For those who like rubber, here's a large selection of sizes made from strong and durable Buna-N rubber, 3/16 inch (5mm) thick. They are not stretchy, but they are pliable and have just a bit of give. All sizes are "nominal", meaning that the actual inside diameter may be a tiny fraction less than the stated size. The drop down list shows all the sizes, along with several money-saving sets.
If you are new at wearing rings and not certain of your best personal configuration, these rings are an easy and inexpensive way to experiment with different sizes, before making a more substantial investment in a a heavy metal cock ring. However, our "Silicone Rings", though identical in shape and size, are in many ways superior.