Key difference: The size of A5 is half the size of A4 paper according to the ISO 'A' series of sheet sizes, which is based on a constant width to length ratio of 1: square root of 2 (value approximately= 1.4142).

The International Organization for Standardization (ISO) issues formal standards for measurements of almost everything that is used or made, including paper. In the 1920's, Dr. Porstmann of Germany published a document proposing a standard for paper. Accordingly to the international paper size standard, ISO 216, the papers have been categorized in A-series, B-series and C-series. The paper sizes are all based on the single aspect ratio of 1: √ 2 (square root of 2). This ratio is particularly important for reduction onto microfilm, or reduction and enlargement on the photocopiers. The ratio makes the process of archiving drawings, resizing, and modifying drawings easier.

A4 paper size is most commonly used in the world. It is used as the classic paper sheet in all industrialized countries. Among all the formats, A4 is clearly the most important one for daily office use. The A4 size has become the standard business letter size in English speaking countries such as Australia, New Zealand and the UK, that formerly used British Imperial sizes. Many South American countries such as Mexico, Costa Rica, Chile, Venezuela and many more, have also officially taken A4 paper as standard paper. A4 is considered as the “star” of ISO 216.

The A5 is mainly used for the creation of booklets, leaflets, flyers, etc. It corresponds to half an A4 size. It is rarely found in printers, although most are adaptable. The A5 is obtained by folding an A4 in the direction of its width. This proportion is still preserved, which also keeps the contents of the sheet at the time of its enlargement.

Comparison between A4 and A5 Paper Size:

Image Courtesy: theinternetprinter.com.au