With nearly every kind of paper is associated a basis size, which is determined by the manufacturing process, the purpose of the paper, and industry tradition. The weight in pounds of one ream (500 sheets) of the paper in the basis size is known as the basis weight or substance number. For instance, 500 sheets of 17 x 22 inch "substance twenty" bond paper weighs twenty pounds.
Note that the basis weight depends on the standardized basis size, and is not affected by the actual dimensions into which the sheet is cut. Example: the basis size of ordinary 8.5 x 11 inch bond paper is still 17 x 22 (see chart below), and the basis size of 8.5 x 11 cover stock is 20 x 26; this is true even though 20 x 26 sheets cannot be cut into 8.5 x 11 sections without waste.
These basis sizes are based upon British tradition, although today nearly everyone outside the United States uses grams per square meter ("grammage").
Confusing the issue somewhat is that some manufacturers also give what is called the M weight, which is the weight in pounds of one thousand sheets of paper in the size actually furnished, as distinguished from any basis size.
The basis weight is only a rough indicator of how thick the paper is, because the raw materials vary from stock to stock, and some papers are compressed more than others in manufacturing.
The six common basis sizes in the United States:
|Size (inches)||Area (square inches)||Examples of Paper Using this Basis|
|17 x 22||374||Bond, Ledger, Mimeograph, Photograph, Writing|
|20 x 26||520||Cover Stock|
|22.5 x 28.5||641.25||Bristol Board|
|25.5 x 30.5||777.75||Index Stock|
|24 x 36||864||Tag, Newsprint, Tissue, Wallpaper, Wax Paper, Wrapping Paper|
|25 x 38||950||Text, Offset, Book, India|
Basis sizes less frequently used:
|Size (inches)||Area (square inches)||Comments|
|12.75 x 16||204||United States Currency|
|20 x 20||400||Chemical Filter Paper|
|19 x 24||456||Blotter Paper|
|20.5 x 24.75||507.375||The former Index basis|
|18 x 31||558||Manuscript Cover|
|20 x 30||600||Carbon Paper|
|21 x 33||693||Wedding Invitations|
|22.5 x 35||787.5||Occasionally used for Bristol|
From time to time, a basis size of 32 x 44 inches is encountered; it has the special property that the number of pounds in the basis weight is equal to the number of grams per square meter.
Equivalence Chart for Basis Weights
Read across each row to find different designations for the same weight of paper. For example, 28-pound bond, 58-pound index and 70-pound text all weigh 104 grams per square meter.
Underscored are the most popular designations of printing or writing paper; utility papers run anywhere between 20 and 500 grams per square meter.
These figures are consistent within three percent. We have not attempted a greater precision because:
|Basis Weight in Pounds||Grams|
17 x 22
20 x 26
22.5 x 28.5
25.5 x 30.5
24 x 36
25 x 38