Leather Ounces to Millimeters and Inches

Here's the whole trick. Multiply the ounces by about 0.4 and you have millimetres. Put the ounces over 64 and you have inches¹.

Where the rule comes from

A leather supplier in Australia says one ounce of leather equals 0.4 mm or 1/64 of an inch, so 9 ounces is 3.6 mm or 9/64 of an inch¹. A US supplier agrees, giving 1 oz as about 1/64 inch or about 0.4 mm, which makes a 4 oz piece about 1/16 inch, or 1.6 mm².

The ASTM standard for measuring leather is a bit more exact. It gives one ounce as 1/64 inch, which is 0.0156 inch or 0.396 mm³. Rounding that to 0.4 mm is fine for craft work. You'll see the tiny gap only on very heavy leather.

Common weights

A reference table from a bookbinding and conservation dictionary lists the finished thickness for each ounce value⁴. Here are the first few rows, with the millimetres as that table gives them.

Notice the pattern. Each step up adds the same small amount, so you can work out any weight in between without a chart.

Going the other way

If you've measured a piece in millimetres, divide by 0.4 to find the ounces. A reading of 1.6 mm lands on 4 oz in the table above⁴.

Most pieces won't land exactly on a row. A reading between two rows just means the leather sits between two weights. That's normal. Leather is a natural material, so you shouldn't expect it to match a table to the last tenth of a millimetre. The page on variation goes into it.

Here's a quick example. Say your caliper reads 2.2 mm. That sits between the 5 oz and 6 oz rows, at 2.0 and 2.4 mm⁴. You'd call it a light 6 oz or a heavy 5 oz, and either is fair.

Check the unit on the label

Some sellers list ounces, some list millimetres, and some list both. If both appear and they don't agree with the table, go with a millimetre figure you've checked yourself. The page on measuring at home shows how.

Irons are a different unit again. One iron is 1/48 inch⁴, so an iron is a little thicker than an ounce. For that conversion, read about irons in leather.

One last habit worth having: write the millimetre figure next to the ounce figure in your project notes. When you reorder leather later, you'll compare like with like, even if the new seller lists the other unit.