Have you ever seen a pattern that you would like to do and the terminology is strange to you, see if there is a match here in the Yarn Conversions and the Needle size Conversions

Yarn Conversions 
 USA UK
Sock, fingering 2 ply, 3 ply
Sport, baby 4 ply
Light Worsted Double
Worsted, Afghan       Aran

 

Bulky, Chunky                  Chunky
Super Bulky Super Chunky

Needles

Old UK Metric USA
14 2mm 0
13 2.25mm 1
12 2.75mm 2
11 3mm -

10                                       3.25mm 3                            
- 3.5mm 4
9 3.75mm 5
8 4mm 6
7 4.5mm 7

 

6          5mm 8
5 5.5mm 9
4 6mm 10
3 6.5mm 10.5
2 7mm 10.5

7.5mm 11   
0 8mm 11
000 9mm 13
0000 10mm 15

Yarn Needle
2-ply,    3-ply 2mm, 2,25mm
4-ply 3mm
Double knitting 4mm
Aran, Chunky 5mm, 6mm
If there are any mistakes here then please let me know