Paper sizes: all dimensions

How the A series works

The ISO 216 standard starts from A0 (841 × 1189 mm, exactly one square meter): each next size is made by halving the long side. That's why two A5 sheets side by side make an A4, two A4 make an A3, and so on. The side ratio is always √2.

What the pixels are for

If you're preparing a file for print, multiply inches by DPI: an A4 at 300 DPI (print quality) is 2480 × 3508 pixels. At 72 DPI you get «screen» dimensions instead.

Why the ratio is the square root of two

It's the choice that makes the A series different from all the others: if the ratio between the sides is 1 to 1.414, halving the long side gives a sheet with the same ratio. It's the only number for which this works, and it's why two A4s make an A3 with nothing left over, and why «print two pages per sheet» crops nothing and leaves no white edges.

The most useful practical consequence concerns photocopying: from A4 to A3 you enlarge to 141%, from A3 to A4 you reduce to 71%. Those are the two numbers on the buttons of photocopiers all over the world, and that's where they come from.

The other two series you meet

Alongside the A series there are B and C, and they exist for one precise purpose: containing. A C sheet sits exactly between its A and its B, and indeed the C series is the one for envelopes: a C4 takes a flat A4, a C5 an A4 folded in half, a C6 an A4 folded in three. The B series is used for posters, books and boards, where you need a size between two A measures.

A4 and Letter are not interchangeable

The American Letter format is 216 × 279 mm: compared with A4 it's 6 mm wider and 18 mm shorter. It sounds like little and it's enough to ruin a document: a file laid out for A4 printed on Letter loses a line at the bottom or gets scaled down, and the other way round leaves white bands. If the document has to travel between Europe and the United States, the safe way is a PDF with margins at least a centimetre and a half wide.

Before sending it to print

Two numbers that avoid a redo. The DPI: printing wants 300 dots per inch, so an A4 is 2480 × 3508 pixels; at 150 DPI the graininess shows on photos, at 72 it's only good for screens. And the bleed: if colour has to reach the edge, the file must extend 3 mm beyond the format on each side, because the guillotine is never accurate to the millimetre.

Nearby tools

For measures in millimetres and inches there is Length converter, and to prepare an image at the right size Resize photo. To print several pages on one sheet the page is Several PDF pages per sheet, and for a passport photo Passport photo.