Leather thickness is normally measured in ounces, with one ounce equal to 0.4 mm. We use what is called a leather gauge to measure our leather when listing it, but we use millimetres as here in Australia most people are more familiar with the metric system. You'll notice some leathers may have a range of mm listed (eg 1.2-1.4 mm). Because leather is a natural material, it can vary across the full width of a hide by small amounts.
In reality this slight variation of the leather thickness should not affect the outcome of your leathercraft projects.