A4 vs. US Letter: Which Page Size Should Your PDF Use?
The two most common paper sizes differ by only a few millimetres, but choosing the wrong one can cut off content or leave odd margins. Here's how they compare and which to choose.
When you create a PDF from images or scans, one of the first questions is the page size: A4 or US Letter? They look almost the same, and on screen the difference hardly matters. When the document is printed, sent abroad or uploaded to a form with strict layout rules, it can. This guide explains the difference and helps you choose.
The two sizes at a glance
| A4 | US Letter | |
|---|---|---|
| Millimetres | 210 × 297 mm | 215.9 × 279.4 mm |
| Inches | 8.27 × 11.69 in | 8.5 × 11 in |
| Shape | Narrower and taller | Wider and shorter |
| Used in | Almost every country in the world | United States, Canada, Mexico, the Philippines and some other countries in the Americas |
| Standard | ISO 216 | ANSI (North American) |
A4 is about 6 mm narrower and 18 mm taller than Letter. That's enough to matter: a full-page layout designed for one will either be cut off or end up with uneven margins on the other.
Where the A sizes come from
A4 belongs to the ISO 216 series, built on a clever idea: every size has the same proportions, a ratio of 1 to the square root of 2. Folding an A4 sheet in half gives A5 with exactly the same shape; two A4 sheets side by side make A3. That's why you can enlarge or reduce A-size documents on a copier without changing the layout. US Letter has no such system; its dimensions come from traditional paper-making sizes.
Which one should you choose?
- Follow the recipient. If the document will be printed or filed in North America, use Letter; almost everywhere else, use A4.
- Follow explicit instructions. Application forms, publishers and courts sometimes specify a paper size. That rule always wins.
- Screen only? If the PDF will only be viewed on screens, the size doesn't matter much. In Image to PDF, "Fit to Image Size" makes each page exactly the size of its picture, with no white border at all.
What happens when you print on the "wrong" paper?
Printing an A4 PDF on Letter paper, or the other way round, is usually fine thanks to the "Fit to page" or "Scale to fit" option in the print dialog. The page is scaled down by a few percent so that nothing is cut off, at the cost of slightly larger margins on two sides. If you choose "Actual size" instead, content near the edges may be cut off: on Letter paper, the bottom of an A4 page; on A4 paper, the sides of a Letter page.
How page size works in our tools
In Image to PDF you choose A4, US Letter or Fit to Image Size, plus portrait or landscape orientation and a margin. Each image is scaled to fit inside the page and centred, keeping its proportions, so it's never stretched or cropped. With a margin of 10 or 20 mm, the picture stays clear of the edges where many printers can't print.
In Scan to PDF you choose between A4 and Letter; the orientation is chosen automatically for each photo. Pick the size of the paper you photographed: an A4 letter scanned onto a Letter page gains small white bands, but nothing is lost.
Merge PDF and Split PDF never change the page size. Every page keeps the size it had in its source file, so one document can safely mix A4, Letter and landscape pages.
Other sizes you may encounter
- A5 (148 × 210 mm): half of A4, common for booklets and notebooks.
- A3 (297 × 420 mm): twice A4, used for posters and drawings.
- US Legal (8.5 × 14 in): longer than Letter, used for some legal documents in the United States.
Summary
Use A4 unless the document is meant for North America or the recipient asks for Letter. For on-screen documents, let the page fit the image. And when printing on a different paper size, choose "Fit to page" in the print dialog so nothing gets cut off.