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There have been many standard sizes of paper at different times and in different countries.

The international standard is ISO 216, which defines amongst other, A4. ISO paper sizes are all based on a single aspect ratio of the square root of two, 1:1.4142. The largest standard size, A0, has an area of 1 . A1 is formed by cutting a piece of A0 in half, which retains the aspect ratio. This particular measurement system was chosen in order to allow folding of one standard size into another, which cannot be accomplished with traditional paper sizes.

The ISO B series is a scaling of the A series, B1 is half way between A0 and A1 in size. The C series is half way between the A and B series of the same number, for instance, C0 is half way between A0 and B0. This way C0 is slightly larger than A0, and B0 slightly larger than C0. This was intended to allow one to fit inside an envelope of the other. For instance, a letter written on A4 paper, the standard for this role, fits inside a C4 envelope. A C4 envelope can fit inside a B4 envelope.

Sizes larger than A0, B0 and C0 are defined by putting a number in front of the letter (so 1A is twice the size of A0).

International standard paper sizes are now used in all countries except the United States and Canada.

International Paper Sizes

NameSize (mm)
A0841 × 1189 B01000 × 1414 C0917 × 1297
A1594 × 841 B1707 × 1000 C1648 × 917 D1545 × 779
A2420 × 594 B2500 × 707 C2458 × 648 D2385 × 545
A3297 × 420 B3353 × 500 C3324 × 458 D3272 × 385 E3400 × 560
A4210 × 297 B4250 × 353 C4229 × 324 D4192 × 272 E4280 × 400
A5148 × 210 B5176 × 250 C5162 × 229 D5136 × 192 E5200 × 280
A6105 × 148 B6125 × 176 C6114 × 162 D6 96 × 136 E6140 × 200
A774 × 105 B788 × 125 C781 × 114 D768 × 96
A852 × 74 B862 × 88 C857 × 81
A937 × 52 B944 × 62 C940 × 57
A1026 × 37 B1031 × 44 C1028 × 40

US Paper Sizes

NameSize (inches)Size (mm)
Quarto10 × 8254.0 × 203.2
Foolscap13 × 8330.2 × 203.2
Letter8.5 × 11215.9 × 279.4
Legal8.5 × 14215.9 × 355.6
Ledger / Tabloid11 × 17279.4 × 431.8
Executive7.25 × 10.5184.15 × 266.7
Post15.5 × 19.25393.7 × 488.95
Crown15 × 20381.0 × 508.0
Large post16.5 × 21419.1 × 533.4
Demy17.5 × 22.5444.5 × 571.5
Medium18 × 23457.2 × 584.2
Royal20 × 25508.0 × 635.0
Elephant23 × 28584.2 × 711.2
Double Demy23.5 × 35596.9 × 889.0
Quad Demy35 × 45889.0 × 1143.0
A8.5 × 11215.9 × 279.4
B11 × 17279.4 × 431.8
C17 × 22431.8 × 558.8
D22 × 34558.8 × 863.6
E34 × 44863.6 × 1117.6