Sometimes we get patterns from a site in the US and then we need to know how to convert the measures to the ones we use for yarns in Australia and New Zealand.
Here is a yarn conversion chart that might help you.

Conversion tables from US to Metric Needle sizes

US Metric mm
0 2
1 2.25
2 2.75
N/A 3
3 3.25
4 3.5
5 3.75
6 4
7 4.5
8 5
9 5.5
10 6
10 ½ 6.5
N/A 7
11 8
13 9
15 10

American Yarn Weight Conversion

America Australia
Lace Weight 1 ply
Baby or fingering 2 ply & 3 Ply
Sport weight 4 ply
Worsted Weight 8 ply double knit
Medium 10 ply
Bulky 12 to 14 ply