A4 vs US Letter paper: sizes, history and printing
By Dexaloop · Updated 27 October 2026 · Checked against the sources listed below
A4 measures 210 x 297 mm (about 8.27 x 11.69 in). US Letter measures 8.5 x 11 in (215.9 x 279.4 mm). So A4 is about 6 mm narrower and about 18 mm taller than Letter. A4 is the standard office sheet in most of the world. Letter is the norm in the United States and Canada. The two are close enough to confuse, and different enough to cut off a page footer or shift a ruler that is meant to print true to size.
The exact sizes side by side
Both sizes are fixed by standards. A4 belongs to the ISO 216 A series. Letter is a traditional North American size that the drawing standard ANSI/ASME Y14.1 calls size A. Since 1959 the inch has been exactly 25.4 mm, so converting between them is exact, not an estimate. NIST puts it this way: the inch "is exactly equivalent to 25.4 mm."
| Size | Millimeters | Inches | Shape (height / width) |
|---|---|---|---|
| A4 | 210 x 297 | 8.27 x 11.69 | 1.414 |
| US Letter (ANSI A) | 215.9 x 279.4 | 8.5 x 11 | 1.294 |
| A3 | 297 x 420 | 11.69 x 16.54 | 1.414 |
| Tabloid (ANSI B) | 279.4 x 431.8 | 11 x 17 | 1.545 |
| A5 | 148 x 210 | 5.83 x 8.27 | 1.419 |
The last column is the key difference. Every A size has the same shape. US sizes do not share one shape, so moving between Letter and Tabloid changes the proportions of the page.
Why A4 has such odd numbers
The ISO system is built on two rules. First, every sheet has a height to width ratio equal to the square root of 2, about 1.4142. Second, the largest sheet, A0, has an area of one square meter. As the Cambridge reference page on ISO paper sizes puts it, "Format A0 has an area of one square meter."
The square root of 2 ratio has a useful property. Fold or cut a sheet in half across its long side and each half has exactly the same shape as the original. A0 halves into two A1 sheets, A1 into two A2 sheets, and so on down to A4. That is why you can enlarge two A4 pages onto one A3 sheet, or shrink A4 to A5, with no white bands and no cropping.
The catch is that a sheet with that ratio and an area of one square meter cannot have round sides. A0 works out at 841 x 1189 mm after rounding. Halving four times gives 210 x 297 mm for A4. The odd numbers are a side effect of keeping the shape the same at every size.
Where the two sizes came from
The A series was adopted in Germany as DIN 476 in 1922. It became the international standard ISO 216 and the official United Nations document format in 1975, according to the same Cambridge source. Today most countries use it for everyday paper.
Letter has no such neat rule. It grew out of older paper making and office habits in North America, and it stayed in use while the rest of the world moved to ISO sizes. The Cambridge page names the United States, Canada and, in part, Mexico as the industrialized nations where ISO sizes are not yet widely used. For engineering drawings, ANSI/ASME Y14.1 defines a series that starts with A at 8.5 x 11 in and continues with B (11 x 17), C (17 x 22), D (22 x 34) and E (34 x 44).
Who uses which size
- A4: Europe, the UK, the Middle East, Africa, Asia, Australia, New Zealand and most of South America. Schools, offices, governments and home printers in these places default to A4.
- Letter: the United States and Canada, plus businesses elsewhere that deal mostly with them.
- Both: international companies, universities that accept work from abroad, and anyone sharing PDFs across borders. This is where the trouble starts.
What happens when you print one on the other
An A4 page on Letter paper
A4 is 297 mm tall and Letter is 279.4 mm tall. Printed at 100%, about 17.6 mm of the A4 page has nowhere to go. Most printers center the page, so you lose roughly 9 mm at the top and 9 mm at the bottom, plus whatever the printer's own unprintable edge removes. Page numbers and footers are the usual casualties.
To fit the whole height, the page must shrink to 279.4 / 297, which is 94.1%. Everything gets about 6% smaller, including text and any printed scale.
A Letter page on A4 paper
Letter is 215.9 mm wide and A4 is 210 mm wide. At 100%, about 5.9 mm of width is lost, about 3 mm per side. Content near the side margins may be clipped. To fit the width, scale to 210 / 215.9, which is 97.3%. The A4 sheet then has spare space at the top and bottom, because A4 is taller.
The Cambridge page gives the same rounded figures, 94% and 97%, and warns that word processing documents with an A4 layout "can often not be printed without loss of information" on Letter paper.
Worked example: a 5 cm check ruler
Say a printable sheet of graph paper is drawn for A4 with a 5 cm check line at the bottom. You print it on Letter with "fit to page" turned on. The printer shrinks the page to about 94.1%. The 5 cm line now measures 50 x 0.941 = 47.0 mm, and 5 mm squares come out at about 4.7 mm. Nothing looks wrong at a glance, but a ruler shows it at once.
The fix is not to scale at all. Make the sheet for the paper you actually have. A file generated for Letter prints true to size on Letter, and an A4 file prints true to size on A4.
Common mistakes
- Leaving the paper size on the default. Your software may default to A4 or Letter whatever paper is actually in the tray. Check it in the print dialog.
- Trusting "fit to page" for anything measured. It is fine for a letter or an essay. It is wrong for graph paper, templates, sewing patterns or anything with a scale.
- Designing to the edge. Microsoft notes that many printers need a minimum margin of around 0.25 in (6.35 mm) on all sides. Keep important content well inside the margins on either size.
- Assuming the shape is the same. A4 is longer and thinner. A layout that fills a Letter page will not fill an A4 page the same way, even after scaling.
Which size should you use?
Use the size your reader will print on. If you are sending a document to the US or Canada, Letter is the safe choice. Everywhere else, A4. If you do not know, keep generous margins of at least 20 mm on every side. Then the page survives being printed on either size at 100% with nothing cut off.
Make lined paper on A4 or Letter: print.dexaloop.com/paper/lined/. Free, no sign-up, and it runs on your device.
Questions
Is A4 bigger than US Letter?
A4 is taller but narrower. It is 210 x 297 mm, about 8.27 x 11.69 inches. Letter is 8.5 x 11 inches, or 215.9 x 279.4 mm. A4 has slightly more area overall.
Can I print an A4 PDF on Letter paper?
Yes, but at 100% the bottom of the page can be cut off, because A4 is about 18 mm taller. Either scale it to about 94% so the full height fits, or accept that the page is shrunk and check any measurements on it.
Which countries use US Letter?
Letter is the usual office size in the United States and Canada, and in part in Mexico. Most other countries use the ISO A sizes, with A4 as the everyday sheet.
Why is A4 210 x 297 mm and not a round number?
The A series keeps a height to width ratio of the square root of 2, and A0 has an area of one square meter. Both rules together give sides that cannot both be round numbers, so A4 comes out at 210 x 297 mm.
What is ANSI A paper?
ANSI A is the name ANSI/ASME Y14.1 gives the 8.5 x 11 inch sheet in its series of technical drawing sizes. It is the same size as Letter. ANSI B is 11 x 17 inches.
Sources
- Markus Kuhn, University of Cambridge: International standard paper sizes
- NIST: SI units, length (the inch is exactly 25.4 mm)
- Microsoft Support: Print in Microsoft Edge (margins)
This guide is general information, not legal, tax or financial advice. Rules change; check the official source for your own case. Found a mistake? Email support@dexaloop.com.