Why A4 folds perfectly and Letter does not
The A series is built on one decision: every sheet has an aspect ratio of √2, about 1.414 to one. That number is not arbitrary and not a convention that could have gone another way. It is the only ratio a rectangle can have such that cutting it in half across the long edge gives a rectangle of the same proportions.
Work it through. A sheet is w by h. Halve it and you have h/2 by w. For those to be the same shape you need h/w = w/(h/2), which rearranges to h² = 2w², so h/w = √2. Any other starting ratio and the halves are a different shape from the original.
Everything the A series is good at follows from that one property. A4 folded is A5 with nothing trimmed. Two A4 pages fit an A3 sheet at 100%, which is why printing two-up needs no scale factor. Enlarging A4 artwork to A3 is a single 141% step with no cropping, and shrinking A3 to A4 is 71%, and those two numbers are the same two numbers for every neighbouring pair in the series. The whole thing is then anchored by defining A0 as one square metre, which is also why paper weight is quoted in grams per square metre: an A0 sheet of 80 gsm paper weighs 80 grams, and an A4 sheet weighs a sixteenth of that.
US Letter is 8.5 by 11 inches, a ratio of about 1.294. Fold it and you get 5.5 by 8.5, a ratio of about 1.545. Not the same shape — so Half Letter is not a smaller Letter, there is no series descending from it, and none of the conveniences above apply. Two-up printing on Letter always involves a scale factor and margins that do not match on the two axes.
A4 against Letter, measured
The reason this pairing causes trouble is that neither size contains the other. A4 is 210 by 297 mm. Letter is 215.9 by 279.4 mm. So A4 is about 6 mm narrower and about 18 mm taller. Each one sticks out past the other on a different axis, which means there is no direction in which one simply drops onto the other.
A4 and US Letter compared, and what happens in each direction| Measure | A4 | US Letter |
|---|
| Millimetres | 210 × 297 | 215.9 × 279.4 |
|---|
| Inches | 8.268 × 11.693 | 8.5 × 11 |
|---|
| Aspect ratio | 1.4143 (√2) | 1.2941 |
|---|
| Pixels at 300 dpi | 2480 × 3508 | 2550 × 3300 |
|---|
| Fits the other at | 93.9% on Letter | 97.2% on A4 |
|---|
Both directions need shrinking, which surprises people who expect one of the two to be simply bigger. Print an A4 document on Letter at 100% and the bottom 18 mm is cut off — which in practice means a footer, a page number, or the last line of a signature block, all of which are easy to miss on screen. Print a Letter document on A4 at 100% and the right edge overhangs by about 6 mm, which clips wide tables and right-aligned figures.
This is worth knowing before you send a file rather than after. If a document is going to be printed in both regions, the practical answer is to keep margins generous enough to absorb the difference — around 20 mm on every edge survives both sheets without reflowing — rather than to pick one size and hope.
How big is A4 in pixels
It has no answer on its own. A paper size is a physical measurement and a pixel is not, so the question needs a resolution before it means anything. A4 is 595 pixels wide at 72 dpi, 794 at 96, 2480 at 300 and 4961 at 600 — all of them the same piece of paper.
The DPI worth choosing depends on what happens to the file. 72 or 96 dpi is screen resolution and is what a PDF viewer assumes; 150 is acceptable for a draft or an internal document; 300 is the normal figure for anything printed properly and is what most print shops ask for; 600 or above is for fine line work and small type. Exporting at 72 and printing at 300 is the usual cause of a design that looked crisp on screen and came back soft.
One detail this page is careful about, because it is the sort of thing that quietly goes wrong: the pixel figures are computed from whichever unit each size is actually defined in. Letter is defined as 8.5 inches, so at 300 dpi it is exactly 2550 pixels. Round 8.5 inches to 216 mm first and convert back and you get 2551 — one pixel out, from a rounding that looked harmless.
Which envelope takes which sheet
The C series exists for one reason: to be slightly larger than the A series, so that an A sheet fits inside a C envelope. The sizes interleave deliberately — a C4 envelope takes an unfolded A4, a C5 takes an A5 or an A4 folded once, and a C6 takes an A4 folded twice.
DL is the exception and the one people get wrong. It is 110 by 220 mm and is not part of the C proportions at all — it is sized for an A4 folded in three, which gives a strip 99 mm deep, with a little clearance. That is the standard business envelope in most of Europe and the reason letterheads there are laid out with their fold marks a third of the way down. A DL envelope will not take an unfolded A5, even though A5 is smaller in area, because A5 is 148 mm wide and DL is 110.
The US system has no equivalent interlocking scheme. A #10 envelope, the standard business size there, is 4.125 by 9.5 inches and takes a Letter sheet folded in three — the same idea, arrived at separately, with no series behind it and no name that tells you what fits.
Where the two systems are used
ISO 216 is the standard almost everywhere. The United Kingdom, the European Union and India all use A4 as the default document size, along with most of Asia, Africa, South America and Oceania. The United States and Canada use Letter, and so do a few countries with close US trade ties — Mexico, Chile, the Philippines and Colombia among them, though several of those have adopted A4 as the official standard while Letter remains what is on the shelf.
Two consequences follow that are easy to trip over. Software defaults follow the operating system locale rather than the document, so a template created in one region opens with the other region’s page size and silently reflows — which changes line breaks, page counts and anything anchored to a page. And US Legal has no ISO equivalent at all: it is Letter’s width with three extra inches, a shape nothing in the A series is close to, so a Legal document printed in Europe has to shrink to 83.5% to fit A4.
If you are sending a document across that line, sending a PDF with the page size fixed avoids the reflow entirely. The recipient may still have to scale it at the printer, but the layout that arrives is the layout you made.
Sources and methodology
Two standards bodies define the two systems, and they define them in different units — which is the source of most of the confusion this page exists to clear up. ISO 216 specifies the A and B series in whole millimetres. The US sizes are specified in inches. This page therefore stores each size in the unit its own standard uses and converts outward, rather than storing everything in millimetres: rounding Letter’s 8.5 inches to 216 mm and converting back gives 2551 pixels at 300 dpi instead of the correct 2550, and a stray pixel in an export dialog is exactly the kind of error that is hard to trace later.