This information is reprinted from the Cranberry Corner column of Hand Papermaking Newsletter #58 (April, 2002).
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From the earliest days of papermaking in Europe the Basis Weight of paper for various grades has been measured by the weight in pounds of a given number of sheets of a given dimension, which is called a Ream (rame in French, ries in German, resma in Spanish, from rizmah [a bundle of paper] in Arabic.)1
Each grade of paper had its own name, end-uses and ream size, such as the following examples used in England:2
Emperor 48”x72” Writing
Antiquarian 31”x53” Writing & Drawing
Elephant 23”x28” Writing, Drawing & Cartridge
Imperial 22”x30” Writing, Drawing & Printing
Cartridge 21”x26” Cartridge
Royal 19”x24” Writing & Drawing
Foolscap 13.25”x16.5” Writing & Drawing
Other countries had their own ream names and sizes.
The number of sheets in a ream also varied according to the grade and was normally 480 or 500 sheets. The 480 number is equivalent to twenty quires of 24 sheets each.1,3
The ream size of modern art papers is 22”x30”--500 sheets.
In recent times with the more widespread adoption of the metric system (more correctly known as the “S.I.” or systeme internationale), except for certain fine art and handmade papers, Basis Weight has been replaced by grammage as the unit of measurement of the weight per unit of paper.
Grammage is simply the weight in grams of one square metre of paper, and is expressed as grams per square metre or “gsm.”
To convert to grammage in grams per square metre from Basis Weight in pounds per ream of 500 sheets, 22”x30”, multiply by 2.1305. Each ream size has its specific conversion factor.
References: 1Papermaking by Dard Hunter, Alfred A Knopf Inc., New York, 1947. 2The Paper Trade Diary Directory of Great Britain, Trade Journals Ltd., London, 1935. 3Modern Papermaking by Robert H. Clapperton and William Henderson, Basil Blackwell, Oxford, 1941.