Aussie Sapphire provides the following information for your assistance. This chart assist in the conversion of US and Canadian number sizes to the "Wheat sheaf" system of letter sizes used in the UK and Australia. Diameters and circumferences are also given in both inches and millimeters. Please note that these measurements are exact for the American system and approximate for the other sizing scales, since the intervals in each system do not correspond exactly.
Ring sizing gauges and finger gauges as well as published charts do not always agree. The other problem is that there are two conflicting methods of reading ring gauges so it is quite common for measured ring sizes to vary slightly between sellers. The method used by Aussie Sapphire for quoting ring sizes is the "True Diameter" method. However, given this discrepancy in measuring techniques and variation in sizing charts, all ring sizes quoted should be regarded as our best estimate only.
Ring re-sizing is best done by your local jeweller who can measure your required size accurately and adjust the fit perfectly. This is usually a relatively simple and inexpensive job.