Our custom sheet labels are 8.76 mils +/- 10% (0.00876 inches or 191~ microns) thick.
Without the paper backing, the sheet labels are approximately 4.5 mils thick including adhesive.
Label: 4.3 mils (.0043 inches +/- 10%) Adhesive: 0.46 mils (.00046 inches +/-10%) Backing: 4.5 mils (.0045 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.