A: The dimensions of the 3/4" Smooth Ring is in the product description: "Approximately 18mm OD and 11mm ID." The inside diameter is 11 mm. For assistance with metric conversions, click on the "Size Guide" tab next to the Description and Q&A tabs, and scroll down for a handy chart; 11mm = 7/16 of an inch. If you click through the pictures, one is next to a ruler for better visual sizing ideas. Thanks for writing. Please let us know if you need further assistance.