Item Surface Plate Long Form Calibration Certificate No Material Granite Application Grade Laboratory Color Red Edge Style Flat Flatness 0.000075 in/0.0019 mm Grade AA Length 24 in Surface Size 18 in x 24 in 450 mm x 600 mm Thickness 4 in Standards ASME B89.3.7 2013 U.S. Federal Specification GGG-P-463c Surface Size (In.) 18 in x 24 in Surface Size (mm) 450 mm x 600 mm 18 in