Did you know that leather comes in different thicknesses?
It's interchangeably referred to as thickness or weight (I originally learned it in weights, so that's how I think of it now). Here's a chart showing the conversion of weight in ounces to thickness in millimeters:
The chart above also shows the different thicknesses and projects that work well at each weight. Different products work well with certain weights. There are several reasons for that. A thinner leather may be used when multiple layers are involved and you want to avoid it getting too bulky (like a wallet). A thicker leather will typically have more stiffness to it supplying more structure (such as a belt or holster).
In order to achieve a specific thickness, leather goes through a process called splitting. Simply put, the bottom layer is cut off to achieve the weight or thickness desired. Here I've shown a variety of thicknesses from very thin to very thick (you can see the layers of the hide!):
Now you know that leather can come in different thicknesses, you can determine the thickness used for your leather good!