Yarn Weight Conversion Chart

UK	USA	Suggested	Typical uses
		Hook
2ply	Baby	2.25 to 3.5mm	Baby clothes
	Fingering
3ply	Fingering	3 to 4mm	Baby clothes,
	Sock		shawls, socks
4ply	Sport	3.5 to 4.5mm	Baby clothes,
			shawls, socks,
			lightweight sweaters
Double Knitting	Double Knitting	4 to 5mm	Children's clothes,
	Light Worsted		toys, adult sweaters
Aran	Fisherman,	5.5 to 6.5mm	Adult Sweaters
	Worsted
Chunky	Bulky	6.5 to 9mm	Adult Sweaters,
			Bags
Extra Chunky	Super Bulky	9mm-12mm	Bags, Rugs

 
Yarn comes in a wide variety of thicknesses, and it can be quite daunting when you first start crocheting to know which yarn to use. Most patterns will specify a yarn, and give an approximate gauge / tension size, usually how many stitches and rows over a 4 x 4 inch square. (Tension is the traditional term used in knitting in the UK, but the US term is gauge).

It's important to check your gauge / tension before starting a project, to make sure you have the correct yarn and hook size, especially when making something that needs to fit, like clothing. Once you've been crocheting for a while, you'll be able to play around with different weight yarns and not just stick to the yarn weight specified in a pattern.

The names for different weight yarns vary between the UK and USA. Bear in mind that even yarns in the same weight can vary a great deal, eg Paton's Diploma Gold DK is much thinner than say, a budget acrylic DK like Paton's Fab. 

I've included the recommended hook sizes for each yarn, although again, this varies depending on what you are making. For a bag you would use a smaller hook to create a dense, tight stitch, but for something drapey like a shawl or scarf you would use a larger hook. A crochet pattern will always tell you which hook size to use.

Happy crocheting!