Why does my document print with the wrong margins?
A4 is 210 by 297 mm and US Letter is 216 by 279. Letter is wider and shorter, so neither fits inside the other — printing one on the other clips an edge or leaves an uneven margin.
Updated 2026-08-22
The two systems
Almost every country uses the ISO 216 series — A4, A3, A5 and the rest. The United States, Canada and a few others use their own set, of which Letter and Legal are the common ones. The two were designed independently and share no dimension.
A4 is 210 by 297 millimetres, which is 8.27 by 11.69 inches. US Letter is 8.5 by 11 inches, or 216 by 279 millimetres. Letter is about 6 millimetres wider and 18 millimetres shorter. Neither contains the other, which is the whole problem: a document laid out for one and printed on the other either loses something off an edge or gains a margin where it was not expected.
The difference is small enough that a page often looks fine and subtly wrong — a footer creeping onto a second page, a table losing its right-hand rule, a header sitting closer to the top than the designer intended. It is large enough that it cannot be ignored on anything with a fixed layout.
Legal size is the other one worth knowing: 8.5 by 14 inches, the same width as Letter and three inches taller. It is used mainly for contracts and court filings in the United States, and it has no ISO equivalent at all.
Why A4 has those odd numbers
The ISO series looks arbitrary and is the more rigorously designed of the two. Every A size has the same aspect ratio: the square root of two, approximately 1.414 to 1.
That ratio is the only one with a particular property — cut it in half across the long side and both halves have the same ratio as the original. This is why A4 folded in half is exactly A5, two A4 sheets side by side make A3, and so on up and down the series without distortion. Scaling a document between sizes never changes its proportions, which is why photocopiers have A4-to-A3 buttons that simply work.
The series is anchored by area rather than by a round dimension. A0 is defined as one square metre at that aspect ratio, which gives 841 by 1189 millimetres, and each subsequent size is half the area of the one before. A4 is one sixteenth of a square metre. The odd-looking millimetre figures are a consequence of that definition rather than a choice.
The area anchor has a practical payoff: paper weight is quoted in grams per square metre, so the weight of a sheet can be calculated directly. An A4 sheet of 80 gsm paper weighs exactly five grams, because sixteen of them make a square metre. That is why postage bands and envelope weights work out in whole numbers in metric countries.
Fixing a mismatched document
If the document is a PDF, rescaling it is the reliable fix. Each page is drawn onto a sheet of the target size and scaled to fit, which preserves proportions and turns any leftover space into margin. Text and vector graphics stay sharp because nothing is rasterised.
The alternative offered by most print dialogues — "fit to page" or "shrink to printable area" — does the same thing at print time without changing the file. That is fine for a one-off and unhelpful when you need to send the document to someone else, because they will face the same dialogue.
What not to do is stretch the page to fill both dimensions independently. It distorts everything on it, and because the distortion is only a few percent it does not look obviously wrong, just slightly off. Uniform scaling with a margin is always the better outcome.
For documents you are still writing, the better fix is upstream. Set the page size before laying anything out, and avoid designing to the very edge of the paper — a layout with generous margins survives being printed on either size, which is why most templates intended for international use are built that way.
Practical notes
When sending a document internationally, PDF is the safe format precisely because it fixes the page size. A word processor document reflows to whatever the recipient's default paper is, which can move page breaks and repaginate a contract without anyone noticing.
Printers themselves are usually not the problem. Nearly all of them accept both sizes; the mismatch is between what the document says and what is in the tray. An error about paper size is generally the printer being correct rather than being difficult.
Envelope sizes follow the paper. A DL envelope takes an A4 sheet folded in three, and C5 takes it folded once — both derived from the same series. American envelope sizes are named separately and do not correspond.
There is a related trap in image and slide dimensions worth mentioning, because it catches the same people. Presentation software defaults to a screen aspect ratio — 16:9 — which is nothing like either paper size, so a deck printed as handouts gets letterboxed heavily whichever paper it lands on. Setting the slide size to A4 or Letter before building the deck is the only clean fix; rescaling afterwards moves every element that was positioned by eye.
The same reasoning applies to anything designed at a fixed pixel size and later printed. Pixels have no physical dimension until a resolution is chosen, so a 1200-pixel-wide image is 4 inches across at 300 dpi and 12.5 inches at 96. A layout that looks correct on screen and prints at the wrong size almost always comes down to that assumption being made implicitly somewhere.
One conversion worth remembering: A4 is 8.27 by 11.69 inches, so a document designed with one-inch margins in Letter has slightly less room on the sides and slightly more vertically when moved to A4. That is the direction most cross-Atlantic documents travel, and it is why a table that fitted in Letter sometimes wraps in A4.