Hook size

Size F crochet hook: 3.75 mm (US F-5)

A size F crochet hook is 3.75 mm, labelled US F-5, and has no old UK number. It is the recommended hook range for fine (2) yarn, and crocheters typically choose it for sport-weight garments, baby blankets and doilies in fine cotton. 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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3.75 mm crochet hook conversions
Metric	US	Old UK	Suits yarn weights
3.75 mm	F-5	—	#2 Fine

What is a 3.75 mm crochet hook in US and UK sizes?

A 3.75 mm hook is US size F-5 and has no old UK equivalent. 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 3.75 mm
Metric	US	Old UK	Yarn weights
3.25 mm	D-3	10	#1
3.5 mm	E-4	9	#1, #2
3.75 mm (this hook)	F-5	—	#2
4 mm	G-6	8	#2
4.5 mm	7	7	#2, #3

What yarn weight goes with a 3.75 mm hook?

A 3.75 mm hook is typically used with fine (#2) yarn. Fine yarn gives about 16–20 sc per 4 in / 10 cm and suits baby blankets, summer tops, lightweight cardigans.

Yarn weights for a 3.75 mm hook
Weight	Also called	Crochet gauge	Recommended hooks
#2 Fine	Sport, Baby, 4-ply (UK/AU, approximate)	16–20 sc per 4 in / 10 cm	3.5–4.5 mm

What can you make with a 3.75 mm hook?

19 patterns in our library use a 3.75 mm hook, including flower afghan (1951), granny afghan (1951), granny afghan (no. 1745), blue and white shell stitch dress, child’s lily cap (1915), durbar afghan (no. 2774).

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.