Leather Thickness Converter

Leather thickness is measured differently depending on the country and trade. In the United States, leather thickness is traditionally measured in ounces (oz), while most other countries use millimeters (mm). The old British system uses irons.

Conversion formulas:

• 1 oz of leather = 1/64 inch = 0.3969 mm (approximately 0.4 mm)
• To convert oz to mm: mm = oz x 0.3969
• To convert mm to oz: oz = mm / 0.3969

A warning about irons: there are two conventions, and they differ by 14%. The iron is an old British unit and it is not standardised the way the ounce is. Shoemaking references overwhelmingly define it as 1/48 inch (0.529 mm), and that is the figure to use for sole thickness. A number of leathercraft charts, including the reference table below, are built on 1/42 inch (0.605 mm) instead. Neither is a typo; they are different traditions that happen to share a name.

Fourteen per cent does not sound like much until you are ordering sole leather, where it is the difference between 5 irons at 2.6 mm and 5 irons at 3.0 mm. The converter above lets you pick which convention you are working in, and it defaults to the 1/42 figure so it agrees with the table on this page. Ask your supplier which one they quote, and if you can, order in millimetres and sidestep the question entirely.

Note about ounce measurements: Leather ounces refer to the weight of one square foot of leather at that thickness, not a volume measurement. A piece of leather that weighs 4 ounces per square foot is called “4 oz leather.”

Common leather thickness reference table:

Practical tips for leathercrafters:

• Most projects use leather in the 3-7 oz range, which covers wallets, bags, and belts.
• When buying leather, the ounce rating is often given as a range (e.g., 4-5 oz) because natural leather varies slightly in thickness across a single hide.
• Leather can be split or skived to reduce thickness for specific areas of a project, such as fold lines or edges.
• Vegetable-tanned leather is best for tooling and carving, while chrome-tanned leather is softer and better for garments.