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What does this converter do?
Crochet hook converter: translate hook sizes between US letters, UK numbers, and metric millimeters for any yarn pattern. Free, private, runs in your browser.
- US E-4 = 3.5 mm; US H-8 = 5.0 mm; US K-10.5 = 6.5 mm.
- Steel hooks for thread work use a separate numeric scale (sizes 0-14, finer as numbers rise).
- Hook labels are nominal — a '5.0 mm' hook can measure 4.9-5.1 mm and still be in standard.
How this calculator works
Formula
US letter sizes map to mm (B = 2.25 mm ... K = 6.5 mm) with the same no-formula convention as knitting needles; metric sizing is the primary standard.
Sources
Sources support the method, terminology, or planning context. Editable assumptions remain your responsibility.
Assumptions to verify
- Manufacturer charts vary; conversion outputs are the standard-mapping midpoint.
- Measurements are body measurements unless the tool labels them as garment measurements.
- Between-size results round per the rule stated in the FAQ.
Frequently asked questions
Why do patterns give both letters and mm?
US patterns use letters, European patterns use mm. Gauge again decides: match the mm the pattern's gauge assumes, and treat the letter as its nickname.
Cite this tool
BoringToolsKit. “Crochet Hook Converter.” boringtoolskit.com/crochet-hook-converter/ (reviewed 2026-09-20). Free to reference in articles, syllabi, and answer posts with a link.
Privacy: Inputs and results stay in this browser. Conversion runs locally in your browser; review the output before relying on it.
A worked example: converting a US size 7 to metric
Suppose a pattern calls for a US 7 hook and you want the exact metric size to match a European hook in your drawer. Type 7 into the converter. The array lookup takes position 7: the letters array runs B, C, D, E, F, G, H, I, J, K, so index 7 is letter I, and the millimeter array at the same index is 5. The result shows US I and 5 mm, which matches the published standard for a US 7 hook.
Now try index 3, a US E. Position 3 in the letters array is E, and position 3 in the mm array is 3.5, so the output reads US E, 3.5 mm. If you instead entered 5, position 5 gives letter H and 4.0 mm, so a US H-8 is a 5 mm hook, not a 5.5 mm one. That distinction matters when buying hooks: a 5 mm and a 5.5 mm hook differ by only half a millimeter in diameter, yet they sit two US sizes apart (H-8 vs I-9) and produce visibly different stitch gauges.
The lookup structure also explains why some sizes are skipped. The mm array moves in small steps early on (2.25, 2.75, 3.25, 3.5, 3.75) and jumps from 4 mm to 5 mm between H and I. There is no standard US letter for a 4.5 mm hook; in practice knitters and crocheters borrow the knitting needle convention and just call it 4.5 mm or a US 7 knitting-needle equivalent. So the converter will never return 4.5 mm for any index between 0 and 11, and that gap is a real property of the sizing system, not a missing entry.
What the input boundaries mean in practice
The converter accepts only integer indexes 0 through 11, and the boundaries behave in specific ways. At index 0 you get US B and 2.25 mm, the smallest standard hook in the table; this size is fine work territory, used for thread doilies and bead crochet where stitch counts run in the hundreds. At index 11 you get US K and 6.5 mm, the largest in the table; beyond K the sizing system continues with sizes marked by millimeter labels alone (L-11 is roughly 8 mm, and giant hooks for jumbo yarn run 15 to 25 mm with no letter at all). Entering anything outside 0 to 11, including a negative number, returns the check-the-input message instead of a conversion, so a stray decimal or a mistyped 12 will never silently produce a wrong hook size.
One systematic limitation: the converter only handles the standard aluminum and plastic hook line. Steel hooks, used for thread crochet, use a different scale where higher numbers mean smaller hooks (a US 12 steel is about 1 mm, a US 0 steel is about 2.7 mm), and none of those sizes appear in this table. The result panel flags this by reminding you hooks also come in steel sizes, but if your pattern specifies a steel size, treat the output as a starting reference rather than a match. A second practical note: older or imported patterns occasionally label hooks by UK size, where a UK 4 corresponds to the US F-5 at 3.75 mm, so verify which system a source uses before trusting a single conversion.