboo hooks, ranging from 0.6 mm to about 2.0 mm. They use their own US numbering system where larger numbers mean smaller hooks.
Pro tips for hook size decisions
- Trust the metric measurement. Modern hooks all show the mm size. US letter/number and UK number systems can be confusing — the mm number is universal.
- Start with the yarn label's recommendation. The label prints a hook range. Use the middle of the range as your starting point, then adjust based on your swatch.
- Use a smaller hook for amigurumi. Go down 1-2 mm from the yarn label's recommendation — produces dense fabric that hides stuffing.
- Beware UK numbers running opposite from US. UK Size 4 is larger than UK Size 8 — the reverse of US lettering.
- Different brands at the same mm can feel different. Inline hooks produce slightly tighter stitches than tapered. Stick with one brand per project.
Worked examples
UK to US: a UK pattern calls for a Size 4 hook. The conversion: UK 4 = US J/10 = 6.0 mm. Match by mm.
Vintage US: an old pattern says "Size H." Modern US H/8 = 5.0 mm — use a 5.0 mm hook.
Common mistakes when converting hook sizes
These are the errors that most often send a calculated result off course. Each one changes the number the calculator gives you.
-
Assuming UK numbers climb with size like US ones.
They run the opposite way. A UK 6 is larger than a UK 9. Reading a UK pattern with US instincts gets you a hook two or three sizes off.
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Trusting the letter over the millimetre.
Letters are not standardised between brands — one maker's H is another's 5.25 mm. The millimetre figure is the physical measurement of the shaft, so treat it as the source of truth and the letter as a label.
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Reading a steel hook against the aluminium scale.
Steel thread hooks use their own numbering, and it also runs backwards: a steel 14 is much finer than a steel 0. They are not interchangeable with the regular scale.
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Thinking an ergonomic handle means a different size.
The grip changes comfort, not the shaft. A 5.0 mm ergonomic hook makes the same stitch as a 5.0 mm aluminium one, though a more relaxed grip can loosen your tension slightly.
Direct answers.
Why are crochet hook sizes so confusing?
Three systems developed independently: metric (universal, measures diameter in mm), US (letters A–S, larger letters mean larger hooks), and UK/Canadian (numbers, smaller numbers mean larger hooks — the reverse of US). Modern hooks worldwide are standardised on metric; you'll see mm size stamped on every modern hook.
What's the difference between inline and tapered hooks?
Inline hooks (Susan Bates style) have a throat the same width as the shaft — yarn doesn't slip back. Tapered hooks (Boye style) have a wider throat that narrows to the head, which some crocheters find easier to insert into stitches. Both produce the same gauge at the same mm size; the difference is purely tactile.
Do I need different hooks for different yarn weights?
Yes. Each yarn weight has a recommended hook range printed on its label. Use a too-small hook with bulky yarn and your fabric will be stiff and dense; use a too-large hook with fine yarn and the fabric will be loose and floppy.
Are ergonomic hooks the same sizes as regular hooks?
Yes. Ergonomic hooks (Clover Amour, Tulip Etimo, Furls Odyssey) use the same standard millimetre sizing as regular hooks — a 5.0 mm ergonomic produces the same gauge as a 5.0 mm aluminium. The handle is the only difference, designed to reduce hand fatigue.
What are steel crochet hooks?
Steel hooks are used for thread crochet (doilies, fine lace, tatted edgings). They range from 0.6 mm to about 2.0 mm — much smaller than yarn hooks. The US numbering for steel hooks is reversed: larger numbers mean smaller hooks (a #14 steel hook is smaller than a #8).
Sources & further reading
- Craft Yarn Council — Crochet Hook and Knitting Needle Sizes — the metric-to-US equivalents used in the conversion table on this page.
- Craft Yarn Council — Standard Yarn Weight System — the recommended hook range for each yarn weight category.
Millimetres are the reference measurement; US letters and UK numbers are labels mapped onto it. Those mappings are conventions rather than a legally defined standard, and letter sizing can differ slightly between manufacturers, so a hook's stamped millimetre value is the value to trust when two brands disagree.