18 gauge aluminum sheet has a thickness of approximately 0.0403 inches or 1.02362 millimeters. Gauge is a unit of measurement used to describe the thickness of a material, and it is often used for metal sheets. The gauge number is inversely proportional to the thickness of the material, meaning that as the gauge number increases, the thickness of the material decreases.
For example, a 16 gauge aluminum sheet is thicker than an 18 gauge aluminum sheet, while a 20 gauge aluminum sheet is thinner than an 18 gauge sheet. It's important to note that different aluminum alloys may have slightly different thicknesses for the same gauge number.