What an Iron Is in Leather Thickness

An iron is 1/48 inch of leather thickness¹. An ounce is 1/64 inch. So an iron is a bit thicker than an ounce.

Where irons are used

The ASTM method for measuring leather says thickness may be given in ounces or irons. It adds that irons are generally used for sole leather, while ounces are generally used for shoe upper leather².

One Australian supplier's chart puts men's sole bends at 4.5 to 5.4 mm, and its heavy men's sole bends at 5.5 to 6.4 mm³.

Irons in inches and millimetres

The ASTM figure for one iron is 0.0208 inch, or 0.53 mm². For one ounce it gives 0.0156 inch, or 0.396 mm². That's why an iron is a little bigger.

Converting between irons and ounces

The ratio is simple. One iron is 1/48 inch and one ounce is 1/64 inch, so irons times four thirds gives ounces¹. Going back, ounces times three quarters gives irons.

A table from the book conservation field lines the units up¹:

So 3 irons matches 4 oz, 6 irons matches 8 oz, and 9 irons matches 12 oz¹. Each pair follows the same four-to-three pattern.

Using the number

If a maker gives irons, convert to millimetres first, then measure the piece. A caliper or gauge reads millimetres or inches directly, and the page on checking a piece yourself covers how. The ounce conversion has the other half of the maths.

Here is a worked example from the table above. A label says 6 irons. The table lines 6 irons up with 8 oz and 3.2 mm¹. So you would look for a piece that reads about 3.2 mm on your caliper, then check it with the routine on the measuring page. If the reading is close, the label and the leather agree. If it is far off, ask the seller which unit they meant.

Don't mix the units in one calculation. Pick one, convert everything to it, and keep a note of which one you used.

Say you see a bare "9" on a sole leather label. Nine ounces would be 3.6 mm, but nine irons matches 12 oz, or 4.8 mm¹. That's a big gap, so always check which unit the seller means.

ASTM says irons are generally used for sole leather and ounces for shoe upper leather². That's a general pattern, so confirm the unit with the seller.

If a label doesn't say which unit it uses, ask the seller before you buy. Write the answer down next to your measured reading so you can compare later.