Paper density

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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]

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 80 g/m², therefore a typical A4 sheet (¹⁄₁₆ 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.^{[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 (560 mm) paper, and in the latter, 500 sheets of 24 by 36-inch (910 mm) 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	22½	× 28½	- 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	25½	× 30½	- 500 sheets
Paperboard (all types)	12	× 12	- 1000 sheets (1,000 square feet per ream)

Sheets 17 by 22 inches (560 mm) can be cut into four 8½ by 11-inch (280 mm) sheets, a standard for business stationery known conventionally as letter sized paper. So, the 17 by 22-inch (560 mm) ream became commonly used. The 25 by 38-inch (970 mm) book-paper ream developed because such a size can easily be cut into sixteen 6 by 9-inch (230 mm) book-sized sheets without significant waste.

Early newsprint presses printed sheets 2 by 3 feet (0.91 m) in size, and so the ream dimensions for newsprint became 24 by 36 inches (910 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 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# 8.5 by 11-inch (280 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 (560 mm) paper.

Conversions

The following chart^[2] shows standard conversions for bond/writing/ledger size paper (17" x 22"). These are a guide only, and are not necessarily always accurate.

Basis Weight (lb)	Caliper (inches)	Thickness (mm)	Grammage (g/m²)
16	.0032	0.081	60.20
18	.0036	0.092	67.72
20	.0038	0.097	75.20
24	.0048	0.120	90.30
28	.0058	0.147	105.35
29	.0060	0.152	109.11
31	.0061	0.155	116.63
35	.0062	0.157	131.68
36	.0068	0.173	135.45
39	.0072	0.183	146.73
40	.0073	0.185	150.50
43	.0074	0.188	161.78
44	.0076	0.193	165.55
47	.0078	0.198	176.83
53	.0085	0.216	199.41
54	.009	0.229	203.17
58	.0092	0.234	218.22
65	.0095	0.241	244.56
67	.010	0.250	252.08
72	.011	0.289	270.90
76	.013	0.330	285.95
82	.014	0.356	308.52
87	.015	0.380	312.00
105	.0175	0.445	385.06

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 = 25.4 µm)

See also

• Paper and ink testing
• Photographic printing — standard photographic print sizes
• Punchhole — filing holes
• Envelope size
• Index card
• Book size
• Paper size

External links

• Paper Weights Demystified
• Paper Weight - Conversion Chart
• Understanding Paper Weights
• Understanding paper weight (Staples, Inc.)
• M-weight Calculator

References

1. ^ International Standard ISO 536: Paper and board – Determination of grammage. International Organization for Standardization, Geneva.
2. ^ 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.