As an American with a computer printer, I was vaguely aware of European metric paper, with its “scientific” naming scheme (e.g., A4) and slightly different proportions. International Standard Paper Sizes explains it all:
ISO 216 defines the A series of paper sizes based on these simple principles:
- The height divided by the width of all formats is the square root of two (1.4142).
- Format A0 has an area of one square meter.
- Format A1 is A0 cut into two equal pieces. In other words, the height of A1 is the width of A0 and the width of A1 is half the height of A0.
- All smaller A series formats are defined in the same way. If you cut format An parallel to its shorter side into two equal pieces of paper, these will have format A(n 1).
- The standardized height and width of the paper formats is a rounded number of millimeters.
A4 paper is 210 mm ? 297 mm. US “letter” paper is 216 mm ? 279 mm — 6 mm wider and 18 mm shorter.
A benefit of the constant square-root-of-two aspect ratio is that you can always fit two pages of one size onto the next larger size, side by side — or two pages, reduced, on the same size paper, with no loss. And you can always scale a page up or down to a larger or smaller paper size, with no loss.