ISO Paper Sizes

The most common system of paper sizes in Europe is the ISO standard. Most people are familiar with the A series which includes A4 (highlighted on the chart) the usual letterhead size. The C series is for envelopes — A C4 envelope being ideal for holding an A4 sheet. There is also a B series which provides intermediate sizes for the A series but this is rarely used. DL is a special size for envelopes designed to accept A4 paper folded in three.

The aspect ratio of ISO paper sheets is 1 to 1.4142 (The square root of 2). This gives them a unique property: If you cut a sheet into two the resulting halves are the same proportion as the original. In other words a sheet of A4 when halved gives you two sheets of A5. All A size papers have the same proportions. The largest sheet in this series is A0 which is 841mm x 1189mm and just happens to be one square metre in area (ISO paper sizes are rounded to the nearest millimetre).

Two other series which you may come across are RA and SRA which are used by printers. They are slightly larger than the A series to provide for grip, trim and bleed

Of course you can use any size of paper you choose. However, most paper merchants supply ISO sized stock so it is most cost effective to stay with A sizes.