Crochet hook size converter

Crochet hooks carry a letter, a number, or both, and the pairing is the part people get wrong. H-8 is one hook, not two; the letter and the number are two names for the same 5 mm shaft. It falls apart at the edges, though — G names a 4 mm hook in one company's range and a 4.25 mm hook in another's, and the official chart records both. This converter works from millimetres to letters and back, and tells you when a letter means more than one thing.

How it is worked out

size → row in the Craft Yarn Council chart → the size in the other system

Hook sizing is a published table, not a calculation. The letters run alphabetically with diameter and the numbers run upward too, but neither sequence is regular: there is no A, the 4.50 mm hook is called 7 with no letter at all, and past P the chart runs P/Q, Q, S, then T/U/X. Looking the size up is the only method that survives those gaps.

Source: Craft Yarn Council — Hooks & Needles conversion chart: crochet hook sizes, 2.25 mm to 30 mm

The whole chart

Every row the Craft Yarn Council publishes, with the yarn weights each size is recommended for. A dash means the standard lists no size there rather than that we left it out.

Questions people ask

Is H the same as 8?

Yes — H-8 is a single hook of 5 mm, written with both the letter and the number because U.S. manufacturers have used both systems. A pattern that calls for an H hook and one that calls for an 8 are asking for the same tool. Type either into this converter and you will get 5 mm.

Why does the letter G appear twice in the chart?

Because two different diameters have been sold under it: 4 mm as G-6 and 4.25 mm as plain G. A quarter of a millimetre changes the fabric enough to matter over a whole garment, so when a pattern says G and the gauge is tight, check the millimetre marking on the hook you actually own.

Why are steel hooks numbered backwards?

Steel crochet hooks are used with crochet thread and lace-weight yarn, and they follow their own scale where the higher the number, the smaller the hook — the reverse of regular hook sizing. They are a separate table in the standard and are not covered by this converter, which handles regular hooks only.

What about UK hook sizes?

The Craft Yarn Council chart maps millimetres to U.S. sizes and stops there, so this tool does the same. UK hooks are sold by millimetre today; older UK patterns use a numbering that runs opposite to the metric size and was never standardised against the U.S. one. Measure in millimetres and the ambiguity disappears.

Which hook should I use with my yarn?

The chart gives a recommended range per yarn weight, and this converter shows which weights each hook serves — a 5 mm hook, for instance, sits in the Light range. Treat it as a starting point and work a gauge swatch: the pattern's stitch count decides the hook, not the label on the yarn.

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