ISO A paper sizes

The A paper size system was formally adopted in Europe in the 19th century and has since spread around the world. It is now used in nearly every country apart from the USA and Canada. A4 is the most common standard business letter size used in English speaking countries.

The A-series consists of a logical set of paper sizes that are defined by the ISO 216 standard. The largest size A0 measures one square meter. The height/width ratio remains constant (1:1.41) for all sizes. This simple means you get the A1 size by folding an A0 paper in two along its short side. Then fold the A1 size in two to get an A2 size paper, and so on.

ISO B paper sizes

The dimensions of the B series paper sizes are defined by the ISO 216 international paper size standard. The B series is not as common as the A series. The B series uses an aspect ratio of 1:√2. The area of the B series paper sheets is the geometric mean of the A series sheets. For example, B1 is between A0 and A1 in size.

ISO C paper sizes

The dimensions of the C series sizes are defined by the ISO 269 paper size standard. The C series is most commonly used for envelopes. The aspect ratio of C series envelopes is 1:√2, and this means that an A4 sheet of paper when folded in half, parallel to its smaller sides, will fit nicely into a C5 envelope.