Guide
A4 vs Letter: paper sizes explained
A4 is 210 x 297 mm (8.27 x 11.69 in). US Letter is 215.9 x 279.4 mm (8.5 x 11 in). A4 is 5.9 mm narrower and 17.6 mm taller, which is enough to matter: printing an A4 page onto Letter needs about 94 percent scaling, and a Letter page onto A4 about 97 percent. Print either at 100 percent on the wrong sheet and part of it falls off the paper. Below are the exact numbers in every unit you might need, why two systems exist at all, and what to do when you have the wrong one.
What is the difference between A4 and Letter paper size?
The same two sheets, measured every way that matters. Points are the DTP point of exactly 1/72 inch, which is the unit PDF pages are actually defined in, so those figures are what your PDF reader reports.
| Measure | A4 | Letter |
|---|---|---|
| Millimetres | 210 x 297 | 215.9 x 279.4 |
| Inches | 8.27 x 11.69 | 8.5 x 11 |
| Centimetres | 21 x 29.7 | 21.59 x 27.94 |
| Points (1/72 in) | 595 x 842 | 612 x 792 |
| Picas (12 pt) | 49.6 x 70.2 | 51 x 66 |
| Pixels at 300 DPI | 2480 x 3508 | 2550 x 3300 |
| Pixels at 96 DPI | 794 x 1123 | 816 x 1056 |
| Aspect ratio | 1 : 1.4142 (square root of 2) | 1 : 1.2941 |
| Area | 623.7 cm2 | 603.2 cm2 |
| Standard | ISO 216, A series | ANSI A (ASME Y14.1) |
| Everyday use in | Almost every country | United States and Canada |
Two things fall out of that table. A4 holds about 3.4 percent more paper than Letter despite being narrower, because the extra 17.6 mm of height more than repays the 5.9 mm of lost width. And the proportions genuinely differ: A4 is a noticeably slimmer rectangle, which is why a photo or layout designed for one always leaves an odd band of white space on the other.
Why are there two paper size systems?
The A series is a designed system. The ratio of 1 to the square root of 2 was described by the physicist Georg Christoph Lichtenberg in a letter in 1786, reinvented and turned into the German standard DIN 476 by Walter Porstmann in 1922, and adopted internationally as ISO 216 in 1975. Letter is an inherited one: 8.5 by 11 inches was already the ordinary commercial sheet in North America, and it was standardised there as ANSI A in the ANSI and ASME Y14.1 series rather than replaced.
The practical cost of that history is that the North American sizes do not scale cleanly. As Markus Kuhn's reference on the ISO standard notes, the US series alternates between two aspect ratios, 17/11 = 1.545 and 22/17 = 1.294, so there is no single magnification that takes you from one size to the next. Every ISO A step, by contrast, is exactly the same factor.
Why does the A series halve so neatly?
Because of that square root of 2 proportion. Cut a sheet with sides in the ratio 1 : 1.4142 in half across its long side and each half has sides in the ratio 1 : 1.4142 again. Nothing else does this. It means an A4 page reduced to fit A5 keeps its proportions exactly, with no cropping and no white bands, and it is why photocopiers carry 71 percent and 141 percent buttons: 71 percent is 1 divided by the square root of 2 (one size down) and 141 percent is the square root of 2 (one size up).
The series is anchored at A0, which has an area of one square metre. Halve it repeatedly, rounding each dimension down to whole millimetres, and you get the whole ladder.
| Size | Millimetres | Inches | Compared to A4 |
|---|---|---|---|
| A0 | 841 x 1189 | 33.11 x 46.81 | 16 x the area |
| A1 | 594 x 841 | 23.39 x 33.11 | 8 x the area |
| A2 | 420 x 594 | 16.54 x 23.39 | 4 x the area |
| A3 | 297 x 420 | 11.69 x 16.54 | 2 x the area |
| A4 | 210 x 297 | 8.27 x 11.69 | The reference size |
| A5 | 148 x 210 | 5.83 x 8.27 | Half the area |
| A6 | 105 x 148 | 4.13 x 5.83 | A quarter of the area |
| A7 | 74 x 105 | 2.91 x 4.13 | An eighth of the area |
What happens if you print A4 on Letter paper?
At 100 percent scale, the bottom 17.6 mm of the A4 page simply runs off the end of the Letter sheet, and because Letter is wider, the whole layout sits 5.9 mm further from the right edge than it should. Going the other way, a Letter page printed at 100 percent on A4 loses 5.9 mm off the right side and leaves 17.6 mm of unused paper at the bottom.
If you let the printer fit the page instead, these are the exact factors it applies. The limiting dimension is whichever one is larger on the source size.
| Printing | Scale factor | Why |
|---|---|---|
| A4 page on Letter paper | 94 percent | 279.4 / 297 = 0.9407, height is the constraint |
| Letter page on A4 paper | 97 percent | 210 / 215.9 = 0.9727, width is the constraint |
| A4 page on A3 paper | 141 percent | Square root of 2, one full ISO step up |
| A4 page on A5 paper | 71 percent | 1 / square root of 2, one full ISO step down |
That 94 percent is the number to remember, and it is exactly why rescaling ruins measured paper. A 5 mm grid printed at 94 percent becomes a 4.7 mm grid. Isometric paper stops matching a 30 degree set square. A page of Cornell notes no longer lines up with a punched binder. If the accuracy of the ruling matters, generate the PDF in the size your printer actually holds rather than scaling afterwards.
Rule of thumb. Generate the PDF in the same size as the paper in your tray, then print at 100 percent scale with Fit to page switched off. That keeps every grid and ruling true. See the print settings guide for the exact dialog options.
Which countries use A4 and which use Letter?
A4 is the default in almost every country in the world, and ISO 216 is the format the United Nations uses for its documents. Letter is the everyday size in the United States and Canada, and remains common in Mexico, the Philippines and parts of Central and South America. If you are sharing a printable with an international audience and have to pick one, A4 reaches more people; if your reader is in North America, give them Letter.
The other North American sizes are worth knowing if you meet them. Tabloid, also called Ledger, is 11 x 17 in (279.4 x 431.8 mm) and is ANSI B, the equivalent step up from Letter that A3 is from A4. Legal is 8.5 x 14 in (215.9 x 355.6 mm), a Letter-width sheet stretched three inches longer, and it sits outside the ANSI series rather than inside it.
Which should you choose for a printable?
- Pick the size your printer actually holds. Choosing A4 and printing on Letter, or the reverse, forces a rescale and quietly changes every measurement on the page.
- Match your binder, not your habit. A4 punched paper and Letter three-ring paper use different hole patterns and different spacing, so the sheet has to match the folder it is going into.
- Think about the destination, not the source. If you are emailing a planner page to someone else to print, ask which side of the Atlantic they are on.
- For measured paper, never scale. Graph, dot grid and isometric sheets are only useful at true size.
Frequently asked questions
Is A4 bigger than Letter?
A4 is taller and narrower. A4 is 210 x 297 mm, Letter is 215.9 x 279.4 mm, so A4 is 5.9 mm narrower and 17.6 mm taller. By area A4 wins: 623.7 cm2 against 603.2 cm2, about 3.4 percent more paper.
What is A4 in inches?
A4 is 8.27 x 11.69 inches, converted exactly from 210 x 297 mm at 25.4 mm to the inch. Letter is defined the other way round, as exactly 8.5 x 11 inches, which is 215.9 x 279.4 mm.
What scale do I use to print A4 on Letter paper?
About 94 percent. The limiting dimension is height: 279.4 divided by 297 is 0.9407. Going the other way, a Letter page fits on A4 at about 97 percent, because 210 divided by 215.9 is 0.9727.
What happens if I print A4 on Letter at 100 percent?
The bottom 17.6 mm of the A4 page falls off the sheet, and the layout sits 5.9 mm further from the right edge than intended. Printing Letter on A4 at 100 percent clips 5.9 mm off the right side instead and leaves 17.6 mm of extra space at the bottom.
Why is A4 210 by 297 mm?
Because A0 has an area of one square metre and every A size keeps the same 1 to square root of 2 proportion. Halving A0 four times gives A4, and rounding each dimension down to whole millimetres gives 210 x 297 mm.
Which countries use Letter instead of A4?
Letter is the everyday default in the United States and Canada, and is still widely used in Mexico, the Philippines and parts of Central and South America. Almost everywhere else uses the ISO 216 A series, which Markus Kuhn's reference on the standard puts at roughly 95 percent of the world's population.
How many pixels is an A4 page?
It depends on the resolution. At 300 DPI, print quality, A4 is 2480 x 3508 pixels and Letter is 2550 x 3300. At the 96 DPI CSS reference resolution, A4 is 794 x 1123 and Letter is 816 x 1056.
Sources
- ISO 216, the international standard defining the A and B paper series, published by the International Organization for Standardization.
- Markus Kuhn, University of Cambridge, International standard paper sizes, for the history of DIN 476 and ISO 216, the alternating US aspect ratios and the A4 to Letter scaling factor.
Try it out
Every generator lets you switch between A4 and Letter before you download. Start with the paper generator or browse all printables. Filling a notebook with printed pages? Read our bullet journal guide to put dot grid pages to work.