In this now-digital world, paper sizes are a subject people deal with less frequently these days. But when you do, it’s good to understand the difference between various sizes. What’s the difference between A4 and A5 paper size, for instance, and where do they measure up against the U.S. standard of “letter size” paper? And why are letter and A4 different at all, anyhow? To understand more about all this, it helps to take a deep dive into the history of standardization and why international standards matter.

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A3, A4 and A5 papers are all different paper sizes used in most countries of the world, with A4 being the size closest to America's letter-sized 8.5 by 11-inch paper.

Paper: America vs. the World

When it comes to North America versus the rest of the planet, it’s the lone regional holdout on adopting the A-series of paper sizes as the default standard. Instead, you’ll hear paper sizes like letter, legal, tabloid and so on. Everywhere else has been moving toward the A-series ever since the 1920s. In fact, the most commonly used paper size among Earth’s 7.5 billion people is A4. It’s only in Canada, the United States, Mexico, the Philippines and the Dominican Republic that the age of the letter-sized paper still prevails.

Why the Americas have been reticent in adopting the A-series sizes is up for debate. Is it because of the resistance toward the metric system? It’s hard to pinpoint why the switch has yet to be made, but whatever the reason, it can mean complications when taking documents created in Asia, Europe or elsewhere in the world and trying to print them in North America without undergoing some formatting changes.

In 1992, the American National Standards Institute — popularly known as ANSI — finally decided on fixed dimensions of legal- and letter-sized paper, officially ending all discussion about adopting the rest of the world’s A-series. So, why does everyone else use the A-series?

An Introduction to ISO

“ISO” is the International Organization for Standardization, and it’s an international body geared toward making sure that national borders don’t get in the way of functionality. Over the last half-century, the push toward standardizing products around the world has increased because society and trade are both becoming more global.

It was 1946 when delegates from 25 nations met at London’s Institute of Civil Engineers in an effort to “facilitate the international coordination and unification of industrial standards.” Since then, more than 164 countries have joined the ISO’s work in creating over 23,000 international standards.

These standards dictate everything from the specifications for a wheelchair (ISO 7176) to quality management processes for manufacturing (ISO 9000) and even simpler things, like how to abbreviate a country’s name, found in ISO 3166. So, no matter where you are in the world, if you see a country abbreviated as “MX” or “MEX,” you know they’re talking about Mexico, because it’s an internationally agreed abbreviation that’s been outlined in ISO 3166.

An Intro to A-Series Paper

It’s ISO 216 that handles the standardized sizes for paper, and with its international purview, those sizes are all listed in metric measurements. Math is helpful when it comes to understanding how the A-series sizes all work, since every A-series paper size is a function of the square root of two. It all starts with the A0 size, which is one square meter in size. After that, every size down is exactly one-half of the previous size.

The primary A-series of paper sizes starts at A0, the largest at 33.1 inches by 46.8 inches, and works its way down to A10, which is just 1 inch by 1.5 inches. But, of course, those aren’t the precise measurements, because metric is much more exacting in dimensions than imperial measurements are.

Take any A-sized paper and you’ll find that the length divided by the width is 1.4142 (or the square root of 2). This is also called the aspect ratio, and its consistency from size to size is what makes the A-series so much more convenient for print shops and graphics artists. (Conversely, U.S. paper sizes have different aspect ratios, depending on the size and type of paper.)

A4 vs A5 and Other Sizes

For Americans, it’s A4 paper that is closest to the most commonly used paper size in the U.S., letter-sized, at roughly 8.3 inches by 11.7 inches. A5, on the other hand, is seldom used by the average office worker, as it’s typically used only for creating booklets, leaflets, flyers and other promotional materials. A5 is approximately 5.8 by 8.3 inches in size, and it’s the closest in size to the U.S. paper size called the “half-letter” size, at 5.5 by 8.5 inches.

But here’s how all the A-series sizes break down:

  • A0 paper is 841 by 1189 millimeters, approximately 33.1 by 46.8 inches
  • A1 paper is 594 by 841 millimeters, approximately 23.4 by 33.1 inches
  • A2 paper is 420 by 594 millimeters, approximately 16.5 by 23.4 inches
  • A3 paper is 297 by 420 millimeters, approximately 11.7 by 16.5 inches
  • A4 paper is 210 by 296 millimeters, approximately 8.3 by 11.7 inches
  • A5 paper is 148 by 210 millimeters, approximately 5.8 by 8.3 inches
  • A6 paper is 105 by 148 millimeters, approximately 4.1 by 5.8 inches
  • A7 paper is 74 by 105 millimeters, approximately 2.9 by 4.1 inches
  • A8 paper is 52 by 74 millimeters, approximately 2.0 by 2.9 inches
  • A9 paper is 37 by 53 millimeters, approximately 1.5 by 2.0 inches
  • A10 paper is 26 by 37 millimeters, approximately 1 by 1.5 inches

ANSI Paper Sizes

America is complicated when it comes to papers, because there are the commonly used sizes you may know — half-letter, letter, legal, junior legal and tabloid/ledger — but then there are ANSI sizes, which use the letter-sized paper (8.5 by 11) as its base format. They come in sizes A through E.

  • A is 216 x 279 millimeters, or 8.5 x 11 inches, with an aspect ratio of 1:1.2941. Its closest IS0 216 paper size is A4.

  • B is 279 x 432 millimeters, or 11 x 17 inches, with an aspect ratio of 1:1.5455. Its closest IS0 216 paper size is A3.

  • C is 432 x 559 millimeters, or 17 x 22 inches, with an aspect ratio of 1:1.2941. Its closest IS0 216 paper size is A2.

  • D is 559 x 864 millimeters, or 22 x 34 inches, with an aspect ratio of 1:1.5455. Its closest IS0 216 paper size is A1.

  • E is 864 x 1118 millimeters, or 34 x 44 inches, with an aspect ratio of 1:1.2941. Its closest IS0 216 paper size is A0.

Commonly Used American Paper Sizes

While these are technically standardized, these do not have shared aspect ratios. Only the ledger/tabloid paper corresponds with anything else, and that’s because letter-sized paper is exactly half the size of tabloid paper. It’s the lack of a common aspect ratio that makes life tough for graphic designers, marketers and others who use paper dimensions daily in their work.

  • Half-Letter: Measures 140 by 216 millimeters, or 5.5 inches by 8.5 inches, with an aspect ratio of 1:1.5455, and this size does not correspond to any ANSI size, but is closest to the ISO 216 size of A5.

  • Letter: Measures 216 by 279 millimeters, or 8.5 inches by 11 inches, with an aspect ratio of 1:1.2941, the same as size A in ANSI.

  • Legal: Measures 216 by 356 millimeters, or 8.5 inches by 14 inches, with an aspect ratio of 1:16471, and this corresponds to no other size in either ISO 216 or ANSI papers.

  • Junior Legal: Measures 127 by 203 millimeters, or 5 inches by 8 inches, with an aspect ratio of 1:1.6000, and this, too, does not correspond to any other paper size with ISO 216 or ANSI.

  • Ledger/Tabloid: Measures 279 by 432 millimeters, or 11 inches by 17 inches, with an aspect ratio of 1:1.5455, the same as ANSI size B and similar to ISO 216’s size A3.

Why Should You Use ISO 216?

If you’re a business student just whipping up a report for a class, it doesn’t really matter what you use, provided you use the right paper size for how you’ve laid out your document. Letter to letter, A4 to A4. That said, there’s no shortage of international students who’ve come to the USA and panicked about why they can’t get their document to print the full text — and it’s because their document is set up to print on A4, which is 0.2 inches narrower but 0.7 inches longer — just enough to monkey with the margins and leave some text unprinted.

But if you do business internationally and are creating documents to be used in satellite offices around the world, be mindful of the fact that ISO 216 has been the standard for most nations for more than half a century, and they won’t even think about American paper sizes when they’re trying to problem-solve their document printing woes.

The consistent aspect ratio of ISO 216 is the reason you should always, always use A-series sizes when you’re doing any kind of design work for marketing purposes. Say you create a fantastic poster for an event, and now you decide you want a small card for handing out to passersby on the street. If you’ve made a tabloid/ledger-sized poster, your only option for the same aspect ratio is the letter-sized paper. But if you’d designed with an ISO 216 size in mind, like A1, then sizing down to something like an A6-sized flyer will be easily achieved.

It’s All About the Aspect

Whoever devised the ISO 216 sizes for paper did a brilliant job, because the aspect ratio being the same across all the sizes makes your life easier for all kinds of designing and printing applications. Marketing materials become infinitely more adaptable when all you need to do is change size parameters rather than having to resize elements to fit a whole new document’s different aspect ratio.

Now, you’re probably thinking this all sounds great, but what’s the point if America’s just doing business with letter- and legal- and ledger-sized papers? That’s the great thing — it’s not. Despite the proliferation of America’s standard sizes out there on the shelves, it's a global economy and nearly every major office supplies company — even stores like Walmart – will have ISO 216 sizes, like A4, as regular stock items.

If you do business overseas, it’s time to adopt ISO 216 document sizes so your foreign colleagues don’t experience printing issues — and vice versa. And for those wanting to learn more, ISO 216 also has the B and C series paper sizes — B is for envelopes, book sizes and even passports. C series are the least common, but envelopes fall in this category, and yes, the aspect ratios are still at play here, too.