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The paper density of a type of paper or cardboard is the mass of the product per unit of area. The term density here is used somewhat incorrectly, as density is mass by volume. More precisely, "paper density" is a measure of the area density.
Two ways of expressing paper density are commonly used:
Expressed in grams per square metre (g/m2), paper density is also known as grammage.[1] 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.[1]
Contents
1 Grammage
2 Basis weight
3 Conversions
4 Caliper
5 See also
6 External links
7 References
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Grammage
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 grammage 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 80g/m2, therefore a typical A4 sheet (1?16 m2) weighs 5g.
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.[citation needed]
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 (560mm) paper, and in the latter, 500 sheets of 24 by 36-inch (910mm) paper. Here are some basic ream sizes for various types of paper. Units are inches except where noted.
Paper Type
Paper Size
- Number of sheets
Bond, writing, ledger
17
22
- 500 sheets
Manuscript cover
18
31
- 500 sheets
Blotting
19
24
- 500 sheets
Box cover
20
24
- 500 sheets
Cover
20
26
- 500 or 1000 sheets
Bristol and tag
22?
28?
- 500 sheets
Tissue
24
36
- 480 sheets
Newsprint
24
36
- 500 sheets
Hanging, waxing, bag, etc.
24
36
- 500 sheets
Book, Text, Offset
25
38
- 500 sheets
Index bristol
25?
30?
- 500 sheets
Paperboard (all types)
12
12
- 1000 sheets (1,000 square feet per ream)
Sheets 17 by 22inches (560mm) can be cut into four 8? by 11-inch (280mm) sheets, a standard for business stationery known conventionally as letter sized paper. So, the 17 by 22-inch (560mm) ream became commonly used. The 25 by 38-inch (970mm) book-paper ream developed because such a size can easily be cut into sixteen 6 by 9-inch (230mm) book-sized sheets without significant waste.
Early newsprint presses printed sheets...(and so on)
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