Here’s a great guide guide to leather thickness and buying leather from Tandy Leather Factory showing the different cuts of leather, hide thicknesses, and suitable uses. Bear in mind that these are general guidelines – the actual thickness you choose for your project can vary a bit, depending on what you’re making, how durable you want the product to be, if you’ll be tooling the leather, etc.
http://www.tandyleatherfactory.com/en-usd/home/infoandservices/leatherguide/leatherguide.aspx