Why the sheet is decided before the photograph
A4 is a promise about paper, not about pictures. When a college portal, a visa centre or an employer asks for A4, they are describing what will come out of a printer at the far end, and every photograph in the document has to be arranged to suit that rather than the other way round. So this page starts from 210 by 297 millimetres and fits the photographs into it.
The margin is part of the same promise. Most office printers cannot put ink within about 5 mm of the paper edge, and a document with no margin at all will be either clipped or silently shrunk by the print dialogue — which is how a signature ends up half a centimetre from where it was meant to be. Twenty millimetres is comfortably clear of any printer, leaves room for a staple down the left, and is what this page opens with.
A4 in the three units people use
| Millimetres | Inches | Pixels at 200 DPI | Pixels at 300 DPI |
|---|---|---|---|
| 210 × 297 | 8.27 × 11.69 | 1654 × 2339 | 2480 × 3508 |
| 170 × 257 (inside a 20 mm margin) | 6.69 × 10.12 | 1339 × 2024 | 2008 × 3035 |
Those last two columns are the useful ones. If a photograph is 1600 pixels across and it is going into a 170 mm box, it is delivering about 239 dots per inch, and a 300 DPI ceiling will not improve it — a ceiling never invents pixels. The DPI guide works through that in more detail, and the one on paper sizes compares A4 against Letter and Legal properly.
Questions about A4 in particular
Is A4 the same as Letter?
No, and the difference bites in both directions. A4 is 210 by 297 mm; Letter is 216 by 279 mm. A4 is 6 mm narrower and 18 mm taller. A document laid out for one and printed on the other loses a strip off an edge or gains a band of white, which is why a portal that says A4 means A4.
What does a 20 mm margin actually cost me?
Twenty millimetres on all four sides of A4 leaves a box of 170 by 257 mm. That is 68 per cent of the paper's area for the photograph and the rest for white. A 10 mm margin leaves 190 by 277 mm, which is 84 per cent. Both are perfectly ordinary; 20 mm is the safer one if the page will ever go through a printer that cannot reach the edge.
My photograph is landscape and A4 is upright. What happens?
By default it is fitted into the upright box whole, with white above and below it. If you would rather the sheet followed the photograph, set orientation to Auto and a landscape shot gets a landscape sheet. With two or four photographs to a sheet the paper stays upright, because four pictures cannot agree on which way the page should go.