Basis weight of paper is the density of paper expressed in terms of the mass (in pound) of a
ream of paper of given dimensions and a sheet count. The American and the traditional British systems are largely the same, with only minor differences: the paper dimensions and the sheet count are different.
United States In the US system, the weight is specified in
avoirdupois pounds and the sheet count of a
paper ream is usually 500 sheets. However, the mass specified is not the mass of the ream that is sold to the customer. Instead, it is the mass of the uncut "basis ream" in which the sheets have some larger size (parent size). Often, that is a size used during the manufacturing process before the paper is cut to the dimensions in which it is sold. So, to compute the mass per area, one must know • the mass of the basis ream, • the number of sheets in that ream, and • the dimensions of an "uncut" sheet in that ream. The standard dimensions and sheet count of a ream vary according to the type of paper. These "uncut" basis sizes are not normally labelled on the product, are not formally standardized, and therefore have to be guessed or inferred somehow from trading practice. Historically, this convention is the product of pragmatic considerations such as the size of a sheet mold. By using the same basis sheet size for the same type of paper, consumers can easily compare papers of differing brands. Twenty-pound bond paper is always lighter and thinner than 32-pound bond, no matter what its cut size, and 20-pound bond
letter size and 20-pound bond
legal size papers are the same weight paper with a different cut size. However, a sheet of common copy paper that has a basis weight of does not have the same mass as the same size sheet of coarse paper (newsprint). In the former case, the standard ream is 500 sheets of paper, and in the latter, 500 sheets of paper. Here are some basic ream sizes for various types of paper. Units are inches except where noted. : Sheets can be cut into four sheets, a standard for business stationery known conventionally as
letter sized paper. So, the ream became commonly used. The book-paper ream developed because such a size can easily be
cut into sixteen book sized sheets without significant waste (nominally before trimming and binding). Early newsprint presses printed sheets in size, and so the ream dimensions for newsprint became , with 500 sheets to a ream. Newsprint was made from ground wood pulp, and ground wood hanging paper (
wallpaper) was made on newsprint machines. Newsprint was used as wrapping paper, and the first paper bags were made from newsprint. The newsprint ream standard also became the standard for packaging papers, even though in packaging papers
kraft pulp, rather than ground wood, was used for greater strength. Paper weight is sometimes stated using the "
#" symbol. For example, "20#" means "20 pounds per basis ream of that kind of paper". When the density of a ream of paper is given in pounds, it is often accompanied by its "M weight" (M is 1000 in
Roman numerals). The M weight is the weight (in pounds) of 1000 cut sheets. Paper suppliers will often charge by M weight, since it is always consistent within a specific paper size, and because it allows a simple weight calculation for shipping charges. For example, a 500-sheet ream of 20# copy paper may be specified "10 M". 1000 cut sheets (or two reams) will weigh , half of the four reams of cut paper resulting from the 20# basis ream of paper.
United Kingdom Overview of the traditional British system Like the American system, the traditional British system also involves the use of basis weight to express the area density of paper (and, indirectly, paper thickness) in the course of buying and selling paper. The British paper industry exclusively used basis weight before 1968. In the traditional British system, basis weight is also expressed in terms of the weight of an imaginary ream of paper of a certain size. The underlying concept is that a ream of paper of the stated paper size and of the same thickness as the paper the parties are dealing with would have the stated weight. The weight is expressed in
pound. As to paper sizes (basis sizes), they can, theoretically, be any sizes, but, by custom, a number of common sizes emerged. In the 1960s, even the metric A1 and A2 had been used as basis sizes alongside the British ones. The following are common British basis sizes (sizes of British uncut paper): One ream in the context of basis weight (basis ream) can be 480 sheets, 500 sheets, 504 sheets, or 516 sheets: there is no fixed rule on when a certain definition was applied; however, starting from the 1950s, Below is a list of common basis weights of paper stocked by British paper merchants:
Conversion between British basis weights, American basis weights, and grammage It is possible to convert between different basis weights – both British and American – and grammage using different conversion formulae. To convert between the basis weights of different paper with the same basis‑ream size (e. g., from to ), the following formula can be employed: Area of one sheet of paper in the target ream × Weight of the source ream ÷ Area of one sheet of paper in the source ream To convert between the basis weights of different paper with different basis‑ream sizes (e. g., from to ), the relevant formula is below: (Number of sheets of paper in the target ream × Area of one sheet of paper in the target ream) × Weight of the source ream ÷ (Number of sheets of paper in the source ream × Area of one sheet of paper in the source ream) To convert between the basis weights of the same paper with different basis‑ream sizes (e. g., from to ), one may either use the previous formula or the one below: Weight of the source ream × Applicable conversion factor from the table below To convert from a basis weight to its equivalent grammage, the following is the applicable formula: Weight of the basis ream × 703,125 ÷ Area of one sheet of paper in the basis ream in square inch × Number of sheets of paper in the basis ream To convert from grammage to the equivalent basis weight, the formula below can be used: personal and business letter paper, especially that with engraved letterheads, is typically 24–29 (100–120gsm); greetings cards are usually 85–106 (200–250gsm); newspaper paper is typically 15–23 (35–55gsm); business cards are usually 128–149 (300–350gsm); two‑ply toilet rolls and paper kitchen towels are 7 (31gsm) (both layers combined) and 17 (40gsm), respectively; and the typical department‑store wrapping paper is commonly 46–65 (50–70gsm). In the traditional British system, large post, double crown, and double imperial are common British basis sizes for writing paper, printing paper and other paper that is not writing or wrapping paper, and wrapping paper, respectively. '500' is omitted because it is clear from the context that a 'ream' is 500 sheets. ==Caliper==