The dimensions, side by side
| Size | Millimetres | Inches | Standard |
|---|---|---|---|
| A4 | 210 × 297 | 8.27 × 11.69 | ISO 216 |
| US Letter | 215.9 × 279.4 | 8.5 × 11 | ANSI/ASME Y14.1 |
| US Legal | 215.9 × 355.6 | 8.5 × 14 | ANSI/ASME Y14.1 |
| Tabloid / Ledger | 279.4 × 431.8 | 11 × 17 | ANSI/ASME Y14.1 |
A4 is narrower and longer than Letter: 5.1mm narrower, 17.6mm longer. Neither dimension matches, which is the whole source of the trouble. A document laid out for one and printed on the other does not simply scale, since the two shapes are not proportional to each other at all.
Why the two standards diverged
Letter descends from a US paper tradition with roots in colonial-era papermaking sizes, and was formalised into its current exact dimensions by ANSI only decades after A4 was already an international standard. A4 is one step in the ISO 216 series, derived mathematically from a one-square-metre A0 sheet in a fixed 1:√2 ratio. The two were never designed to relate to one another; they solve the same problem from two unrelated starting points, and the small mismatch between them is a side effect rather than a design choice on either side.
What actually goes wrong when they meet
The most common failure is printing an A4 PDF at "actual size" on a printer loaded with Letter stock, or the reverse. Because A4 is longer than Letter, content near the bottom of an A4 page can be cut off when printed on the shorter Letter sheet. Because Letter is wider than A4, a document formatted for Letter can leave more blank margin down the side when printed on the narrower A4 sheet. Neither is a printer fault; the paper genuinely does not match the layout.
"Fit to page" or "scale to fit" settings, offered by almost every printer driver and PDF viewer, avoid the problem by shrinking the content slightly to guarantee it fits within whichever sheet is loaded, at the cost of the document no longer being printed at its exact original scale. For anything where exact scale matters, a technical drawing or a form with precisely spaced fields, it is worth printing on the size the document was actually built for rather than relying on autoscaling.
Working across both, deliberately
Software that needs to work in both markets commonly designs for whichever is smaller in the relevant dimension, and leaves extra margin on the other, so the same file prints cleanly on both without adjustment. A form built with 20mm margins on every side, for instance, has enough headroom to absorb the difference between A4 and Letter without any content moving. If you are the one creating a document meant to travel internationally, checking margins against both sizes before finalising it avoids the "fit to page" shrink entirely.
Telling them apart at a glance
Held side by side, A4 is visibly narrower and taller than Letter, though the difference is easy to miss without a direct comparison, since both are close to 21 by 28 centimetres and neither looks obviously wrong on its own. The Paper Size Guide draws both to your screen's calibrated scale side by side, which is a faster way to see the gap than reading the numbers.