Paper × 841 mm). A1 is half the size of a sheet of A0 (i.e., 594 mm × 841 mm), such that two sheets of A1 placed side by side are equal to one sheet of A0. A2 is half the size of a sheet of A1, and so forth. Common sizes used in the office and the home are A4 and A3 (A3 is the size of two A4 sheets). The density of paper ranges from for tissue paper to for some speciality paper. Printing paper is about . Paper may be classified into seven categories: Some paper types include:
Mathematics of paper folding to form developable surfaces that are not flat. Wet-folding origami allows an even greater range of shapes. The maximum number of times an incompressible material can be folded has been derived. With each fold a certain amount of paper is lost to potential folding. The loss function for folding paper in half in a single direction was given to be formula_4, where "L" is the minimum length of the paper (or other material), "t" is the material's thickness, and "n" is the number of folds possible. The distances "L" and "t" must be expressed in the same units, such as