Join our CAD community forums where over 25,000 users interact to solve day to day problems and share ideas. We encourage you to visit, invite you to participate and look forward to your input and opinions. Acrobat 3D, AutoCAD, Catia, Inventor, IronCAD, Creo, Pro/ENGINEER, Solid Edge, SolidWorks, and others.
When you are dimensioning a closed circle the default would be diameter and If you are dimensioning an open arc, the default will be radius. So when you dimension that, it will drop in a radius dim. Then do a RMB on it, select properties, and check the box for Diameter dimension.