Our custom magnets are 15 mils (.38 mm) thick.
Keep in mind, mil is not short for millimeter. Mil is a commonly used measurement for products like magnets and stickers.
For reference, our vinyl stickers without the liner are 7 +/- mils thick. (0.007 inches or 177.8 microns). When still on the paper backing, our stickers are 15.45 mils thick (0.015 inches or 392.43 microns).