Paper density is a term used in the pulp and paper industry and also for fabric industry
to denote a measure of mass of the product per unit of area for a type of fabric, paper
or paperboard. The term "density" is not used in its traditional sense of mass per unit
volume. "Paper density", rather, is a measure of the area density. Paper products that let
little or no light pass through are considered dense or heavy. Paper products that allow
some light to pass through are considered lightweight. In the pulp and paper industry,
it is common to set a commercial paper machine to produce paper to a target paper density.
Paper density can also be used to distinguish paper from paperboard as the latter usually
has a grammage greater than 134 g/m2. Two ways of expressing grammage are commonly
used: Expressed in grams per square meter, paper
density is also known as grammage. This is the measure used in most parts of the world.
Expressed in terms of the mass per number of sheets, it is known as basis weight. The
convention used in the United States and a few other countries using US paper sizes is
pounds of a ream of 500 sheets of a given basis size. Japanese paper is expressed as
the weight in kg of 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 meter. 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 80 g/m2, therefore
a typical A4 sheet weighs 5 g. The unofficial unit symbol "gsm" instead of the standard
"g/m2" is also widely encountered in English speaking countries.
Typically grammage is measured in paper mill on-line by 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 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 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. 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 8.5-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 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# 8.5-by-11-inch copy paper may be specified "10 M". 1000 cut
sheets will weigh 10 lb, half of the four reams of cut paper resulting from the 20#
basis ream of 17-by-22-inch 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. In common measure
"20-lb bond varies from about 0.0038 to 0.0045 in. thick".
See also
References
External links Paper Weight - Conversion Chart
Understanding Paper Weights Understanding paper weight
M-weight Calculator Paper Weight Calculator