Convert any crochet hook size between US letter sizes, UK numbers, metric mm and Japanese sizing. Includes recommended yarn weight for each hook.
Built and maintained by Paul Clark, Redmoon Software · Checked against what this tool computes on
| Metric (mm) | US | UK | Japan | Typical yarn |
|---|
Hook sizing is a lookup, not a formula. Metric mm is the only true measurement — US, UK and Japanese systems each use letters or numbers that map to a hook diameter. The chart below uses Craft Yarn Council standards.
UK numbers go DOWN as the hook gets bigger (opposite of metric). Always check gauge on your project — different manufacturers vary by ±0.1 mm.
There is no formula here, because hook sizing is a lookup: 21 rows running from 2.0 mm up to 19.0 mm, with millimetres as the only true measurement. Each row pairs a diameter with its US letter and number (B/1 at 2.25 mm, G/6 at 4.0 mm, H/8 at 5.0 mm, K/10.5 at 6.5 mm), its UK number, its Japanese size and a typical yarn weight. The mapping follows the Craft Yarn Council chart.
Choose your Input system first, from Metric (mm), US letter, UK number or Japan, and the Size dropdown rebuilds itself with only the sizes that actually exist in that system, since none of the four covers every diameter. Seven metric preset buttons jump straight to 2.25, 3.5, 4.0, 5.0, 6.0, 8.0 and 10.0 mm, switching the system back to metric as they do. Your selection is highlighted in the full table.
The gaps in the chart are real rather than omissions: the UK system has no entry at 2.75 or 3.75 mm, and the Japanese column covers only a narrow middle band, so those cells show a dash and drop out of the dropdown entirely. Manufacturers also vary by roughly a tenth of a millimetre on the same nominal size, and handle shape and head style influence your tension more than that difference does. Always swatch.
4.0 mm, listed as G/6. Its neighbours are F/5 at 3.75 mm and plain 7 at 4.5 mm, which carries no letter at all, one of several US crochet sizes identified by a number only rather than a letter and number pair.
UK numbering counts downward as the hook gets thicker: 14 is 2.0 mm, 8 is 4.0 mm, 4 is 6.0 mm and 000 is 10.0 mm. It runs opposite to the metric scale, which is why converting from memory goes wrong so often.
The chart points to 5.0 to 6.0 mm, H/8 through J/10, for worsted and worsted-aran. DK sits around 4.5 to 5.0 mm, sport around 3.5 to 4.0 mm, and bulky from 6.5 mm upward. Treat all of these as swatching starting points.
No. Japanese sizes appear only for 2.5 mm through 5.5 mm, plus a single entry at 7.0 mm, so selecting Japan gives a much shorter dropdown than metric does. Diameters outside that band show a dash and are left out.
Not in the letter systems. This chart covers crochet hooks only: US crochet sizes use letters such as H/8, whereas US knitting needles use plain numbers, and the same numeral means a different diameter in each craft entirely.
US letter sizes, UK numbers and Japanese sizes all map imperfectly onto millimetres, and the mappings have changed over time and differ between manufacturers. A US "H" is nominally 5 mm and not every H hook measures it.
The metric figure is the physical measurement, which is why any conversion should be treated as identifying a millimetre size rather than as an exact equivalence between letter systems.
Two hooks of identical millimetre size can produce noticeably different gauge, because head shape and throat depth differ between brands — inline hooks and tapered hooks pick up yarn differently and the resulting stitches are not the same size.
That is why gauge must be swatched with the actual hook, and why substituting a nominally equivalent hook from another brand mid-project can produce a visible change in the fabric.
Vintage patterns are a further trap, since the size systems they reference may not match modern equivalents at all.
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