You can easily measure the length and width of a rectangular plate with a metric ruler. However, a direct measurement of the third plate dimension (thickness) will not be accurate if the plate is thin enough. You can calculate the plate thickness as a ratio of the volume of the plate to its surface area.
Measure or find out elsewhere the length and width of the plate.
Multiply the length and width, if they are given in inches, by the factor 2.54 to convert the dimensions into centimeters. For example, the plate dimensions are 5-by-3 inches that will be converted to 12.7 and 7.62 cm.
Multiply the length by its width to calculate the plate surface area in square centimeters. In this example, the surface area is 12.7 x 7.62 or 96.774 square cm.
Calculate or measure the volume of the plate; if the plate density is known, divide the plate weight by the density. For example, the plate is made of aluminum (the density 2.7 g/cubic cm) and weighs 41.85 g. Then the plate volume is 41.85 / 2.7 = 15.5 cubic cm.
Divide the plate volume by the surface area to calculate the thickness. In this example, the thickness is 15.5 cubic cm / 96.774 square cm = 0.16 cm or 1.6 mm.