The system of 'A' paper sizes was created by the International Standards Organization (ISO) to standardize the dimensions of paper. The smaller the number, the larger the sheet. So, for instance, an A1 sheet of paper is larger than an A2 piece, and A3 is bigger than A4.
It can be a little confusing at first as you might instinctively think the larger the number, the larger the piece of paper, but in fact it's the other way around. The larger the number, the smaller the paper.
The sizes are all relative to one another. Each sheet is the equivalent in size to two of the next smallest size in the series. So, for instance, two pieces of A1 make up an A0 sheet, or two pieces of A5 make up an A4 sheet. Or, put another way, each sheet is twice the size of the next in the series. If you tear a piece of A4 in half, you've two pieces of A5, or if you tear a piece of A3 in half, you've two pieces of A4.
Table of 'A' Paper Size -- size in millimeters -- approx inches
2A0 -- 1,189 x 1,682 mm -- 46.8 x 66.2 in
A0 -- 841 x 1,189 mm -- 33.1 x 46.8 in
A1 -- 594 x 841 mm -- 23.4 x 33.1 in
A2 -- 420 x 594 mm -- 16.5 x 23.4 in
A3 -- 297 x 420 mm -- 11.7 x 16.5 in
A4 -- 210 x 297 mm -- 8.3 x 11.7 in
A5 -- 148 x 210 mm -- 5.8 x 8.3 in
A6 -- 105 x 148 mm -- 4.1 x 5.8 in
A7 -- 74 x 105 mm -- 2.9 x 4.1 in
2A0 is the largest piece of paper measured by this system, and A7 the smallest. A4 size is the paper commonly used in computer printers. The dimensions are set in millimeters, so the inch equivalents given in the table below are only approximations.
For the mathematically minded: the height-to-width ration of ISO A paper sizes is based on the square root of two (1.4142 : 1) and a sheet of A0 is defined as having an area of a square meter.