Conversion Chart for Yarn
American yarns vs European yarns
Weight	Sts Per 4" (10 cm)	Suggested Needles	
Category 1  Superfine  Fingering  Baby  4 Ply	28-32	0-3 US  2-3.25mm	
Category 2  Fine  Sportweight  5 Ply	24	3-6 US  3.25-4.25mm	
Category 3  Light  DK  8 Ply	22	4-6 US  3.50-4.25mm	
Category 4  Medium  Worsted  10 Ply	20	6-9 US  4.25-5.50mm	
Heavy Worsted  Aran  12 Ply	16-18	8-10 US  5-6mm	
Category 5  Bulky  Chunky  13 Ply	12-14	10-11 US  6-8mm	
Category 6  Super Bulky  14 Ply	8-10	11-15 US  8-10mm	

2-Ply
Either 32 stitches and 40 rows to 10 cm. (4 inches) on a 3¼ mm (US 3) needle, or 34 stitches and 42 rows to 10 cm. (4 inches) on a 3¼ mm (US 3) needle, depending upon the manufacturer. Check the ball band and pattern.
3-Ply
32 stitches and 40 rows to 10 cm. (4 inches) on a 3¼ mm (US 3) needle.
4-Ply Fingering
28 stitches and 36 rows to 10 cm. (4 inches) on a 3¼ mm (US 3)needle. In the US the term 4-Ply is used for Knitting Worsted. Check your pattern carefully to determine which yarn you need.
5-Ply
Used in Australia and New Zealand. 26 stitches to 10 cm (4 inches) on a 3.75 mm (US 5) needle. See also Quickerknit
8-Ply
Used in Australia and New Zealand for Double Knitting or DK.
10-Ply
Used in Australia and New Zealand for Worsted Weight Yarn.
12-Ply
Used in Australia and New Zealand. 17 to 18 stitches to 10 cm (4 inches) on a 5 to 5.5 mm (US 8 to 9) needle. See Aran .
Aran
18 stitches and 24 rows to 10 cm (4 inches) on a 5 mm (US 8) needle. Some knitters use Knitting Worsted instead, but check your tension carefully first!
Baby
Usually used to refer to 3-Ply but check the tension on the pattern first.
Bulky
14 stitches and 19 rows to 10 cm. (4 inches) on a 6 mm (US 10) needle.
Chunky
15 stitches and 20 rows to 10 cm. (4 inches) on a 6 mm (US 10) needle. In the UK what is referred to as a chunky is what North Americans call a Bulky. Again check your pattern and tension carefully.
Double Knitting (DK)
22 stitches and 30 rows to 10 cm. (4 inches) on a 4 mm (US 6) needle.
Jumper Weight
Usually a Four Ply Fingering Weight.
Knitting Worsted
20 stitches and 26 rows to 10 cm. (4 inches) on a 4 1/2 mm (US 7) needle.
Mohair
Mohair generally knits up as a Chunky but some mohairs knit slightly different, so check your pattern and tension first!
Quickerknit
26 stitches to 10 cm (4 inches) on a 3.75 mm (US 5) needle. Popular yarn in the UK, mainly for baby and toddler outfits. Not generally available in North America.
Sports weight Yarn
24 stitches and 34 rows to 10 cm. (4 inches) on a 3 3/4 mm (US 5) needle. Some yarns such as Astra knit up as Sports weight or a Double Knit. It should be possible to interchange these two weights, but check your tension first!
As seen at http://www.yarnfwd.com/tension.html#worsted#worsted

Australian and UK Conversion Chart
Hi, I have been looking at your impressive website, but I have found one rather awkward mistake on the page that has the abbreviations.
At the bottom of the page there is a ply conversion for 2 ply, 3ply, 4 ply and 5 ply.
Actually 2 ply wool is finer than fingering and used for lace making or fine shawls, 3 ply is a light baby wool, like fingering, 4 ply is a little heavier (double the thickness of 2 ply), 5 ply is sport weight, 8 ply is double the thickness of 4 ply and about worsted weight, then there is 10 ply - Aran weight and the 12 ply which is chunky (and 3 times the thickness of 4 ply, or 6 times the thickness of 2 ply).
Knitting is normally done on 12 ply with 6.5mm needles, 8 ply done on 4mm needles, 5 ply done on 3.75mm needles and 2 ply knitted on 2.25mm needles.
It is very difficult to get exact conversions, but if you only go to 5 ply as a bulky wool patterns will be very badly out of size.
Websites for English wool manufacturers like Sirdar will give you a recommended needle size for the thickness of wool, and English styled "DK" or double knitting is equivalent to 8 ply, Aran = 10 ply and Chunky = 12ply.

http://www.knitting-crochet.com/found/ausanduk.htm