Most of the world prints on A4. The United States, Canada and some of Latin America print on Letter. The two look almost identical on a desk, and the difference only shows up when something goes wrong: a footer cut off, a page shrunk with a wide white margin, a portal that rejects your upload because "the document must be A4". This guide explains the sizes and how to move between them.
The sizes
| Size | Millimetres | Inches | Used for |
|---|---|---|---|
| A4 | 210 × 297 | 8.27 × 11.69 | Standard paper almost everywhere |
| US Letter | 215.9 × 279.4 | 8.5 × 11 | Standard paper in the US and Canada |
| US Legal | 215.9 × 355.6 | 8.5 × 14 | Some legal and real-estate forms |
| A3 | 297 × 420 | 11.69 × 16.54 | Posters, drawings, spreadsheets |
| A5 | 148 × 210 | 5.83 × 8.27 | Booklets, notebooks, flyers |
Letter is about 6 mm wider than A4 and about 18 mm shorter.
Why the A sizes fit together
The A series follows the ISO 216 standard, which has a neat property: each size is exactly half the one above. Fold A3 in half and you get A4; fold A4 and you get A5. This works because every A size has the same shape, with sides in the ratio 1 to the square root of 2. The practical benefit is that a document scales perfectly from one A size to another: printing an A4 page on A3, or two A4 pages side by side on one A4 sheet, wastes no space. Letter has no such relationship with any other size.
Printing Letter on A4, and A4 on Letter
A Letter document on A4 paper: the page is slightly too wide. With "Fit to page" the printer shrinks it by about 3%, which nobody notices. At "Actual size" a few millimetres at each side can be cut off.
An A4 document on Letter paper: the page is too tall. "Fit to page" shrinks it to about 94%; at actual size the footer, often holding page numbers, is cut off.
So when printing a document made for the other standard, choose Fit to page or Shrink oversized pages in the print dialog.
Mixed page sizes in one PDF
Documents assembled from several sources often mix sizes: A4 forms, a Letter reference letter from abroad, phone photos at odd dimensions, a long thin receipt. On screen this looks untidy; some upload portals reject it outright, and printing it requires the printer to make a decision for every page.
Resize pages makes every page the same standard size. Each page is scaled to fit and centred, never stretched, so a receipt sits in the middle of an A4 page rather than being distorted to fill it. If pages carry large empty borders, trim them first with Crop PDF so the content is printed larger.
Setting the size at the source
The cleanest fix is to create documents at the right size in the first place. Word and Google Docs default to the paper size of the region the software was installed for, which is how people in A4 countries end up with Letter documents. Check Layout, Size in Word, or File, Page setup in Google Docs, before you start writing, and set it to the paper your recipient will use.
When turning photos into a PDF, choose A4 as the page size in JPG to PDF rather than "fit to image", so every page comes out a standard size.