A4 and US Letter compared, down to the point

The exact dimensions

A4 measures 210 by 297 millimetres, or about 8.27 by 11.69 inches. US Letter measures 8.5 by 11 inches, or 215.9 by 279.4 millimetres. So Letter is about 6 mm wider and about 18 mm shorter than A4. A4 is the standard almost everywhere; Letter is standard in the United States, Canada and parts of Latin America.

Inside a PDF, page sizes are recorded in points, at 72 points to the inch:

• A4: 595 x 842 points (595.28 x 841.89 to be exact), 210 x 297 mm, 8.27 x 11.69 in
• US Letter: 612 x 792 points, 215.9 x 279.4 mm, 8.5 x 11 in
• US Legal, for comparison: 612 x 1008 points, 8.5 x 14 in
• Aspect ratio: A4 is 1 : 1.414; Letter is 1 : 1.294, noticeably squatter

Where A4 comes from: the square root of two

The A series is built on one idea. A0 is a sheet with an area of one square metre and sides in the ratio 1 to the square root of two. Cut it in half across the long side and you get A1, which has exactly the same proportions. Halve that and you get A2, and so on down to A4, which is A0 halved four times.

The square root of two is the only ratio with that property, and it is why A-series sizes scale into each other perfectly: two A4 pages side by side make exactly one A3, and enlarging A4 to A3 on a photocopier is a clean 141 percent with nothing cropped. This is standardised as ISO 216, which is why the size is sometimes called ISO A4.

US Letter has no such system. Its dimensions are a historical convention, and the sizes around it, such as Legal and Tabloid, do not share its proportions. Halving a Letter sheet gives 5.5 by 8.5 inches, a different shape.

Why documents print with gaps or cut-off edges

Because neither size fits inside the other. A4 is narrower and taller; Letter is wider and shorter. Whichever way a document travels, something has to give.

Print an A4 document on Letter paper at actual size and the extra 18 mm of height has nowhere to go. Headers, footers and page numbers are usually the casualties, clipped at the top or bottom. Print a Letter document on A4 at actual size and it fits vertically with spare white space at the bottom, but the extra 6 mm of width can push content into the printer's unprintable margin on either side.

Most print dialogues offer to scale to fit, which avoids the clipping by shrinking the page uniformly until both dimensions fit. The cost is a slightly smaller page with blank margins on the sides that had room to spare. Uniform scaling never distorts the proportions, but it cannot make two different shapes match, so the margins are unavoidable.

Forms are where this hurts most. A form designed with fields close to the edge, or with boxes meant to align with pre-printed stationery, can become unusable after a size change that would be invisible in an ordinary letter.

How much scaling each direction needs

Fitting a page into the other size means scaling it by whichever dimension is the tighter fit:

• A4 to Letter: the height is the constraint, so the page shrinks to about 94 percent. The width then falls about 52 points short, leaving a strip of roughly 18 mm of blank space across the page.
• Letter to A4: the width is the constraint, so the page shrinks to about 97 percent. The height then falls about 72 points short, leaving roughly an inch of blank space.

Converting a PDF between the two

If a document is going to be printed in the other region repeatedly, it is better to change the page size in the file once than to rely on everyone's print settings. World of PDF's Crop & Resize tool does that in the browser: choose Resize, pick A4 or Letter, and select which pages to change. Each page's content is scaled uniformly to fit the new size and the page dimensions are set to the target, so nothing is cut off and nothing is stretched. The leftover space described above stays blank.

Because a PDF's text and lines are usually stored as drawing instructions rather than pixels, scaling them down to 94 or 97 percent costs no sharpness. Scanned pages are images and will be resampled when printed, but a few percent is not visible in practice.

The same tool can crop instead, trimming a margin from every page by changing the visible area without deleting anything underneath. That is useful for removing white space from a scan, and the wrong choice for a size conversion, since cropping does not change the shape of the content.

If you have the source document, there is a better option still: change the paper size in the word processor and export again. Text then reflows to the new page instead of being shrunk, and margins come out even.

Frequently asked questions