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.

A quarter sheet is the page halved in both directions, the size you get from folding a sheet twice and cutting along the folds. A quarter of Letter is exactly 4.25 x 5.5 inches. A quarter of A4 is 105 x 148.5 mm, and because ISO 216 rounds every A size down to whole millimetres, the named size A6 is half a millimetre shorter at 105 x 148 mm. One common mix-up: 8.5 x 11 inches is US Letter, not A4. A4 in inches is 8.27 x 11.69, or 8.268 x 11.693 before rounding, which some charts cut down to 8.26 x 11.69.

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.

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.

The B series, and where you meet it

The B series sits between the A sizes. Each B size is the geometric mean of the two A sizes around it, which makes B5 a comfortable book page: larger than A5 but smaller than A4. B0 is defined as one metre wide, and the same square root of 2 proportion runs the whole way down. You will meet B5 in paperbacks and notebooks, B4 in some Asian office paper, and the larger B sizes in posters.

The ANSI series and the other North American sizes

ANSI A is Letter. Each step up doubles the area by joining two sheets along the long edge, exactly as the A series does, but because the starting rectangle is not root two the proportions alternate rather than repeat. Legal and Executive sit outside the series altogether: they are commercial sheet sizes that were standardised in practice rather than derived from anything.

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.

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.

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

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. Consulted 5 August 2026.
• ANSI/ASME Y14.1, Decimal Inch Drawing Sheet Size and Format, for the ANSI A to ANSI E dimensions, which are defined in inches and converted here at exactly 25.4 mm to the inch.
• ISO 32000-1 (PDF 1.7), for the point of 1/72 inch used in the PDF points column. See how we measure for what that means in practice.

Take the data with you

Every table on this page has its own anchor and a copy button, so you can link to the A series ladder or the scale factors directly rather than to the page. The same 23 sizes are also available as JSON at the paper sizes API, free to use under CC BY 4.0.

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.