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International Paper Sizes: A4, Letter, and the ISO 216 System

Most of the world sizes its paper by one elegant folding rule; the US and a few other countries use a different system entirely. Here is how both actually work.

A4: 210 x 297 millimeters

A4 is the most common paper size worldwide for everyday documents, letters, and printing, used by nearly every country except the United States, Canada, and a handful of others that still use US Letter and Legal sizes.

The ISO 216 system halves each size to make the next one

Every "A" size is defined so that folding it exactly in half along its longer side produces the next size down: A3 folded in half makes two A4 sheets, A4 folded in half makes two A5 sheets, and so on. This is why a single aspect ratio works at every size in the series.

The A-series aspect ratio is 1:root-2, not a simple fraction

The width-to-height ratio of every A-size sheet is 1 to the square root of 2 (about 1:1.414), the one ratio that stays constant when a rectangle is folded exactly in half. US Letter uses a plainer 8.5:11 ratio that does not have this self-repeating property.

A0 is defined to have an area of exactly one square meter

The whole series starts from A0, sized so its area equals one square meter (841 x 1189 mm). Every smaller A size in the series is simply A0 halved repeatedly, which is why A4 (one-sixteenth of A0) ends up at almost exactly 210 x 297 mm.

B and C series exist for other jobs

The ISO system also defines a B series, used for things like posters and passports, and a C series, sized specifically so a C4 envelope fits an unfolded A4 sheet and a C6 envelope fits an A4 sheet folded twice.

Why the US never adopted A4

The US Letter size predates the international ISO 216 standard and was already deeply embedded in American printing, office equipment, and government forms by the time the metric-based A-series was formalized internationally in the 20th century, similar to why the US has kept its customary units more broadly.

Why this trips up international paperwork

A4 is close enough to US Letter that a document formatted for one can look almost right, but not quite, when printed on the other, sometimes cutting off margins or leaving extra white space. Software that lets you choose a paper size when printing or exporting a PDF exists specifically to handle this mismatch.

Frequently Asked Questions

Which is bigger, A4 or US Letter?

US Letter is very slightly wider and shorter than A4 (8.5 x 11 in versus roughly 8.27 x 11.69 in), so the two are close in total area but not interchangeable without adjusting margins.

What is A5 used for?

A5 (half of A4) is a common size for notebooks, flyers, and greeting cards, small enough to handle easily but larger than a typical postcard.

Does the ISO system have a size bigger than A0?

Yes, a less commonly used "4A0" and "2A0" exist above A0 for large-format printing, though A0 itself already covers most poster and technical-drawing needs.