Traditionally, the thinnest types of wallets are minimalist front-pocket wallets. These wallets usually consist of a pouch with several card slots and either a small space for bills or an attached money clip.
Two of the thinnest wallets we’ve seen are the ultraslim offerings from European company DUN and American company SlimFold. Both are 5 mm thick, but they also have enough room for a few credit cards and some bills. At this point, it looks like 5 mm is the minimum for a wallet that has more functionality than a rubber band holding your stuff together.
Keep in mind that these measurements are for empty wallets. Any cards, bills, or coins will make the wallets thicker.