Grammage


Grammage and basis weight, in the pulp and paper and the fabric industries, are the area density of a paper or fabric product, that is, its mass per unit of area. Two ways of expressing grammage are commonly used:
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. This quantity is commonly called in both English and French, though printers in most English-speaking countries still refer to the "weight" of paper.

Grammage = Weight*10000/Length * Width
Typical office paper has, therefore a typical A4 sheet weighs. The unofficial 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

In countries that use US paper sizes, a less direct measure known as basis weight is used in addition to or instead of grammage. The basis weight of paper is the density of paper expressed in terms of the mass of a ream of given dimensions and a sheet count. 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. Often, this is a size used during the manufacturing process before the paper was cut to the dimensions in which it is sold. So, to compute the mass per area, one must know
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 mould.
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 having 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. 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.
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 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 500 sheets". When the density of a ream of paper is given in pounds, it is often accompanied by its "M weight". The M weight is the weight 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 will weigh, half of the four reams of cut paper resulting from the 20# basis ream of paper.

Caliper

Paper thickness, or caliper, is a common measurement specified and required for certain printing applications. Since a paper's density is typically not directly known or specified, the thickness of any sheet of paper cannot be calculated by any method. Instead, it is measured and specified separately as its caliper. However, paper thickness for most typical business papers might be similar across comparable brands. If thickness is not specified for a paper in question, it must be either measured or guessed based on a comparable paper's specification.
Caliper is usually measured in micrometres, or in the US also in mils. Commonly, 20-pound bond ranges between roughly in thickness.