The diameter is about 17mm, so I believe it's a size 13. Please note that there may be some error as it has not been measured with a ring sizer. Also, as this is a pre-owned item, there may be minor scratches from use. A simple silver ring from Tiffany & Co. - Brand: Tiffany & Co. - Material: Silver - Design: Simple band ring - Engraving: Tiffany & Co. - Condition: No noticeable scratches or dirt Thank you for looking.