Paper sizes developed over many years and across many different countries into the common international standards we use today.

Paper weights and sizes

There are two main systems in use: the ISO international standard and the North American/Canadian sizes. We will concentrate on the ISO international standard as this is the UK standard. A sheet of paper with the longest side vertical is called 'portrait' orientation. A sheet of paper with the longest side horizontal is called 'landscape' orientation.

The international paper size standard is scaleable: the base format is a sheet of paper measuring 1 m² in area (A0 paper size). Smaller paper sizes are calculated by halving the original paper size parallel to its shorter side. For example, an A0 landscape sheet is the equivalent of two A1 portrait sheets adjacent side by side; and an A1 portrait sheet is the equivalent of two A2 landscape sheets adjacent top and bottom. Because of the choice of lengths, the aspect ratio is the same for A1 as for A0 (as it is for A2, A3, etc).

This particular measurement system was chosen to allow folding of one standard size into another, which cannot be accomplished with traditional paper sizes. Brochures are made by using material at the next size up i.e. material at landscape A3 is folded to make A4 brochures. Similarly, material at landscape A4 is folded to make A5 brochures. The system also allows scaling up and down without any loss of image. Thus an A4 portrait image can be enlarged to A3 portrait size and retain the exact proportions of the original document.

When talking about paper sizes, you should specify height followed by width e.g. A4 portrait 297x210, A4 landscape 210x297.

Less commonly used is the B series of paper measurement. The area of B series sheets is the geometric mean of successive A series sheets. B1 is between A0 and A1 in size, having one side 1-metre long. Smaller B sizes therefore have one side that is a half, quarter or eighth of a metre. Many posters use B-series paper or a close approximation, such as 500 mm× 700 mm; B5 is a relatively common choice for books.

The C series is used only for envelopes. The area of C series sheets is the geometric mean of the areas of the A and B series sheets of the same number. Simply put, a C4 sheet area is the geometric mean of the areas of an A4 sheet and a B4 sheet. C4 comes out slightly larger than A4, so for practical purposes an A4 page fits inside a C4 envelope, and a C4 envelope fits inside a B4 envelope.

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PAPER WEIGHTS & THICKNESS

Paper density is expressed in grams per square metre (gsm or g/m²), paper density is also known as grammage. This is the measure used in most parts of the world.

In the metric system, the density of all types of paper and paperboard is expressed in terms of grams per square metre (gsm or g/m²). This quantity is commonly called grammage in both English and French (ISO 536), though printers in most English-speaking countries still refer to the "weight" of paper.

The unofficial unit symbol "gsm" instead of the standard "g/m²" is also widely encountered in English speaking countries.

While paper is measured by weight, card is also measured by thickness in microns.