# Paper density

Paper density

The paper density of a type of paper or cardboard is the mass of the product per unit of area. Two ways of expressing paper density are commonly used:
* Expressed in grams per square metre (g/m²), paper density is also known as grammage.International Standard ISO 536: Paper and board – Determination of grammage. International Organization for Standardization, Geneva.] This is the measure used in most parts of the world.
* Expressed in terms of the mass (in pounds) of a ream of 500 (or in some cases 1000) sheets of a given (raw, still uncut) basis size, paper density is known as basis weight. The base size and area used here depends on the product type. This convention is used in the United States, and (to a lesser degree) in a very small number of other countries that use United States paper sizes. Japanese paper is expressed as the weight in kg of 1000 sheets. [http://home.inter.net/eds/paper/jpaper.html#basis]

=Gram

In the metric system, the density of all types of paper and paperboard is expressed in terms of grams per square metre (g/m²). This quantity is commonly called "" in both English and French (ISO 536), though printers in most English-speaking countries still refer to the "weight" of paper.

Typical office paper has 80 g/m², therefore a typical A4 sheet (frac|16 m²) weighs 5 g.

The unofficial unit symbol "gsm" instead of the standard "g/m²" is also widely encountered in English speaking countries.

While paper is measured by weight, card is measured by thickness in micrometres.Fact|date=July 2008

Basis weight

In countries that use United States 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 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. 20 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 20 pounds 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 paper, and in the latter, 500 sheets of 24 by 36 inch paper. Here are some basic ream sizes for various types of paper. Units are inches except where noted.

:

Sheets 17 by 22 inches can be cut into four 8½ by 11 inch sheets, a standard for business stationery known conventionally as "letter sized paper". So, the 17 by 22 inch ream became commonly used. The 25 by 38 inch book-paper ream developed because such a size can easily be cut into sixteen 6 by 9 inch book-sized sheets without significant waste.

Early newsprint presses printed sheets 2 by 3 feet in size, and so the ream dimensions for newsprint became 24 by 36 inches, 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 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 (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 specify "10 M". Therefore, 1000 sheets (or two reams) will weigh 10 lb (≈4.54 kg).

Conversions

The following chart [Copyright Micro Format, Wheeling Illinois. See http://www.paper-paper.com/weight.html. Permission to use data given by Steve Singer of Micro Format on November 27, 2007 to user Wikidsoup.] shows standard conversions for bond/writing/ledger size paper. These are a guide only, and are not necessarily always accurate.

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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, in the United States also in mils. (1 mil = 0.001 inch = 24.5 µm)

ee also

* Photographic printing &mdash; standard photographic print sizes
* Punchhole &mdash; filing holes
* Envelope size
* Index card
* Book size
* Paper size

* [http://www.thepapermillstore.com/pages.php?pageid=236 Paper Weights Demystified]
* [http://www.paper-paper.com/weight.html Paper Weight - Conversion Chart]
* [http://www.paper-papers.com/paper-weights.html Understanding Paper Weights]

References

Wikimedia Foundation. 2010.

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