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
The easiest way to measure a belt is to mark the inside of the belt with a pen, put a mark on a long flat surface, benchtop or garage floor. Line up the two marks and roll the belt (being careful not to let it slip on the flat surface) until the mark on the belt makes contact with the flat surface again. Put another mark on the flat surface at this point. (It is always advisable to do the process again to check your marks are correct.) Measure the distance between the two marks, and you have the belt length.
Sandpaper Discs are measured across the diameter.
That is, a 150 mm diameter disc measures 150 mm across the diameter.
If a disc has a measurement of 100 mm x 16 mm, this would mean it is 100 mm in diameter and has a 16 mm centre hole. This would typically be a Fibre disc to fit an Angle Grinder and is attached by a nut secured through the 16 mm centre hole.
A sanding disc with dust extraction holes would be written as follows:
125 mm diameter x 120 grit 8 hole, or 150 mm diameter x 400 grit 7 hole.
Sandpaper Rolls are measured by the width x length x grit.
That is, a 115 mm x 50 m x 80 grit, is 115 mm wide x 50 metres in length, and 80 grit.
Sandpaper Sheets are measured by the width x length x grit.
That is, a 230 x 280 mm x 240 grit, is 230 mm wide x 280 mm in length, and 240 grit.
Sandpaper Flap Wheels are measured by the diameter x width x shaft diameter x grit.
So if a Flap Wheel is listed 40 mm x 20 mm x 6 mm x 80 grit, it is 40 mm in diameter x 20 mm wide x 6 mm shaft x 80 grit.