The Definitive Guide to Paper Sizes Around the World

Updated on January 8, 2026

Why Paper Size Is Not a Trivial Question

Every few months, someone in a print shop or a corporate office discovers the hard way that the document they carefully formatted in one country looks catastrophically wrong when printed in another. Margins collapse. Text gets cropped. PDFs that looked fine on screen suddenly don't fit the paper in the tray. The root cause, almost always, is the silent incompatibility between two fundamentally different philosophies of standardizing paper: the ISO metric system used by most of the world, and the inch-based ANSI/legacy system used predominantly in the United States and Canada.

Understanding how these systems work — and, crucially, why they work the way they do — will save you time, money, and a fair amount of frustration.

The ISO 216 Standard: Built on a Mathematical Insight

The ISO A series is not arbitrary. It is built on a single elegant constraint: every sheet in the series must have an aspect ratio of 1:√2, approximately 1:1.4142. This seemingly abstract requirement has a profoundly practical consequence — when you fold a sheet in half along its longer axis, the resulting half-sheet has exactly the same aspect ratio as the original. This property is called self-similarity upon halving, and it is the entire reason the ISO system is so powerful for document workflows.

The series starts at A0, which is defined as having an area of exactly one square meter, with dimensions 841 × 1189 mm. Every subsequent size is derived by cutting the previous one in half across its longer dimension:

  • A0: 841 × 1189 mm
  • A1: 594 × 841 mm
  • A2: 420 × 594 mm
  • A3: 297 × 420 mm
  • A4: 210 × 297 mm — the global standard for correspondence and office documents
  • A5: 148 × 210 mm — common for booklets and notepads
  • A6: 105 × 148 mm — postcard size

The rounding at each step is done to the nearest millimeter, which introduces tiny cumulative errors, but these are negligible in practice. What matters is that the system is coherent: scaling an A4 document to A3 always produces a clean 141% enlargement. There is no guesswork.

The ISO B Series: Filling the Gaps

The B series exists to provide intermediate sizes between A series sheets. Where the A series has A4 (210 × 297 mm) and A3 (297 × 420 mm), the B series inserts B4 at 250 × 353 mm. Each B size is defined as the geometric mean between the A size of the same number and the A size one step larger.

B series paper sees heavy use in specific applications. B5 (176 × 250 mm) is extremely common for book publishing in Japan, Scandinavia, and parts of Europe — it sits neatly between A4 and A5, making it a comfortable reading size. B4 is widely used in Japanese newspapers and for certain spreadsheet-heavy business documents. B0 and B1 appear in poster printing, where the slightly larger dimensions relative to A-series allow for bleeds and borders without overprinting onto A0.

There is also a C series (C4, C5, C6 envelopes), which uses the same geometric logic and is specifically sized so that A-series documents fit inside C-series envelopes without folding or with a single fold. An A4 sheet folded once fits a C5 envelope. Folded twice, it fits a C6. This is not coincidence — it is the system working exactly as designed.

US Letter and Legal: A Different Tradition Entirely

The United States never adopted ISO 216, and the reasons are partly historical inertia and partly the fact that the US had already standardized around different dimensions before the metric system gained international traction.

US Letter (8.5 × 11 inches, or 215.9 × 279.4 mm) has been the de facto American standard since at least the early twentieth century. It is close to A4 but not equivalent — A4 is narrower and taller (210 × 297 mm). The difference seems small: A4 is 6mm narrower and 18mm taller. But that 18mm height difference is enough to cause layout problems when documents are shared internationally without adjustment.

US Legal (8.5 × 14 inches, or 215.9 × 355.6 mm) is used primarily for legal contracts, court filings, and certain government documents. It has no ISO equivalent of equivalent dimensions.

The American National Standards Institute also defines larger formats:

  • Tabloid / Ledger (ANSI B): 11 × 17 inches — used for spreads, blueprints, and newspapers
  • ANSI C: 17 × 22 inches
  • ANSI D: 22 × 34 inches
  • ANSI E: 34 × 44 inches — large-format engineering drawings

Here is where the US system reveals its weakness: doubling a Letter sheet gives you Tabloid, and doubling Tabloid gives you ANSI C, but the aspect ratio shifts with each step. US Letter is approximately 1:1.294, while Tabloid is 1:1.545. There is no consistent geometric relationship. Scaling a document from one ANSI size to another requires manual adjustment or produces distorted output. The ISO system does not have this problem.

The Aspect Ratio Math in Plain Terms

To understand why 1:√2 is the "correct" aspect ratio for a halving system, consider this: if you have a rectangle with width w and height h, and you want cutting it in half (along the long axis) to produce a rectangle with the same proportions, then the half-rectangle has dimensions h/2 × w... wait, that's not right. Let's be precise.

Start with a rectangle where the long side is h and the short side is w. Fold it in half along the long dimension. The new rectangle has short side h/2 and long side w. For self-similarity: w / (h/2) = h / w. This gives w² = h²/2, or h/w = √2. That is the proof. The ratio 1:√2 is the unique ratio that preserves proportions when halved. Any other ratio — including 1:1.294 (Letter) — breaks this property.

This is not just mathematical elegance. It is practical: print shops can halve or double jobs without re-imposing content. Photocopiers can scale between A sizes using simple preset percentages (A4 to A3 = 141%, A3 to A4 = 71%). Software that supports ISO paper sizes can automate these transforms without layout rework.

Which System Should You Use, and When?

The honest answer depends on your audience and your workflow, not on which system is theoretically superior.

  1. If your documents will be printed in the US or Canada: Use Letter (8.5 × 11 in) for standard correspondence and Legal for contracts if your legal team requires it. Setting your document to A4 and sending it to an American printer will result in either cropped content or unwanted white margins, depending on how the print driver handles the mismatch.
  2. If your documents will be printed anywhere else in the world: Use A4 as your default. It is the international standard recognized by ISO, adopted by over 160 countries, and expected by virtually every print service outside North America.
  3. For international documents — reports, whitepapers, technical manuals that may be printed in both regions: Design in A4. A4-formatted content, when printed on US Letter with "fit to page" enabled, loses only a small amount of margin. The reverse — Letter content on A4 — often clips the bottom of pages because Letter is shorter than A4 and content frequently runs to the bottom margin. A4 is the safer default for mixed-region distribution.
  4. For books and publishing: Neither A4 nor Letter is typically used for finished books. Trade paperbacks in the US commonly use 6 × 9 inches. European publishing often uses B5 or custom trim sizes. Know your printer's requirements before you set your page size.
  5. For posters and large-format output: Use the appropriate A or B series size in ISO-standard regions (A1, A0, B1, B0). In the US, large-format print shops typically work in inches and use sizes like 24 × 36 or 18 × 24 inches, which have no clean ISO equivalent.

Practical Conversion Reference

When you need to convert between systems, these are the numbers that actually matter:

  • A4 in inches: 8.27 × 11.69 in (narrower and taller than Letter)
  • US Letter in mm: 215.9 × 279.4 mm (wider and shorter than A4)
  • A3 in inches: 11.69 × 16.54 in (taller but narrower than Tabloid/11×17)
  • Tabloid in mm: 279.4 × 431.8 mm (wider and taller than A3)

When converting a PDF from A4 to Letter in software like Adobe Acrobat or LibreOffice, always use "shrink to printable area" rather than cropping, unless you can verify that all content sits within the US Letter printable region. Cropping an A4 document to Letter without checking first is how footers disappear.

The Takeaway

The ISO 216 system is mathematically coherent, internationally adopted, and designed for real-world workflows involving scaling, folding, and filing. The US system is entrenched in one of the world's largest economies and is not going away. Both coexist, and the professional approach is to understand them well enough to choose the right one for each project — and to convert between them without losing content in the process.

Paper size is one of those details that seems trivial until it costs you a reprint. It rarely needs to.