US I-9 Crochet Hook: Metric Size

I-9 sits at 5.5mm on the metric scale, marking the low end of what worsted weight's chart calls for — I-9 through K-10.5 is the full published span — and it's one of the most frequently reached-for hooks in general crochet.

Where I-9 gets used

Worsted-weight blankets, hats, and amigurumi bases before any size-down adjustment for density. It's the hook most worsted-weight yarn labels are tested and printed against, which makes it a reasonable first swatch attempt any time a pattern simply says "worsted, hook I-9 to K-10.5" without narrowing further.

The two hooks bracketing it

H-8 (5.0mm) is one step finer; J-10 (6.0mm) one step up — a full millimetre jump from I-9 to J-10, noticeably wider than the half-millimetre step from H-8 to I-9 just below it.

Gauge implications

Single crochet at worsted's own range (11-14 stitches per 4 inches) tends to sit at the tighter end on an I-9 hook for most worsted yarns, since I-9 is the bottom of that weight's recommended range rather than its middle. If your worsted-weight project feels denser than a pattern's photos suggest it should, sizing up to J-10 or K-10.5 is a reasonable next swatch.

FAQ

Is I-9 the "default" worsted crochet hook? It's a common starting point specifically because it's the range's lower bound and the size most yarn labels reference, though K-10.5 (the range's upper bound) is an equally valid starting swatch depending on how loose or tight you want the fabric.

What knitting needle size matches I-9 in diameter? A US 9 knitting needle (5.5mm) shares the identical 5.5mm diameter — one more instance of the knit and crochet naming conventions lining up at this particular size.

Does I-9 work for a firmer worsted-weight fabric than K-10.5 would give? Yes — being the smaller of the two hooks in worsted's recommended range, I-9 produces a denser, firmer result than K-10.5 at identical yarn.

Is I-9 a reasonable hook to buy first if I'm just starting worsted-weight crochet? Yes — it's affordable, widely stocked, and gives you a real result to compare against before deciding whether your own tension wants a size up.