Paper Size Guide

A4 and US Letter are close in size but not identical. A4 measures 210 x 297 mm (8.27 x 11.69 inches), while US Letter measures 215.9 x 279.4 mm (8.5 x 11 inches). So Letter is slightly wider but noticeably shorter than A4. This difference matters when printing documents for one size on the other. A document formatted for A4 printed on Letter paper will have the bottom cut off unless you enable shrink-to-fit in your print settings. Going the other direction, a Letter-formatted document printed on A4 will have extra space at the bottom. A4 is the international standard used in virtually every country except the United States, Canada, and parts of Mexico. The difference traces back to two independent standardization efforts that happened on different continents.

The ISO A series is based on a mathematical principle: each size is exactly half the area of the next larger size, achieved by cutting the paper in half along its longer dimension. A0 is defined as having an area of exactly one square meter, with sides in a ratio of 1 to the square root of 2 (approximately 1:1.414). This ratio is special because when you fold or cut the paper in half along the long side, the resulting half sheet has exactly the same proportions as the original. A1 is half of A0, A2 is half of A1, and so on down to A10. The dimensions of A0 are 841 x 1189 mm. This system was first adopted as a German standard in 1922 and later became the international ISO 216 standard. The mathematical elegance means scaling between sizes is straightforward: A4 to A3 is always 141% enlargement, and A4 to A5 is always 71% reduction.

The paper size for posters depends on where and how they will be displayed. For small event posters or flyers meant for bulletin boards, A3 (297 x 420 mm) is a common choice. It is readable from a few feet away while still printable on many office printers. For medium-sized posters in store windows or classrooms, A2 (420 x 594 mm) provides good visibility. Standard movie posters use 27 x 40 inches (686 x 1016 mm), closest to A1 in overall area. For conference or academic poster sessions, A0 (841 x 1189 mm) and ANSI E (34 x 44 inches) are most common. Check with the venue or event organizer for their specific size requirements before designing your poster, as many conferences have strict limits for the poster boards they provide.

Legal size paper measures 8.5 x 14 inches (215.9 x 355.6 mm), which is the same width as US Letter but three inches longer. As the name suggests, Legal paper was historically the standard for legal documents in the United States and Canada, including contracts, court filings, deeds, and other official documents. The extra length provided more space for lengthy text and left room for signature lines, notary stamps, and legal seals. While many law firms have shifted to Letter size for routine documents, Legal size is still used for certain filings, real estate closing documents, and government forms. Most office printers have an adjustable tray that accommodates Legal size. In the design industry, Legal size is occasionally used for brochures and tri-fold pamphlets, since its length provides three roughly equal panels when folded into thirds.

The United States uses different paper sizes primarily because of historical path dependency. American paper sizes were established by custom and industry practice long before international standardization efforts began. The 8.5 x 11 inch Letter size became the standard in the US during the early 20th century, derived from traditional hand papermaking mold dimensions. By the time ISO 216 was published in 1975, American paper mills, printers, filing systems, and office equipment were already around Letter and Legal sizes. Switching to A4 would have required replacing billions of file folders, binders, envelopes, and paper trays across every office in the country. The US government considered adopting A4 in the 1980s and early 1990s, and some federal agencies experimented with it, but the transition never gained enough momentum.

The ISO B series is a companion to the A series that provides intermediate sizes between each pair of A sizes. The dimensions of each B size are the geometric mean of the corresponding A size and the next larger A size. B0 measures 1000 x 1414 mm, larger than A0. The B series is commonly used for specific applications: B5 (176 x 250 mm) is popular for books and journals, particularly in European and Asian publishing. B4 (250 x 353 mm) is used by some Japanese office equipment as a standard print size. B series envelopes are hold A series documents without folding. The B series is less encountered in everyday office work but is standard in printing, publishing, and envelope manufacturing across Europe and Asia.

Setting up your printer for a different paper size involves both a physical step and a software step. Start by loading the correct paper into the tray. Most printers have adjustable guides that slide to accommodate different widths and lengths. Adjust these guides snug against the paper without bending it. Then, in your operating system or application, go to print settings. On Windows, click Print, then Properties and look for Paper Size. On macOS, go to File then Page Setup or look in the Print dialog under Paper Size. Select the correct size from the dropdown. Make sure the document page size matches the paper in the printer, or use Fit to Page if there is a mismatch. Common mistakes include forgetting to change the paper guides in the tray, which causes jams, or setting the document to A4 but loading Letter paper, which results in cropped content.

For international shipping documents, A4 is almost always the correct choice, even if you are based in the United States. Shipping documents like commercial invoices, packing lists, and certificates of origin will be processed by customs officials in the destination country, and most countries outside North America use A4. Customs offices, banks handling letters of credit, and freight forwarders in Europe, Asia, Africa, and South America all work with A4 filing systems. Sending documents on Letter paper can result in them not fitting properly or being partially cut off when photocopied. The extra vertical space on A4 also provides room for the additional information fields that international documents often require. If you regularly handle international shipments, keeping A4 paper alongside your Letter paper is worthwhile.