Leather thicknesses (Weight)
Leather thickness is referred to as the "weight" (although I have seen this referred to as the "substance").
Leather is sold and priced in most cases by the square foot, no matter the unit of measure used for the thickness or the purpose of the leather.
When we are buying leather we often see the leather weight as n ounces.
Some can easily be misled into thinking that the weight (ounces) given for a piece of leather refers to how heavy a particular size weighs, say ounce per square foot - logically, the heavier the thicker.
This is a throw-back from the fabric industry where fabric thickness was denoted by how heavy the fabric was per square yard, the heavier the material (ounces per square yard, the thicker it was. All well and good for material since it is consistent through the whole piece of material.
This principle could not be applied to leather since thickness varies even on the same hide - so the weight became a linear measurement.
1 ounce is 1/16" thick
So a 16 ounce leather is in fact 1/4" (6.4mm) thick.
The ounce is still used today especially in countries using the imperial system.
In countries using the metric system, leather is still (mostly) sold by the square foot, but the weight (thickness) is denoted in Millimetres - ahhh yes, that age old problem of converting systems!
So in the metric system, leather is sold by the square foot, but the weight is measured in Millimetres.
Another old measurement for the weight is Irons. The term is derived from the plates of iron used to measure the thickness of leather used for various parts of shoes by cobblers and cordwainers in Europe. Like the imperial system, the iron is a linear measurement.
1 iron is 1/48" (6.35mm) thick.
For reference, this table below is a useful conversion chart.