Leather is an ancient material, perhaps one of the first materials that mankind learned to make. As such it has a rich history and as a trade working with leather goes back to our oldest civilizations. And as you would guess leather craft has some pretty unusual terms for the tools and techniques involved.
One of the odd terms we use involves how to measure the thickness of leather. In modern times and most of the world we use a very straightforward measurement in millimeters or fractions of an inch. In many places though a more archaic measurement is still used, ounces.
Measuring thickness in ounces is tied to a standard of weight for leather. The weight of any given leather used to be defined by how much the leather weighed for a piece that was 12 inches square. For example, if a square foot of the leather weighed 8 ounces, that would be referred to as 8 oz. leather. This is a thickness that is very commonly used in carved leather belts and it corresponds to about 1/8 inch or 3 mm in thickness. While much of the world has moved on to metric measurements in the US ounces is still the standard unit of thickness for leather.
When talking about leather in ounces the conversion factor is one ounce equals 1/64 of an inch or 0.4 mm.
There is an older and more interesting term for measuring leather: irons. This is an old term that derives from the English shoe making industry. Cobblers (workers who made shoes in pre-industrial times) used the term iron to measure leather thickness and one iron was equivalent to 1/48 of an inch. It is believed that the ‘irons’ probably referred to an iron plate that was stacked up against the leather to measure the thickness. Since shoes are rarely hand made these days the term irons has been relegated to the history books and you would find few leather workers that would be familiar with it.