Two ways of expressing the area density of a paper product 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 (g/m2).
This quantity is commonly called grammage in both English and French,[2] though printers in most English-speaking countries still refer to the "weight" of paper.
The abbreviation "gsm" instead of the standard g/m2 symbol is also widely encountered in English-speaking countries.
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 mold.
When the density of a ream of paper is given in pounds, it is often accompanied by its "M weight" (M is 1000 in Roman numerals).
Since there are basis reams of different sizes, unless the context leaves no room for ambiguity, the number of sheets in the basis ream is also given after the paper size to avoid misunderstanding: in such case, basis weight is expressed in the format of ‘[number]lb [paper size] [number of sheets of paper in the basis ream]’, such as ‘21lb large post 480’ (‘21 pounds per 480‑sheet ream of large post paper’).
In the United Kingdom, ‘#’ has never been used to express paper weight because it is called ‘hash’, instead of ‘pound’, in British English.
Demy traditionally plays the role of the default basis size for expressing basis weights: in the British paper industry, there was a device, called ‘demy scale’, that was used to measure the weights of different paper samples in pound per ream of demy paper (480‑sheet, 500‑sheet, and 516‑sheet reams).
To convert between the basis weights of different paper with the same basis‑ream size (e. g., from lb large post 500 to lb demy 500), the following formula can be employed:[13] Area of one sheet of paper in the target ream × Weight of the source ream ÷ Area of one sheet of paper in the source ream To convert between the basis weights of different paper with different basis‑ream sizes (e. g., from lb large post 480 to lb demy 500), the relevant formula is below: (Number of sheets of paper in the target ream × Area of one sheet of paper in the target ream) × Weight of the source ream ÷ (Number of sheets of paper in the source ream × Area of one sheet of paper in the source ream)[b] To convert between the basis weights of the same paper with different basis‑ream sizes (e. g., from lb large post 480 to lb large post 500), one may either use the previous formula or the one below: Weight of the source ream × Applicable conversion factor from the table below[14] From ↓ To convert from a basis weight to its equivalent grammage, the following is the applicable formula:[15] Weight of the basis ream × 703,125 ÷ Area of one sheet of paper in the basis ream in square inch × Number of sheets of paper in the basis ream To convert from grammage to the equivalent basis weight, the formula below can be used:[15] Area of one sheet of paper in the basis ream in square inch × Number of sheets of paper in the basis ream × Value of gram per square metre ÷ 703,125 Utilising the aforementioned formulae, it is possible to work out the conversion factors for converting between various British and American units of basis weight and the gram per square metre: Note 2:Value of a unit in the left column ÷ Relevant conversion factor = Value of lb demy Note 3:To convert between any two units in the left column, Value of the source unit × (Conversion factor for the target unit ÷ Conversion factor for the source unit) Note 4:Note that, as a matter of mathematical relationship, 1 ‘single’ unit (e. g., lb foolscap) is always equal to 2 ‘double’ unit (e. g., lb double foolscap) and 4 ‘quad’ unit (e. g., lb quad foolscap), respectively.
Paper thickness, or caliper, is a common measurement specified and required for certain printing applications.
[23] The paper density is calculated by dividing the grammage over the caliper, and is usually expressed in grams per cubic centimetre (g/cm3)[24] to cancel out the mathematical need for unit conversions between metres and micrometres (a conversion factor of 1,000,000).