Hook size

Size I crochet hook: 5.5 mm (US I-9)

A size I crochet hook is 5.5 mm, labelled US I-9, and has old UK size 5. It is the recommended hook range for light (3) and medium (4) yarn, and crocheters typically choose it for worsted blankets, beanies and everyday granny squares. Always check gauge with a swatch, because tension varies between crocheters.

By the crochetpatterns.io editorial team · Updated September 20, 2026 · Editorial policy

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5.5 mm crochet hook conversions
Metric	US	Old UK	Suits yarn weights
5.5 mm	I-9	5	#3 Light, #4 Medium

What is a 5.5 mm crochet hook in US and UK sizes?

A 5.5 mm hook is US size I-9 and old UK size 5. Metric millimetres are the only measurement that is the same on every brand, so check the mm size stamped on the hook.

Hook sizes around 5.5 mm
Metric	US	Old UK	Yarn weights
4.5 mm	7	7	#2, #3
5 mm	H-8	6	#3
5.5 mm (this hook)	I-9	5	#3, #4
6 mm	J-10	4	#4
6.5 mm	K-10½	3	#4, #5

What yarn weight goes with a 5.5 mm hook?

A 5.5 mm hook is typically used with light (#3) yarn or medium (#4) yarn. Light yarn gives about 12–17 sc per 4 in / 10 cm and suits garments, baby items, amigurumi. Medium yarn gives about 11–14 sc per 4 in / 10 cm and suits blankets and afghans, beanies, granny squares.

Yarn weights for a 5.5 mm hook
Weight	Also called	Crochet gauge	Recommended hooks
#3 Light	DK, Double knitting, Light worsted, 8-ply (AU/NZ)	12–17 sc per 4 in / 10 cm	4.5–5.5 mm
#4 Medium	Worsted, Afghan, Aran, 10-ply (AU/NZ)	11–14 sc per 4 in / 10 cm	5.5–6.5 mm

What can you make with a 5.5 mm hook?

13 patterns in our library use a 5.5 mm hook, including ribbed twisted crochet ear warmer headband, top-down granny stitch triangle shawl, classic shell stitch crochet cowl, relaxed slouchy crochet beanie with ribbed brim, the twenties flip crochet hat, gypsy bolero.

Sources and scope

Craft Yarn Council: Hooks & Needles
Reference for metric and US hook sizes. Letter and number labels vary between manufacturers; check the millimetre measurement. Old UK sizes are a separate historical convention.
Craft Yarn Council: Standard Yarn Weight System
Source for categories 0–7 and the crochet gauge and hook ranges. These are starting guidelines; use the gauge in your pattern. Regional ply names and yards per 100 g are approximate comparisons, not CYC specifications.