It is always best to have your ring size measured by the professional jeweller and ideally by the jeweller who will be making the final piece. Many factors effect the accuracy of a size, such as the temperature on the day and the width of the ring. Often there is a difference between your left and right hand so be sure to measure the finger where you plan to wear the ring.
Amy is happy to give guidance on size if you are unable to have your finger measured by her or unsure how to obtain an accurate size. In Australia we use an alphabetical ring sizing system. Amy will be happy to help convert your size if you have been measured using another sizing system.