A4 vs US Letter: the two paper sizes and how to print between them

A4 is 210 × 297 mm; US Letter is 8.5 × 11 inches, or 215.9 × 279.4 mm. Letter is about 6 mm wider and A4 about 18 mm taller, which is just enough to clip a footer or add an awkward strip of white space when a document made for one is printed on the other. I built the PDF page size checker after one too many of those surprises, and this guide explains where each size comes from and how to move a PDF between them cleanly.

The exact dimensions

PDF measures pages in points, where one point is 1/72 of an inch. That gives each size three equivalent descriptions:

• A4 — 210 × 297 mm, about 8.27 × 11.69 in, and 595 × 842 pt (595.28 × 841.89 before rounding).
• US Letter — 8.5 × 11 in, 215.9 × 279.4 mm, and exactly 612 × 792 pt.
• US Legal, often confused with Letter — 8.5 × 14 in, or 612 × 1008 pt.

The two everyday sizes cover almost the same area; A4 is roughly 3% larger. The real difference is shape. A4 has a height-to-width ratio of 1.414, while Letter's is 1.294, so Letter is squatter and A4 slimmer.

Why two standards exist

A4 belongs to the ISO 216 series, which is built on a single idea: every sheet has the proportions 1 : √2. Cut a sheet in half across its long side and the two halves have exactly the same shape as the original. A0 is defined as one square metre in area with that ratio (841 × 1189 mm); A1 is half of A0, A2 half of A1, and A4 is one sixteenth of A0. Because every A size is the same shape, an A3 poster shrinks onto A4 with no cropping or extra margins, which is why photocopiers offer exact 141% and 71% enlargement and reduction settings. The paper sizes reference lists the whole A, B and C series alongside the American sizes.

Letter has no such system behind it. Its 8.5 × 11 inch dimensions grew out of traditional hand papermaking and office practice, and the precise origin is not documented. The US federal government standardised on it for its forms in the early 1980s, replacing a slightly smaller government letter size, and it is formally defined as ANSI A.

Who uses which

Letter is the everyday size in the United States, Canada and the Philippines, and in several Latin American countries, including Mexico, Chile and Colombia. Almost everywhere else uses A4. If you send a CV, a form or a report across that divide, the recipient will usually print it on their own paper, so it is worth designing documents that survive either size: generous margins, at least 20 mm at the top and bottom, cope with both.

What happens when you print across

Printing a page at actual size (100%) on the other paper either crops it or leaves a gap. A Letter page on A4 loses a few millimetres at the sides; an A4 page on Letter loses about 18 mm in height, split between top and bottom or all from one edge, depending on the printer. Some office printers simply stop and ask for the other paper. Most print dialogs offer a fit or shrink oversized pages option that scales the page to the paper:

• Letter onto A4 scales to about 97%, leaving a little extra space above and below.
• A4 onto Letter scales to about 94%, leaving narrow bands at the sides.

Both are small enough that body text stays readable. The trouble comes with forms that must be printed at exact size, fine print near the edge, and booklets, where the scaling compounds; How booklet imposition works — why page 1 prints next to the last page covers why.

Checking and fixing a PDF

The page size checker reads every page and shows its width and height in millimetres, inches and points, names the standard size it matches, and flags documents that mix sizes, such as a Letter report with an A4 appendix or one stray landscape page. It treats sizes within 3 points (about 1 mm) of a standard as a match, because different software rounds slightly; anything else is reported as custom, which is common for scans. It reports the media box, the full page size, not the area covered by content, and the file is read on your device.

If the size is wrong, the best fix is to change the page size in the original document and export again, since the layout then reflows properly. When you only have the PDF, PDF fit to page redraws every page onto A4, Letter, Legal, A3 or A5, either fitting the whole page (with small margins) or filling the sheet and cropping slightly. Note that it redraws page content, so links and form fields on the original pages are not carried over. For a quick look at how PDFs store page boxes, see How PDF files work: why some have text and some are pictures.