Ring sizing can be tricky! For those based in Australia, I will happily send a complimentary ring sizer in the post. Please contact me to arrange this. 
Ring size chart Australia & UK 

Size Dimensions (mm) inside diameter
H 14.65
I 15.04
J 15.40
K 15.80
L 16.10
M 16.51
N 16.92
O 17.35
P 17.75
Q 18.19
R 18.61
S 19.10
T 19.51
U 20.02
V 20.32
W 20.76
X 21.18