For many years each country had its own standards for paper measurement. Today two systems of paper measurement are used throughout the world: the international standard (A4 and related sizes) and North American sizes.
A5 paper is part of a set of paper sizes called ISO-A, established by the International Organization for Standardization (ISO). It measures 148 millimeters by 210 millimeters, which is 5.83 inches by 8.27 inches.
A size chart
2A0 = 1189 millimeters x 1682 millimeters = 46.8 inches x 66.2 inches A0 = 841 millimeters x 1189 millimeters = 33.1 inches x 46.8 inches A1 = 594 millimeters x 841 millimeters = 23.4 inches x 33.1 inches A2 = 420 millimeters x 594 millimeters = 16.5 inches x 23.4 inches A3 = 297 millimeters x 420 millimeters = 11.7 inches x 16.5 inches A4 = 210 millimeters x 297 millimeters = 8.3 inches x 11.7 inches A5 = 148 millimeters x 210 millimeters = 5.8 inches x 8.3 inches A6 = 105 millimeters x 148 millimeters = 4.1 inches x 5.8 inches A7 = 74 millimeters x 105 millimeters = 2.9 inches x 4.1 inches
A4 size paper is often used for European letters, magazines, forms, catalogs and copy machine printing. It is the most commonly used A size. A5 is mainly used for notepads and pocket books.
International Organization for Standardization
The height/width ratio of the A paper system is constant. The base format is a sheet of paper which measures 1 meter squared in area (A0 paper size). If you fold an A0 paper in two along the short size you’ll have an A1-sized page. An A1 page folded the same way will result in an A2 size. Today the ISO standard has been adopted by every country except the United States and Canada.
North American sizes
In 1995 the American National Standards Institute created a set of paper sizes, which is used in the United States and Canada. The two most common sizes are "letter" (8.5 inches by 11 inches) and "legal" (8.5 inches by 14 inches).