29mm in Diameter -- Including 12pcs 3-hole binder rings. Each ring metal loose leaf binder is about 1.67" long and it's inner diameter is 1.14" (29 mm), proper size to fit most books or albums, easy to fulfill your needs. Shiny & Not Easy to Fade -- These metal loose-leaf binder rings are made of high-quality iron, won't break easily, have a smooth surface and bright color, and will last you a long time. Locks Perfectly -- The opening of the loose-leaf ring has a gear design for perfect closure even a bit tight. However, you will find it a good thing once you put them on the notebooks. That means they will stay tightly secure. Easy to Open and Close -- Well design metal loose leaf ring can be split into 2 halves and connected with a metal swivel rivet tightly, ensuring that the book binder is locked safely and tightly to prevent the paper from being scattered or loose. Wide Application -- These metal loose leaf binders can be expanded and closed easily, flexible for you to DIY travel diary, photo album, scrap booking, loose-leaf notebook and graduation alumni book.