How to Measure Sandpaper. Sandpaper is measured by the following methods, depending on the item: Sandpaper Belts are measured by the "width of the belt" x "length of belt" x "grit". That is, a 50 mm x 914 mm x 80 grit belt is 50 mm wide, 914 mm around the total circumference, and 80 grit. Measuring a belt length