UK, we use millimeters for thickness. 1 US ounce for leather = 0.4mm thickness. Therefore 3oz = 1.2mm, 4oz = 1.6mm
and remember; when you apply dye to your raw leather and it dries, the leather shrinks. Not only in width x height, but also in thickness. Most often this shrinking in thickness goes unnoticed but it can affect the final out come of your project in some cases