Measuring the thickness of leather has most commonly been done in ounces. 1oz = 1/64 inch. On Buckleguy we sell a range of leather belt straps that range between 8-10oz OR 3.2mm - 4mm. The reason for the range is that the hide often varies in thickness throughout the hide. The average leather belt strap is 7-8oz. Below is a conversion chart that is here to help compare leather ounces to inches and mm. Hope this helps!