Unlike lumber which has a nominal dimension based on 1/4″, leather generally is measured in thickness by ounces. This is not the ounces of weight or volume from the Imperial system, although it may have some historic relationship. It is a special measurement unique to leather supply. Of course, when you have one proprietary measure, why not two? So in addition to ounces there is also irons.
According to Federal Test Method Standard No. 311 – 1101:
1 (leather) ounce = 1/64″ [~ 0.4 mm]*
1 (leather) iron = 1/48″ [~0.53 mm]*
*Both measurements are taken with some pressure on the specimen and will therefor differ slightly from a measurement taken with a different pressure. Overall, the difference really isn’t worth taking into consideration. In one version of the spec, 0.1 mm was given as .25 ounces. However, it’s clear that starts to divert from the thickness in inches quite quickly as the ounces increase. Revisions to the specification remove this in favor of a direct metric conversion based on the SI inch of 25.4mm. The calculator and charts do the same.
From observation: most of the time ounces are sold in whole or half-ounce increments. Thus the charts have been setup this way. It’s common to have 1/2-1 ounce of variation in thickness for leather 4-8 ounces and 1-2 ounce variation for 8+.
Downloads
For the sake of my sanity I created a calculator in OpenOffice Calc (spreadsheet/Excel). I have created an Excel file, but I don’t know if it works due to not owning a copy of Excel.
I also made a chart with lines to demonstrate weight:
Leather Thickness Print Chart [pdf]
Conversion Charts
This chart is based on text from the U.S Fed. Test Method Std. No. 311, in test method 1011 (1971-75). This may or may not be the most reliable source for this information and ounce leather and irons may defined differently per another applicable standard. Please use with caution and cross-reference results with local standards and the samples of material you have from suppliers. You can find testing methodology in the standard described above. There is the possibility that this calculator is correct but the supplier abides by a different standard. If you find another standard that disagrees with 311-1011, please let me know.
There may be errors in the calculations provided. It could be just a single one or systematic. If you catch one please contact the site admin.
Downloadable Chart
Web Chart
Ounces
| Ounces | Inches | mm | Irons |
| 1/ 2 | 0.0078 | 0.20 | 0.4 |
| 1 | 0.0156 | 0.40 | 0.8 |
| 1 1/ 2 | 0.0234 | 0.60 | 1.1 |
| 2 | 0.0313 | 0.79 | 1.5 |
| 2 1/ 2 | 0.0391 | 0.99 | 1.9 |
| 3 | 0.0469 | 1.19 | 2.3 |
| 3 1/ 2 | 0.0547 | 1.39 | 2.6 |
| 4 | 0.0625 | 1.59 | 3.0 |
| 4 1/ 2 | 0.0703 | 1.79 | 3.4 |
| 5 | 0.0781 | 1.98 | 3.8 |
| 5 1/ 2 | 0.0859 | 2.18 | 4.1 |
| 6 | 0.0938 | 2.38 | 4.5 |
| 6 1/ 2 | 0.1016 | 2.58 | 4.9 |
| 7 | 0.1094 | 2.78 | 5.3 |
| 7 1/ 2 | 0.1172 | 2.98 | 5.6 |
| 8 | 0.1250 | 3.18 | 6.0 |
| 8 1/ 2 | 0.1328 | 3.37 | 6.4 |
| 9 | 0.1406 | 3.57 | 6.8 |
| 9 1/ 2 | 0.1484 | 3.77 | 7.1 |
| 10 | 0.1563 | 3.97 | 7.5 |
| 10 1/ 2 | 0.1641 | 4.17 | 7.9 |
| 11 | 0.1719 | 4.37 | 8.3 |
| 11 1/ 2 | 0.1797 | 4.56 | 8.6 |
| 12 | 0.1875 | 4.76 | 9.0 |
| 12 1/ 2 | 0.1953 | 4.96 | 9.4 |
| 13 | 0.2031 | 5.16 | 9.8 |
| 13 1/ 2 | 0.2109 | 5.36 | 10.1 |
| 14 | 0.2188 | 5.56 | 10.5 |
| 14 1/ 2 | 0.2266 | 5.75 | 10.9 |
| 15 | 0.2344 | 5.95 | 11.3 |
| 15 1/ 2 | 0.2422 | 6.15 | 11.6 |
| 16 | 0.2500 | 6.35 | 12.0 |
| 16 1/ 2 | 0.2578 | 6.55 | 12.4 |
| 17 | 0.2656 | 6.75 | 12.8 |
| 18 | 0.2813 | 7.14 | 13.5 |
| 19 | 0.2969 | 7.54 | 14.3 |
| 20 | 0.3125 | 7.94 | 15.0 |
| 21 | 0.3281 | 8.33 | 15.8 |
| 22 | 0.3438 | 8.73 | 16.5 |
| 23 | 0.3594 | 9.13 | 17.3 |
| 24 | 0.3750 | 9.53 | 18.0 |
| 25 | 0.3906 | 9.92 | 18.8 |
| 26 | 0.4063 | 10.32 | 19.5 |
| 27 | 0.4219 | 10.72 | 20.3 |
| 28 | 0.4375 | 11.11 | 21.0 |
Irons
| Irons | Inches | mm | Ounces |
| 5 | 0.1042 | 2.65 | 6.7 |
| 5 1/ 2 | 0.1146 | 2.91 | 7.3 |
| 6 | 0.1250 | 3.18 | 8.0 |
| 6 1/ 2 | 0.1354 | 3.44 | 8.7 |
| 7 | 0.1458 | 3.70 | 9.3 |
| 7 1/ 2 | 0.1563 | 3.97 | 10.0 |
| 8 | 0.1667 | 4.23 | 10.7 |
| 8 1/ 2 | 0.1771 | 4.50 | 11.3 |
| 9 | 0.1875 | 4.76 | 12.0 |
| 9 1/ 2 | 0.1979 | 5.03 | 12.7 |
| 10 | 0.2083 | 5.29 | 13.3 |
| 10 1/ 2 | 0.2188 | 5.56 | 14.0 |
| 11 | 0.2292 | 5.82 | 14.7 |
| 11 1/ 2 | 0.2396 | 6.09 | 15.3 |
| 12 | 0.2500 | 6.35 | 16.0 |
| 12 1/ 2 | 0.2604 | 6.61 | 16.7 |
| 13 | 0.2708 | 6.88 | 17.3 |
| 13 1/ 2 | 0.2813 | 7.14 | 18.0 |
| 14 | 0.2917 | 7.41 | 18.7 |
| 14 1/ 2 | 0.3021 | 7.67 | 19.3 |
| 15 | 0.3125 | 7.94 | 20.0 |
| 15 1/ 2 | 0.3229 | 8.20 | 20.7 |
| 16 | 0.3333 | 8.47 | 21.3 |
| 16 1/ 2 | 0.3438 | 8.73 | 22.0 |
| 17 | 0.3542 | 9.00 | 22.7 |
| 18 | 0.3750 | 9.53 | 24.0 |
| 19 | 0.3958 | 10.05 | 25.3 |
| 20 | 0.4167 | 10.58 | 26.7 |
| 21 | 0.4375 | 11.11 | 28.0 |
| 22 | 0.4583 | 11.64 | 29.3 |
| 23 | 0.4792 | 12.17 | 30.7 |
| 24 | 0.5000 | 12.70 | 32.0 |
| 25 | 0.5208 | 13.23 | 33.3 |
| 26 | 0.5417 | 13.76 | 34.7 |
| 27 | 0.5625 | 14.29 | 36.0 |
| 28 | 0.5833 | 14.82 | 37.3 |
