Leather is usually measured in ounces. Each hide is different and there is always a slight thickness variation throughout the hide. That is why leather is usually shown with a range of thickness, such as, 2-3 oz.
The best weight for making soft-sole shoes is 1.5 to 3 ounces; but you can sew up to 3.5. A good rule of thumb is that 3 ounces is the thickness of a dime, 3.5 ounces in the thickness of a penny. So you shouldn't buy leather thicker than a penny.
When buying leather online you might find measurements other than ounces listed. Here is a partial conversion table from Leather Supply House.
| Ounces | Inches | Dial Caliper | Metric |
| 1 | 1/64 | .016 | .397 |
| 1-1/2 | .023 | .595 | |
| 2 | 1/32 | .031 | .794 |
| 2-1/2 | .039 | .992 | |
| 3 | 3/64 | .047 | 1.191 |
| 3-1/2 | .055 | 1.389 |
Don't forget to check out our page on thread choices for sewing with leather.