Grammage

Grammage and basis weight, in the pulp and paper industry, are the area density of a paper product, that is, its mass per unit of area. Two ways of expressing the area density of a paper product are commonly used:

• Expressed in grams (g) per square metre (g/m²), regardless of its thickness (caliper)^[1] (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.

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,^[2] though printers in most English-speaking countries still refer to the "weight" of paper.

${\displaystyle {\text{Grammage}}={\frac {{\text{mass}}{\text{ (g)}}}{{\text{length}}{\text{ }}({\text{m}})\times {\text{width}}{\text{ }}({\text{m}})}}}$

Typical office paper has 80 g/m² (0.26 oz/sq ft), therefore a typical A4 sheet (⁠1/16⁠ of a square metre) weighs 5 g (0.18 oz). 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 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.

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 20 pounds (9.1 kg) 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 17-by-22-inch (432 by 559 mm) paper, and in the latter, 500 sheets of 24-by-36-inch (610 by 914 mm) paper. Here are some basic ream sizes for various types of paper. Units are inches except where noted.

Paper type	Paper size (inches)		Sheets per ream
Bond, writing, ledger	17	× 22	500
Manuscript cover	18	× 31	500
Blotting	19	× 24	500
Box cover	20	× 24	500
Cover	20	× 26	500 or 1000
Watercolour	22	× 30	500
Bristol and tag	⁠22+1/2⁠	× ⁠28+1/2⁠	500
Tissue	24	× 36	480
Newsprint	24	× 36	500
Hanging, waxing, bag, etc.	24	× 36	500
Book, text, offset	25	× 38	500
Index bristol	⁠25+1/2⁠	× ⁠30+1/2⁠	500
Paperboard (all types)	12	× 12	1000 (1000 sq ft per ream)

Sheets 17 by 22 inches (432 by 559 mm) can be cut into four 8+1⁄2-by-11-inch (216 by 279 mm) sheets, a standard for business stationery known conventionally as letter sized paper. So, the 17-by-22-inch (432 by 559 mm) ream became commonly used. The 25-by-38-inch (635 by 965 mm) book-paper ream developed because such a size can easily be cut into sixteen 6-by-9-inch (152 by 229 mm) book sized sheets without significant waste (nominally 6+1⁄4-by-9+1⁄2-inch (159 by 241 mm) before trimming and binding).

Early newsprint presses printed sheets 2 by 3 feet (610 by 914 mm) in size, and so the ream dimensions for newsprint became 24 by 36 inches (610 by 914 mm), 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# 8+1⁄2-by-11-inch (216 by 279 mm) copy paper may be specified "10 M". 1000 cut sheets (or two reams) will weigh 10 lb (4.5 kg), half of the four reams of cut paper resulting from the 20# basis ream of 17-by-22-inch (432 by 559 mm) paper.

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.^[3]

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^[4] and A2^[5] had been used as basis sizes alongside the British ones.

The following are common^[6][7][8] 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,^[11] the trend of moving towards 500 sheets emerged.

Basis weight is commonly expressed in terms of pound per ream of paper of a certain size, in the format of '[number]lb [paper size]' (e. g., '21lb large post' ('21 pounds per ream of large post paper')). Since there are basis reams of different sizes, unless the context leaves no room for ambiguity, the number of sheets in the basis ream is also given after the paper size to avoid misunderstanding: in such case, basis weight is expressed in the format of '[number]lb [paper size] [number of sheets of paper in the basis ream]', such as '21lb large post 480' ('21 pounds per 480‑sheet ream of large post paper'). In the United Kingdom, '#' has never been used to express paper weight because it is called 'hash', instead of 'pound', in British English.

Demy traditionally plays the role of the default basis size for expressing basis weights: in the British paper industry, there was a device, called 'demy scale', that was used to measure the weights of different paper samples in pound per ream of demy paper (480‑sheet, 500‑sheet, and 516‑sheet reams).^[12]

Below is a list of common^[11] basis weights of paper stocked by British paper merchants: