Look up paper dimensions in millimetres and inches across ISO A/B series and US sizes (Letter, Legal, Tabloid).
Paper • A/B series · US sizes
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The international A-series (A0, A1, A2 … A4) is defined by ISO 216 and built on an elegant idea proposed by Georg Christoph Lichtenberg in 1786: every size has the same aspect ratio of √2 (≈1.414), so folding a sheet in half yields the next size down with identical proportions. A0 is exactly one square metre, which fixes every other size. North America instead uses traditional "Letter", "Legal" and "Tabloid" sizes that pre-date the metric standard.
Knowing the exact dimensions in both millimetres and inches matters for printing, scaling artwork, setting up documents, and choosing frames or envelopes. This converter lists the full A/B series alongside the common US sizes so you can match or resize between systems without guesswork.
The ISO 216 A series is built on a single elegant property: every sheet has an aspect ratio of √2, so halving it across the long side produces two sheets of the same proportions. That is what makes A4 to A5 scaling lossless and why photocopier reduction settings are the tidy 71% (which is 1/√2). A0 is defined as one square metre of area, and each subsequent size halves it. North American Letter and Legal sizes come from a different tradition entirely, with no consistent ratio between them, which is why scaling between A4 and Letter always distorts or crops.
Aspect ratio = 1 : √2 (1.4142) for every A-series sizeA(n+1) = A(n) halved across the long dimensionA0 = 841 × 1189 mm = 1 m²Scaling A4 → A5 = 70.7% linear (1 ÷ √2)where:
Assumptions: A-series dimensions are rounded to whole millimetres, so the halving is very slightly approximate in practice. Printing A4 artwork on Letter requires either 94% scaling or accepting a crop, because the two have different proportions.
SourceISO 216:2007
Demonstrate the halving property, then show why the US comparison has no clean answer.
ResultA4 → A5 scales cleanly at 71%; A4 → Letter needs 94% and loses margin
Letter is 6 mm wider and 18 mm shorter than A4. Documents laid out for one and printed on the other lose either bottom margin or edge content, which is why international documents are usually designed with generous margins rather than to the page edge.