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What 300 dpi actually gives you

300 dpi is not a quality. It is a ruler.

Dots per inch says nothing on its own. It is the rate at which you agree to spend pixels on paper, and it only becomes a size when you know how many pixels there are. A file is not “300 dpi” the way a wine is dry; a file has a pixel count, and 300 dpi is the exchange rate you print it at.

The whole arithmetic is one line:

centimetres = pixels ÷ 300 × 2.54 — or, in inches, inches = pixels ÷ 300.

That is the entire secret of the number on every listing you have ever read. A seller who advertises a print size and a resolution has told you the pixel count whether they meant to or not, and you can check the claim in ten seconds.

The table

Long side of the file, printed at 300 dpi:

Pixels	Centimetres	Inches	Roughly
2 480	20	8.3	A4 width
3 508	29	11.7	A4 long side
4 961	42	16.5	A3 long side
7 016	59	23.4	A2 long side
9 933	84	33.1	A1 long side
12 000	101	40.0	a metre of wall
14 043	118	46.8	A0 long side
16 130	136	53.8	the largest sheet published here

Run it backwards and it answers the question you actually have. A print seventy centimetres on the long side needs 70 ÷ 2.54 × 300, which is 8 268 pixels. If the file has fewer, the number on the listing came from somewhere other than the file.

Why we print two numbers and not one

An old map is a printed image sitting on a larger piece of paper. The engraved plate left a border; outside it there is blank margin, an imprint line, sometimes a sheet number, and the plate mark itself. Those are two different widths, and a single advertised figure has to pick one and hide the other.

So every map here carries both: how wide the map prints, and how wide the full sheet with its margins prints. Across the maps published on this site the sheet runs on average about seven centimetres wider than the map; on one of them the gap is thirty-seven centimetres — ninety-nine against a hundred and thirty-six. If you had only the larger number you would have ordered a frame for a picture that is not that size.

Both numbers are measured, not estimated: the map from the accepted plate border, the sheet from the full frame of the master file, each divided by 300 and rounded down to the whole centimetre. Rounding down matters. Rounding 58.9 up to 59 is a tenth of a centimetre of lying, and the same habit is what turns a 62-centimetre map into “up to 75 cm”.

Where the inflated numbers come from

There are only a few ways a print size gets larger than the file, and none of them add detail.

Printing below 300 dpi and not saying so

The same 7 016-pixel file is 59 cm at 300 dpi, 74 cm at 240 dpi and 89 cm at 200 dpi. Nothing was added; the dots were simply spread further apart. For a photograph at arm's length that is often fine. For an engraving, where the whole subject is a hairline of ink and six-point place names, it is the difference between reading Terra Australis Incognita and seeing a grey smudge where it was.

Resampling the file upwards

Enlarging a 7 000-pixel scan to 12 000 pixels gives you a file that claims a metre of wall. The pixels are real; the detail is invented by an algorithm averaging its neighbours, and on engraved hatching it produces soft, slightly waxy lines that look wrong next to the crisp ones. It is easy to detect: open the file at 100 % and look at a letter.

Quoting the sheet and delivering the map

The honest version of the largest number, and still not what you asked for if you wanted the image that wide.

We do not upscale. If a scan is 8 739 pixels wide, the number on its page is the number that comes out of 8 739, and the file you download is 8 739 pixels wide. Where the source cannot carry a large print, the page says a smaller number rather than a larger file.

How to check any seller in ten seconds

Find the advertised long side in centimetres. Multiply by 300, divide by 2.54. That is the pixel count the file must have.
Ask for the pixel dimensions. A seller who has them will give them; the number is not a secret, it is the first thing any image viewer shows.
Ask which the size refers to — the image or the whole sheet.
Ask what the original is: which library, which shelfmark, which year. A scan with no provenance is a scan that cannot be checked.

None of these are hostile questions. They are the four facts that decide whether the print on your wall will be sharp, and a seller who has done the work will have all four to hand.

What 300 dpi is actually for

It is the point at which a normal eye at reading distance stops resolving individual dots. Further away it is more than you need — a map seen from across a room would survive 150 dpi — but an old map is a thing people walk up to. They lean in to find their own town, to read the cartouche, to see whether the Californian coast is drawn as an island. That is the distance these files are made for, and it is why the number is not negotiable here.

The maps published on this site currently run from seventeen to ninety-nine centimetres on the map's own long side, because that is what the surviving sheets carry. The number changes with the map, and it is stated on the map's own page, with the sheet size beside it.