Paper size matters when you print a document, design a poster, create a PDF or send a file to a print shop. Europe uses the ISO "A" series, while the United States and Canada use Letter and Legal. This guide explains the differences and shows how to convert sizes into millimetres, centimetres, inches, points and pixels.
The ISO 216 standard defines the A series used almost everywhere outside North America. The idea is simple: A0 has an area of one square metre, and each next size is half of the previous one. A1 is half of A0, A2 is half of A1, and so on. Every size has the same proportions (1 : 1.414), so a document keeps its shape when you scale it up or down.
Letter is 215.9 × 279.4 mm (8.5 × 11 inches), slightly wider and shorter than A4. Legal is 215.9 × 355.6 mm (8.5 × 14 inches), and Tabloid is 279.4 × 431.8 mm (11 × 17 inches). If you send a document from Europe to the US, a file made for A4 may be scaled or cut off when printed on Letter.
When you design an image or poster, you need its size in pixels. The number depends on the resolution (dpi, dots per inch):
For US Letter at 300 dpi the size is 2550 × 3300 pixels.
Letters in Europe commonly use the C series and DL envelopes. A DL envelope is 110 × 220 mm and fits an A4 sheet folded in three. C5 (162 × 229 mm) fits an A5 sheet or an A4 sheet folded in half, and C4 (229 × 324 mm) fits an A4 sheet without folding.
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A4 is 210 × 297 mm. Letter is 215.9 × 279.4 mm, so it is wider and shorter.
2480 × 3508 pixels at 300 dpi, or 1240 × 1754 pixels at 150 dpi.
Almost all European countries use the A series, with A4 as the standard office size.
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You want an A3 poster at good print quality. A3 is 297 × 420 mm, which is 11.69 × 16.54 inches. At 300 dpi you need an image of about 3508 × 4961 pixels. If your image is smaller, it will look blurry when printed. At 150 dpi, 1754 × 2480 pixels is acceptable for a poster seen from a distance.
The ISO 216 system is practical: if you fold an A4 sheet in half you get A5, and two A4 sheets side by side make an A3. Photocopiers can enlarge A4 to A3 or reduce A3 to A4 without losing any part of the page. This is why the A series became the standard in Europe, Asia, Africa and Latin America, while the US and Canada keep Letter for historical reasons.
PDF files and design programs often measure in points. One point is 1/72 of an inch, so A4 is about 595 × 842 points. To convert millimetres to inches divide by 25.4; to convert inches to points multiply by 72. The converter does all these calculations for you in one table.