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:

• Paper as it comes from the factory can vary a few percent from the specification.
• When stored in moist climates paper absorbs water from the air, while in dry climates moisture in the paper evaporates.
• In almost any kind of printing process the paper will gain weight from the impressed ink.
• Printing methods that involve high temperatures, such as xerography, cause water to evaporate from the paper.

Basis Weight in Pounds						Grams Per Square Meter
Bond 17 x 22	Cover 20 x 26	Bristol 22.5 x 28.5	Index 25.5 x 30.5	Tag 24 x 36	Text 25 x 38
16	22	27	33	37	40	60
20	28	34	41	46	50	75
24	33	41	50	55	60	89
26	36	44	54	60	65	97
28	39	48	58	64	70	104
32	44	54	66	73	80	119
33	46	57	69	77	84	125
36	50	61	74	82	90	134
39	54	67	81	90	100	147
41	57	71	86	95	105	155
43	60	75	90	100	110	162
45	63	77	94	104	115	169
47	65	80	97	108	119	176
53	74	91	110	125	135	199
58	80	99	120	133	147	217
65	90	111	135	150	165	244
67	93	115	140	155	170	252
72	100	123	149	166	182	270
76	105	130	158	175	192	284
79	110	136	165	183	201	298
86	120	148	179	200	219	324
94	130	161	195	216	238	352