A4 is the most frequently used size of paper. Letters, business documents, forms, brochures, advertisements and much more will be the ISO 216 standard A4 size which is 210mm x 297mm.
The size of A4 paper differs to the commonly used “Letter” size used in North America; by being 6mm (0.24 in) narrower and 18mm (0.71 in) longer.
In the ISO paper size system, the height-to-width ratio of all pages is the square root of two. In other words, the width and the height of a page relate to each other like the side and the diagonal of a square. This aspect ratio is especially convenient for a paper size. If you put two such pages next to each other, or equivalently cut one parallel to its shorter side into two equal pieces, then the resulting page will have again the same width/height ratio.
- A3 is twice the size of A4.
- A2 is twice the size of A3, and four times the size of A4.
- A1 is twice the size of A2, and eight times the size of A4.
- A0 is twice the size of A1, and sixteen times the size of A4.