Reference summary: Two paper-size systems are in daily use worldwide. ISO 216 (the A-series, whose best-known member is A4 at 210 × 297 mm) is the national standard in almost every country on Earth. ANSI/US Letter (8.5 × 11 in / 216 × 279 mm) is the office default in the United States, Canada, and — in mixed use — parts of Latin America and the Philippines. This page documents both systems, the geometry behind ISO 216, and a country-by-country reference table.
Why ISO 216 exists: the √2 aspect ratio
Every sheet in the ISO A-series has the same width-to-height ratio: 1 : √2 (approximately 1 : 1.4142). That single design decision produces the property the system is famous for — cut an A-series sheet in half across its longer side, and each half has exactly the same proportions as the original. A4 halved is A5; A5 halved is A6, and so on, with no leftover margin and no distortion.
The mathematics is straightforward. If a sheet has width w and height h, halving it gives a sheet of width h/2 and height w. For the proportions to be preserved, h/w must equal w/(h/2), which solves to h/w = √2. As Markus Kuhn’s international standard paper sizes reference (University of Cambridge) puts it, width and height relate to each other “like the side and the diagonal of a square.”
The series is anchored by area rather than by an arbitrary dimension: A0 is defined as the sheet of 1 : √2 proportions with an area of one square metre. Each subsequent size halves that area. This is why scaling a document between A-series sizes is lossless — enlarging A4 to A3 is exactly a 141.4% scale, and reducing A3 to A4 is exactly 70.7%, values printed on the control panel of essentially every photocopier sold outside North America.
ISO 216 A-series dimensions
Dimensions in millimetres are the normative values in ISO 216. Inch equivalents below are converted (1 in = 25.4 mm) and rounded to two decimal places — they are not part of the standard.
| Format | Width × Height (mm) | Width × Height (in, converted) | Common use |
|---|---|---|---|
| A0 | 841 × 1189 | 33.11 × 46.81 | Technical drawings, posters (area = 1 m²) |
| A1 | 594 × 841 | 23.39 × 33.11 | Flip charts, large posters |
| A2 | 420 × 594 | 16.54 × 23.39 | Wall art, drawings |
| A3 | 297 × 420 | 11.69 × 16.54 | Spreads, drawings, small posters |
| A4 | 210 × 297 | 8.27 × 11.69 | Standard office and home printing |
| A5 | 148 × 210 | 5.83 × 8.27 | Notepads, planner inserts, booklets |
| A6 | 105 × 148 | 4.13 × 5.83 | Postcards, index cards, pocket planners |
| A7 | 74 × 105 | 2.91 × 4.13 | Small cards, tickets |
Two companion series exist. The B-series sits geometrically between adjacent A sizes (B4 = 250 × 353 mm, B5 = 176 × 250 mm) and is used for books, posters and passports. The C-series is sized for envelopes: a C4 envelope (229 × 324 mm) holds an unfolded A4 sheet, C5 (162 × 229 mm) holds A4 folded once, and C6 (114 × 162 mm) holds A4 folded twice. The relationship is deliberate — C is the geometric mean of the A and B formats of the same number.
North American sizes (ANSI / US Letter)
North American sizes are defined in inches and standardised through ANSI/ASME Y14.1. They do not share a single aspect ratio: Letter is 1 : 1.294, while Tabloid is 1 : 1.545. Scaling between them therefore always leaves unused margin, which is why a document laid out for A4 never lands perfectly on Letter and vice versa.
| Name | Inches | Millimetres | Aspect ratio | Nearest ISO size |
|---|---|---|---|---|
| Letter (ANSI A) | 8.5 × 11 | 216 × 279 | 1 : 1.294 | A4 (210 × 297) |
| Legal | 8.5 × 14 | 216 × 356 | 1 : 1.647 | — (no close match) |
| Executive | 7.25 × 10.5* | 190 × 254 | 1 : 1.336 | Between A5 and A4 |
| Tabloid / Ledger (ANSI B) | 11 × 17 | 279 × 432 | 1 : 1.545 | A3 (297 × 420) |
The practical consequence for anyone distributing printable documents internationally: A4 is 6 mm narrower and 18 mm taller than Letter. Content designed to the full width of Letter will be clipped on A4; content designed to the full height of A4 will be clipped on Letter. The safe approach is a shared “intersection” live area of roughly 210 mm × 279 mm minus your printer’s non-printable margin — which is why well-made printable products ship in both formats rather than one.
Country reference table: which default size is in use
How to read this table. ISO 216 has been adopted as a national standard in the overwhelming majority of countries; the documented exceptions are concentrated in North America, with mixed usage reported in parts of Latin America and the Philippines. Where a country is marked “mixed”, both formats circulate in practice and you should confirm locally rather than assume. This table records the everyday office default, which is not always identical to the formally adopted national standard.
| Country / region | Everyday office default | System |
|---|---|---|
| United States | Letter 8.5 × 11 in | ANSI |
| Canada | Letter 8.5 × 11 in | ANSI (CAN/CGSB mirrors Letter sizes) |
| Mexico | Carta (Letter) — A4 also encountered | Mixed |
| Philippines | Letter / “long bond” (8.5 × 13 in) | Mixed |
| Chile | Carta (Letter) widely used | Mixed |
| Colombia | Carta (Letter) widely used | Mixed |
| United Kingdom | A4 | ISO 216 (BS EN ISO 216) |
| Ireland | A4 | ISO 216 |
| Germany | A4 | ISO 216 (originating DIN 476) |
| France | A4 | ISO 216 |
| Spain | A4 | ISO 216 |
| Italy | A4 | ISO 216 |
| Netherlands | A4 | ISO 216 |
| Belgium | A4 | ISO 216 |
| Austria | A4 | ISO 216 |
| Switzerland | A4 | ISO 216 |
| Poland | A4 | ISO 216 |
| Czechia | A4 | ISO 216 |
| Sweden | A4 | ISO 216 |
| Norway | A4 | ISO 216 |
| Denmark | A4 | ISO 216 |
| Finland | A4 | ISO 216 |
| Portugal | A4 | ISO 216 |
| Greece | A4 | ISO 216 |
| Romania | A4 | ISO 216 |
| Hungary | A4 | ISO 216 |
| Bulgaria | A4 | ISO 216 |
| Croatia | A4 | ISO 216 |
| Türkiye | A4 | ISO 216 |
| Russia | A4 | ISO 216 (GOST equivalents) |
| Ukraine | A4 | ISO 216 |
| South Africa | A4 | ISO 216 |
| Nigeria | A4 | ISO 216 |
| Kenya | A4 | ISO 216 |
| Egypt | A4 | ISO 216 |
| United Arab Emirates | A4 | ISO 216 |
| Saudi Arabia | A4 | ISO 216 |
| India | A4 | ISO 216 (IS 1064) |
| Pakistan | A4 | ISO 216 |
| China | A4 | ISO 216 (GB/T equivalents) |
| Japan | A4 (plus JIS B-series) | ISO 216 + JIS B |
| South Korea | A4 | ISO 216 |
| Singapore | A4 | ISO 216 |
| Malaysia | A4 | ISO 216 |
| Indonesia | A4 (F4/”folio” 210 × 330 mm also common) | ISO 216 + local folio |
| Australia | A4 | ISO 216 (AS 1441) |
| New Zealand | A4 | ISO 216 |
| Brazil | A4 | ISO 216 (ABNT NBR) |
| Argentina | A4 | ISO 216 |
One important nuance: Japan uses a JIS B-series that differs from the ISO B-series. JIS B4 is 257 × 364 mm, whereas ISO B4 is 250 × 353 mm. If you are producing for a Japanese audience and your file is labelled “B5”, state explicitly whether you mean JIS B5 (182 × 257 mm) or ISO B5 (176 × 250 mm).
Practical implications for printable documents
- Ship both formats. A single “US Letter or A4” file that fits neither cleanly is the most common complaint on printable products. Two exports cost you one extra render.
- Design to the intersection. Keep all critical content inside roughly 195 mm × 265 mm of live area and both formats will print without clipping, allowing for typical non-printable printer margins.
- Never rely on “fit to page”. Scaling A4 artwork onto Letter at 94% shrinks every measurement — fatal for anything with a stated size, such as bullet-journal grids, budget envelope templates, or planner discs.
- State dimensions in millimetres and inches. Millimetres are normative for ISO sizes; inches are what a US buyer will check against their printer driver.
If you are producing planner inserts specifically, size selection also has to survive punching and binding. A5 (148 × 210 mm) and Half Letter (5.5 × 8.5 in / 140 × 216 mm) are close but not interchangeable — a 8 mm width difference is enough to misalign a six-ring punch pattern.
Frequently asked questions
Is A4 bigger or smaller than US Letter?
Neither, consistently. A4 (210 × 297 mm) is about 6 mm narrower and about 18 mm taller than Letter (216 × 279 mm). A4 has slightly more total area — roughly 623 cm² against about 603 cm² — but it is narrower, so wide layouts clip on A4 while tall layouts clip on Letter.
What scale factor converts A4 artwork to Letter?
There is no single correct factor, because the aspect ratios differ. Scaling to fit the width requires about 102.9% (216/210); scaling to fit the height requires about 93.9% (279/297). Most print drivers pick the smaller factor, so an A4 file printed on Letter is typically reduced to about 94% of its intended size.
Which countries do not use A4?
The clearly documented non-adopters for everyday office use are the United States and Canada. Mexico, the Philippines, Chile and Colombia are reported as mixed-use, where Letter or a local long format circulates alongside A4. Everywhere else, A4 is the standard office sheet.
Why is A0 exactly one square metre?
Because the series is defined by area, not by a base dimension. Fixing A0 at 1 m² with a 1 : √2 ratio produces 841 × 1189 mm and makes paper weight arithmetic trivial: a sheet of 80 gsm A4 weighs 80 g ÷ 16 = 5 g, since sixteen A4 sheets tile one A0.
Does ISO 216 also cover envelopes?
Envelope formats are the C-series, sized so that a Cn envelope accepts an An sheet. Standard business correspondence uses DL (110 × 220 mm) for a tri-folded A4 sheet, and C5 for a once-folded A4 sheet.
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Pixquo. "Paper Size Guide by Country: A4 vs US Letter Reference Data."
Pixquo, 6 August 2026. https://pixquo.com/paper-size-guide-by-country/Permanent URL: https://pixquo.com/paper-size-guide-by-country/
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