Grammage and basis weight, in the pulp and paper industry, are measures of the area density of a paper product, that is, its mass per unit of area. Two common ways of expressing the area density of a paper product are used: Expressed in grams per square metre (g/m2), regardless of its thickness (caliper)International Standard ISO 536: Paper and board – Determination of grammage. International Organization for Standardization, Geneva. (known as grammage). This is the measure used in most parts of the world. It is often notated as gsm on paper product labels and spec sheets. Expressed in terms of the mass per number of sheets of a specific paper size (known as basis weight). The convention used in the United States and a few other countries using US-standard paper sizes is pounds (lb) per ream of 500 (or in some cases 1000) sheets of a given (raw, still uncut) basis size. The traditional British practice is pounds per ream of 480, 500, 504, or 516 sheets of a given basis size. Japanese paper is expressed as the weight in kilograms (kg) per 1,000 sheets.
Grammage In the metric system, the mass per unit area of all types of paper and paperboard is expressed in terms of grams per square metre (g/m2). This quantity is commonly called grammage in both English and French, though printers in most English-speaking countries still refer to the "weight" of paper.
Typical office paper has , therefore a typical A4 sheet ( of a square metre) weighs . The abbreviation gsm instead of the standard g/m2 symbol is also widely encountered in English-speaking countries.
Typically grammage is measured in paper mill on-line by a quality control system and verified by laboratory measurement.
Basis weight 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.
{| class="wikitable" |- !Paper type !colspan="2"|Paper size(inches) !Sheets per ream |- |Bond, writing, ledger ||style="border-right: none;"|17 ||style="border-left: none;"|× 22 ||500 |- |Manuscript cover ||style="border-right: none;"|18 ||style="border-left: none;"|× 31 ||500 |- |Blotting ||style="border-right: none;"|19 ||style="border-left: none;"|× 24 ||500 |- |Box cover ||style="border-right: none;"|20 ||style="border-left: none;"|× 24 ||500 |- |Cover ||style="border-right: none;"|20 ||style="border-left: none;"|× 26 ||500 or 1000 |- |Watercolour ||style="border-right: none;"|22 ||style="border-left: none;"|× 30 ||500 |- |Bristol and tag ||style="border-right: none;"|||style="border-left:none;"|× ||500 |- |Tissue ||style="border-right: none;"|24 ||style="border-left: none;"|× 36 ||480 |- |Newsprint ||style="border-right: none;"|24 ||style="border-left: none;"|× 36 ||500 |- |Hanging, waxing, bag, etc. ||style="border-right: none;"|24 ||style="border-left: none;"|× 36 ||500 |- |Book, text, offset ||style="border-right: none;"|25 ||style="border-left: none;"|× 38 ||500 |- |Index bristol ||style="border-right: none;"|||style="border-left: none;"|× ||500 |- |Paperboard (all types) ||style="border-right: none;"|12 ||style="border-left: none;"|× 12 ||1000 ( per ream)
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 A1Page 3, July, 1967 edition, Highlight and A2Page 3, October, 1967 edition, Highlight had been used as basis sizes alongside the British ones.
The following are commonPages 136, 138, and 142, Paper and Its Uses (1919 edition)Pages 148, 150, and 171–173, Paper and Its Uses (1939 edition)Page 29, The Papermakers' Pocket Book (1911) British basis sizes (sizes of British uncut paper): +Common British basis sizesNote 1:The Italicised writing‑paper and printing‑paper sizes were in use only prior to the 1925 standardisationPage 168, Paper and Its Uses (1939 edition) of writing‑paper and printing‑paper sizes. The 1925 standardisation did not apply to wrapping‑paper sizes.
Note 2:The underlined writing‑paper and printing‑paper sizes were in use after the 1925 standardisation but were not adopted as standard sizesPages 171–173, Paper and Its Uses (1939 edition) by British Standards Institution specification number 730 of 1937.