With pattern designers not using universal sizing, you can't assume that the size of your day to day garments will be the same size as a sewing pattern. The US & Australian sizing is very different.
This reference card can be printed out, filled in and popped into your purse for next time you are purchasing a pattern. You can refer to your measurements to make sure you are purchasing the correct size.
It will also come in handy when needing to determine the fabric requirements for a particular size on multi sized patterns.
Keep in mind that our bodies change so it will need to be updated. You should also do a proper, current measure before cutting out your pattern.
You can download the free printable here.