A4 vs Letter paper size

A4 is defined metrically: 210 millimetres wide by 297 millimetres tall, which in the PDF point unit comes to approximately 595 by 842 points. US Letter is defined in inches: 8.5 by 11, which is exactly 612 by 792 points, since a point is 1/72 of an inch. Those are the four numbers the whole problem reduces to.

Compare them in the right direction and the relationship is easy to remember: A4 is narrower and taller than Letter. Narrower by roughly 17 points, a little under a quarter of an inch, and taller by about 50 points, close to seven tenths of an inch. Neither dimension is a superset of the other, which is the crux of the incompatibility — there is no orientation or rotation in which one size simply contains the other.

Geographically the split is straightforward. A4 is the standard almost everywhere in the world, part of the ISO 216 paper series. Letter is the standard in the United States and Canada, alongside its relatives Legal and Tabloid. So a document crossing between North America and anywhere else is very likely crossing a paper size boundary at the same time, usually without anybody mentioning it.

The ISO A series is built on a single elegant property: the ratio of the long side to the short side is 1 to the square root of two, approximately 1.414. The consequence is that cutting a sheet in half across its long dimension produces two sheets with exactly the same proportions as the original. A3 halved is A4; A4 halved is A5; A5 halved is A6, and so on up and down the series without ever changing shape.

That property is why scaling between A sizes is clean. Printing two A4 pages side by side on one A3 sheet, or reducing an A3 drawing to A4, is a uniform scale with no cropping and no change of proportion — the whole page fits the whole page. Booklet imposition, poster reduction and the common two-up handout all depend on it, and the factor involved is simply the square root of two or its reciprocal.

Letter has no such property. Its aspect ratio is 8.5 to 11, roughly 1.294, and the North American sizes are not generated by halving one another: Legal is longer at the same width, Tabloid is a different shape again. Placing two Letter pages on one Tabloid sheet therefore involves either empty bands or a non-uniform fit, and there is no neat mathematical family to move around inside. It is a perfectly serviceable size; it just does not scale as a system.

Print A4-formatted content on Letter paper and the height is the problem: the content is about 50 points taller than the sheet. The printer driver responds in one of two ways, and both cost you something. If it prints at actual size, the bottom of the page is clipped — often exactly where page numbers, footnotes and signature lines live. If it scales to fit, everything shrinks by a couple of per cent, which is usually invisible on body text but does shift every margin inward and can matter for anything dimensionally critical.

Print Letter content on A4 and the failure flips to the sides: Letter is wider, so the outer edges risk clipping, or a scale-to-fit leaves a band of unused paper at the bottom. Either way the symptom that gives it away is asymmetry — margins that were even in the source document arriving uneven on paper, or a header sitting lower than you designed it.

The robust habit is to decide the page size at the start and match it to where the document will physically be printed, rather than converting afterwards. When conversion is unavoidable, know which trade-off you are accepting: scaling preserves all content and changes every measurement slightly, while keeping actual size preserves measurements and risks losing content at an edge. Checking a file’s declared page size before sending it to a printer in another country takes seconds and prevents the entire category of problem — and a document whose margins are generous to begin with survives the conversion far better than one designed to the edge.