Paper Sizes And Formats, The Difference Between A4 And Letter

ISO 216

The most convenient and distinguishing characteristic of ISO paper is that each format has an aspect ratio equal to the square root of two (1:4142) which makes it simple to enlarge or reduce a document for printing on another ISO paper format. The most popular series of the ISO standard is the A series. The most widely used paper of this series is the A4 format. All paper sizes of this series have a name that consists of an A followed by a number. The larger that number, the smaller the paper.

The basis for the whole system is the A0 format which has an area of one square meter. With an aspect ratio equal to the square root of two, a sheet of A0 paper ends up being 841 × 1189 millimeters. Figuring out the dimensions of the subsequent paper sizes does not require any real mathematical strain since each ensuing size can be created by simply folding the paper in half with the crease parallel to the shortest sides. If you do this with an A0 sheet of paper, the resulting dimensions will be 594 × 841 millimeters, or the A1 format. Take note that the height of A1 is equal to the width of A0.

Two other ISO paper series exist—B and C. The B series was brought into being to provide a wider range of paper sizes, where as the C series is used only for envelopes. The B paper sizes are a bit larger than their A series counterparts and are based on the geometric mean of two consecutive A series sheets. For example, B4 is between A3 and A4 in size, and B5 is between A4 and A5.

The C series was introduced to provide an envelope with enough space for an A series sheet. The sizes of the A series fit in C series envelopes of the same number. That is, a sheet of A4 easily fits into a C4 envelope. The C sizes are just between the A and B series. Below are tables with the dimensions of all three series. All of them have a height-to-width ratio equal to the square root of two.