Crochet hooks vary widely in how they are labeled. Depending on the brand, where the hook was manufactured and who the target audience was, the size may be indicated by a number, letter, metric mm measurement or any combination of the aforementioned.

I see metric as most standard since it correlates to a measurement that is commonly understood and is often listed as a secondary size on hook packaging. It would seem the industry agrees with me and is trending in the direction of uniformity and confusion reduction. In the meantime however, I put together a chart showing the comparable hook sizes, so that when a pattern says “use G Hook” you can easily find out what that means. I’ve shown the most common yarn weights associated with those hook sizes when talking Amigurumi, which is worked much more tightly and thus uses a crochet hook a few sizes smaller than the manufacturer recommends for the corresponding yarn. Green denotes the typical sizes used for amis.

Yarn Type (for Amigurumi)	Metric	(CN/UK)	(US)	(JP)
SuperFine Yarn Sock, Fingering & Baby weight.	2.0mm	14	-	2/0
	2.25mm	13	B or 1	-
Fine Yarn Sport and Baby weight.	2.3mm	-	-	3/0
	2.5mm	12	-	4/0
Light DK & Light Worsted weight.	2.75mm	-	C or 2	-
	3.0mm	11	-	5/0
	3.25mm	10	D or 3	-
Medium Worsted, Afghan, Aran weight.	3.5mm	9	E or 4	6/0
	3.75mm	-	F or 5	-
	4.0mm	8	G or 6	7/0
Bulky Chunky, Craft, Rug & Novelty.	4.5mm	7	7	7.5/0
	5.0mm	6	H or 8	8/0
	5.5mm	5	I or 9	9/0
Super Bulky Bulky, Roving & Novelty.	6.0mm	4	J or 10	10/0
	6.5mm	3	K	-
	7.0mm	2	-	-
	8.0mm	0	L or 11	-
	9.0mm	00	M or 13	-
	10.0mm	000	N or 15	-

I usually use a 3.75mm hook with medium weight yarn and a 3.5mm hook for smaller pieces (arms that are 6 ST around for example). Thicker bulky and novelty yarns are not typically used in Amigurumi (though they can for a neat effect) and lighter yarns or crochet thread would not usually be used unless you specifically wanted to make a teeny piece.