Our custom sheet labels are 7.5 mils +/- 10% (0.0075 inches or 191~ microns) thick.
Without the paper backing, the sheet labels are approximately 3 mils thick including adhesive.
Label: 2.3 mils (.0023 inches +/- 10%) Adhesive: 0.7 mils (.001 inches +/-10%) Backing: 4.5 mils (.0041 inches +/- 10%)
Keep in mind, mil is not short for millimeter. Mil is a commonly used measurement for products like labels and stickers. It's equivalent to 1/1000 of an inch. You can read more about using mil as a form of measurement on Wikipedia.