Strips of bridle leather in different weights lined up edge-on on a wooden workbench.
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Leather Weight Explained: Ounces, Millimeters and Irons

ByPeter Joeseph 'Briefcase Maker' Kalafatis

An ounce of leather is a thickness. Nobody put anything on a scale.

Leather weight is a measure of thickness, and it has almost nothing to do with how heavy the leather is. A bridle back sold as eight to ten ounces is telling you how thick it runs, roughly, in a unit that stopped making sense a long time ago and never got replaced. Leather has been a hobby of mine since 1995 and my trade since 2010, and the ounce is still the first number on most hide listings. These are the questions people ask about it, answered from tanners, suppliers, a conservation dictionary and a few makers who measured their own leather, with my opinions marked as mine.

Video: The Leather Element: Choosing Leather Weights, from Weaver Leather Supply on YouTube.

Why is leather thickness sold in ounces?

Because a square foot once weighed one. The habit outlived the scale.

Hands measuring bridle leather thickness with a dial gauge away from the edge of the panel.

The old answer sits in a conservator’s dictionary. Bookbinding and the Conservation of Books, Etherington and Roberts’ dictionary of terms hosted on CoOL, defines the ounce as a measure of leather thickness equal to 1/64 inch. It adds the history: the word first meant the thickness of one square foot of finished leather that weighed one ounce avoirdupois.

So it was a weight, once. Then it froze into a thickness.

Popov Leather’s page on weight and thickness puts the same point in shop terms. A hide sold as nine ounces is a thickness reading, not the weight of the hide, and the page says the convention has outlived the logic behind it. Tandy Leather’s chart says it straight too: in leathercraft an ounce measures thickness, and the United States sticks with ounces while most other countries use millimeters.

My opinion: the scale version could never have held up for long. English bridle is stuffed with wax and grease at the tannery, which is why my Leather Choices page calls it waxy, thick, heavy and stiff. A square foot of that would not weigh what a dry square foot of the same thickness weighs. Two hides, same caliper, different ounces on the scale. The trade quit weighing and kept the word.

How do ounces turn into millimeters?

Multiply by 0.4 and you are close. Close is the operative word.

Weaver Leather Supply’s thickness page gives the rule in one line: one ounce equals 1/64 inch. Tandy rounds it to about 0.4 mm per ounce and gives four ounces as about 1/16 inch, or 1.6 mm. The arithmetic is mine: 1/64 inch works out to a hair under 0.4 mm, so a four-ounce strap sits near 1.6 mm, an eight-ounce strap near 3.2 mm, and a ten-ounce panel near 4 mm.

Retail listings show you both units side by side, and they show you the slack. Buckleguy lists a Sedgwick English bridle side at 3.5 to 4 oz with a thickness of 1.4 to 1.6 mm, and a heavier option at 4.5 to 5.5 oz, 1.8 to 2.2 mm. Further down, the same page gives that heavier option as 4.5/5 oz and 1.8 to 2.0 mm. One page, one hide, two answers. Buckleguy’s Hermann Oak bridle back runs 8 to 10 oz, listed as 3.2 to 4.0 mm.

On the British side the ounce mostly drops out. Abbey England sells a Sedgwick stirrup butt as 4.8 mm and gives no ounce figure at all, which tells you who still thinks in sixty-fourths.

Hermann Oak is the heavy one among the tanneries I name. My Leather Choices page says it comes very thick, around 8 to 12 ounces, and that a suede lining makes it thicker still, which is why I usually recommend keeping the natural lining. Glue a lining to a ten-ounce panel and the ounce number on the invoice is no longer the thickness of the case.

What is an iron?

A shoemaker’s unit. Three of them make four ounces.

Macro view of the cut edge of thick bridle leather showing its full thickness and fiber layers.

The iron is the unit most leather people never touch. The CoOL leather substance table sets it down plainly: one iron is 1/48 inch, one ounce is 1/64 inch. Its rows line the three units up, so four ounces equals three irons equals 1/16 inch equals 1.6 mm, and sixteen ounces equals twelve irons, a quarter inch, 6.4 mm. One ounce is three quarters of an iron.

Popov says irons come from shoemaking, where cobblers measured soles against iron plates of known thickness. A 2016 piece on BestLeather.org calls the iron an old standard still used now and then, mostly in shoe manufacturing and especially in the UK, and says the millimeter has become far more common.

It hasn’t died, though. ASTM D1814, the standard test method for measuring the thickness of leather units, still lists millimeters, ounces or irons as the units of record. A standards body keeping three units in the book is the leather trade in one sentence. Nobody throws a tool away.

You will meet irons on sole leather and old shoe paperwork more than on a bridle listing. Know the ratio, convert it, and move on.

Why is it always a range?

Because a hide is a body, not a sheet of steel.

Weaver’s thickness page says hides rarely measure an exact 1/64 inch throughout, that there are always slight variations, and that leather run through a splitting machine is more consistent but still not perfect. Popov is blunter, saying “a tannery that quotes a single number is rounding.” The Heddels guide to leather weights makes the same call from the buyer’s end: because one cut of hide varies, weights usually come as a spread like 4 to 5 ounces.

Where the leather came from on the animal matters as much as the number. Popov notes that the bend along the back is the firmest and most consistent part, and the belly is thin and stretchy. That is the reason the main parts of my cases come only from the bend, where the fiber is tightest. I wrote more about that in reading a bend before you pay for it.

Makers who put calipers on their own leather find the spread fast. In a leatherworker.net thread on Hermann Oak thickness, the original poster measured samples against Tandy’s ranges and found most of them near the top, except that a piece sold as 10/11 oz came in thinner than one sold as 8/9. Another member, fredk, describes a leather sold as 4 mm that measured 2.8 to 3 mm in the store. He also reports that solvent-based dyes and sealants left leather less bulky than water-based ones did.

A range is the honest answer. If a part has to be a set thickness, a splitter takes it there, and splitting and skiving bridle is its own subject. BestLeather describes the same machine as the way leather gets reduced to a target, and says hides vary enough that thickness is usually stated as a range such as 6 to 7 oz.

How do you measure leather yourself?

Away from the edge, jaws snug, and more than once.

An unlined bridle leather briefcase with thick panels and thinner gussets on a workbench.

Sailrite’s guide to measuring leather thickness lays out three tools. The first is a slotted gauge, a flat template with graded cutouts. Leather calipers close two plates on the piece and read out on a dial, digital or analog. Ordinary calipers work the same way but usually read in millimeters, so you convert.

Then the method. Leather is thinnest along the edge, Sailrite says, so measure as far in from the edge as you can, which is why a long-throat gauge beats a template that only fits the edge. Seat the leather snug, not squeezed. Check several spots, moving inward, because the hide changes as you go.

Pressure is where readings go wrong. In the Hermann Oak thread, one member asked the poster how tightly he closed his calipers before trusting the numbers, and the poster said he brought them just to snug. Crush a piece of bridle in the jaws and it reads thin. Leave it loose and it reads thick. Same hide, two numbers, and neither one lied.

The industry side of this is ASTM D1814, which the ASTM page describes as a method for routine thickness checks in production and conformance testing, using a secondary type of gauge built for quick readings. The page says that standard is not meant for measuring test specimens. For a bench, a gauge, a steady hand and a few readings across the bend will tell you more than the label did.

What weight for what job?

The chart gives you a starting point. The job gives you the answer.

The charts agree on the broad strokes. Tandy’s table puts wallet exteriors at 3 to 4 ounces, satchels at 5 to 6, heavy belts at 7 to 8, saddlery at 8 to 9 and harness work at 9 to 10, with structured cases in the 7 to 10 range. On Weaver’s leather weights page, the crafter Chuck Dorsett sorts it into three groups: heavy at 8 to 9 ounces for belts, laptop bags and saddlebags, medium at 4 to 5 for pouches and purses, and light for liners and garments. Weaver also notes the trade-off. Heavier leather is durable and takes tooling, but it molds less well.

I build heavy, and I am biased toward it. A case that has to carry files for years wants thick leather, not thin leather built up with cardboard filler that breaks down once moisture gets in. My gussets run thinner than my panels so the case bends at the sides instead of creasing across the face. Those two rules pull in different directions, and the ounce number has to serve both.

Here is where the case for heavy leather gets tested. Heddels puts it in one line: “a heavyweight measurement doesn’t necessarily define quality.” A thick hide from a bad tannage is still a bad hide, only heavier, and a thick piece cut from the belly is thick and floppy at the same time. In the Hermann Oak thread, one maker said he judges leather by feel and firmness when he starts a project, and wrote of the numbers, “Those numbers really have no meaning to me.” Another, JLSleather, said exact weight matters less for flat goods than firmness and looks, and matters more when the piece has to fit something.

My opinion: they are mostly right, and it costs me something to say so. The ounce number is worth reading first, because it is the only thing a listing gives you before the hide arrives. Once the hide is on the bench, the number goes quiet and the hands take over, thumb on the grain, fingers bending the edge, feeling for where the leather stops pushing back. For a buyer looking at a finished case, bridle against harness leather is a better question than ounces.

The ounce gets you to the right shelf. It does not pick the hide.

Sources

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