Paper sizes are defined by two main standards: ISO 216, which governs the A and B series used in most of the world, and the North American ANSI system, which defines Letter, Legal, and Tabloid. The international A series starts at A0, a sheet with an area of almost exactly one square metre measuring 841 by 1189 millimetres, and every smaller size is created by halving the previous one across its longer side. Every sheet in the A series keeps the same height-to-width ratio, the square root of two (about 1.414), which is why A4 (210 by 297 mm) folds cleanly into two A5 sheets, each still proportional to the original. The B series sits between A sizes and is used for posters and oversized envelopes, while US Letter (8.5 by 11 inches or 215.9 by 279.4 mm) and Tabloid (11 by 17 inches) are defined in inches rather than round metric numbers. You can read every standard size side by side in millimetres, centimetres, and inches, and convert any of them to pixels at any DPI, using the free Paper Sizes Chart.
How the ISO 216 A and B Series Are Built
The international standard for paper, ISO 216, is built on a single geometric trick that makes the entire system behave predictably. Every sheet, from A0 down to A10, has the same height-to-width ratio: the square root of two, roughly 1.414. Halve any sheet across its longer side and you get two sheets of the next smaller size, each still keeping that exact proportion.
The series is anchored by A0, which has an area of almost exactly one square metre and measures 841 by 1189 millimetres. Cutting A0 in half along its longer side gives A1 (594 by 841 mm). Cut A1 in half and you get A2 (420 by 594 mm), then A3 (297 by 420 mm), then the familiar A4 (210 by 297 mm). The same halving continues all the way down to A10 (26 by 37 mm), which is roughly the size of a postage stamp. Because the proportions are preserved, an image or layout designed for A4 can be scaled to A3 or A5 without distortion.
| A Series | Millimetres (W × H) | Centimetres (W × H) | Inches (W × H) |
|---|---|---|---|
| A0 | 841 × 1189 | 84.1 × 118.9 | 33.1 × 46.8 |
| A1 | 594 × 841 | 59.4 × 84.1 | 23.4 × 33.1 |
| A2 | 420 × 594 | 42.0 × 59.4 | 16.5 × 23.4 |
| A3 | 297 × 420 | 29.7 × 42.0 | 11.7 × 16.5 |
| A4 | 210 × 297 | 21.0 × 29.7 | 8.3 × 11.7 |
| A5 | 148 × 210 | 14.8 × 21.0 | 5.8 × 8.3 |
| A6 | 105 × 148 | 10.5 × 14.8 | 4.1 × 5.8 |
The B series is defined the same way, but it is sized between consecutive A sizes. B0 measures 1000 by 1414 millimetres, B1 is 707 by 1000 mm, B2 is 500 by 707 mm, and so on. Because each B size is the geometric mean of two adjacent A sizes, B-series paper is useful whenever a sheet needs to sit comfortably between two A dimensions. Posters, books, and many envelope formats are based on the B series for that reason.
North American Sizes: Letter, Legal, and Tabloid
Outside the ISO 216 world, North America uses its own conventions for everyday paper. The American National Standards Institute defines three sizes that show up in most offices, printers, and copy shops: US Letter, US Legal, and Tabloid, also called Ledger. These sizes are defined in inches rather than millimetres, so their metric values are exact conversions rather than round numbers.
US Letter is 8.5 by 11 inches, which equals 215.9 by 279.4 millimetres. It is the default paper for most printers, letters, schoolwork, and government forms in the United States and Canada. US Legal is the same width as Letter (8.5 inches) but longer at 14 inches, giving 215.9 by 355.6 millimetres, and is used for legal documents and contracts. Tabloid doubles Letter in the short dimension: 11 by 17 inches, or 279.4 by 431.8 millimetres. It is the standard for newspapers, large spreadsheets, and small posters, and serves the same role as A3 in spirit, even though the actual dimensions differ.
A4 vs US Letter: Why They Are Close but Never Identical
A4 and US Letter are often mentioned in the same breath because they look interchangeable at a glance, but they are defined by different standards with slightly different dimensions. A4 comes from ISO 216 and measures 210 by 297 millimetres. US Letter comes from ANSI and measures 8.5 by 11 inches, or 215.9 by 279.4 millimetres. The difference is small but real.
| Format | Origin | Millimetres (W × H) | Inches (W × H) | Shape |
|---|---|---|---|---|
| A4 | ISO 216 | 210 × 297 | 8.27 × 11.69 | Slightly narrower, slightly taller |
| US Letter | ANSI | 215.9 × 279.4 | 8.5 × 11 | Slightly wider, slightly shorter |
| US Legal | ANSI | 215.9 × 355.6 | 8.5 × 14 | Same width as Letter, longer |
| Tabloid | ANSI | 279.4 × 431.8 | 11 × 17 | Letter flipped and doubled, larger |
Because A4 is taller and narrower than Letter, a document formatted for one standard will shift its margins or clip content when printed on the other. If you are designing artwork that may be printed on either side of the Atlantic, it is worth checking the target format before final export. For a closer look at the exact A4 numbers in centimetres, see the dedicated A4 size in cm guide.
Look Up Any Paper Size with the Chart
If you need the exact dimensions of any standard sheet and a quick way to convert that sheet to pixels at your target DPI, the free Paper Sizes Chart puts every format in one place. It runs entirely in your browser, with no uploads and no sign-in, so it is safe to use with sensitive print specifications.
- Scan the reference table to find your paper size in the A series, B series, or US group, with dimensions shown in millimetres, centimetres, and inches.
- In the converter, choose a paper size from the dropdown to load its exact dimensions.
- Type the resolution you need in the DPI field (300 is standard for print, 72 for screen) to see the pixel width and height instantly.
- Read off the pixel result alongside the millimetre, centimetre, and inch values to set up your canvas or export.
This four-step flow is the same one designers, print buyers, students, and developers use to set up documents, prepare files for cutting or binding, and sanity-check export dimensions before sending anything to a printer.
From Millimetres to Pixels: How the DPI Conversion Works
Millimetres, centimetres, and inches are all physical units, so converting between them only requires one constant: one inch equals exactly 25.4 millimetres. From there, millimetres to inches is a simple division by 25.4, and millimetres to centimetres is a division by ten. The chart shows all three side by side so you can copy whichever unit your workflow expects.
Pixels are different. A physical sheet has no fixed pixel count until you choose a resolution. DPI, dots per inch, sometimes written as PPI for pixels per inch, bridges the physical and digital worlds. The pixel width of a sheet equals its width in inches multiplied by the DPI, rounded to a whole pixel.
Worked example, A4 width at 300 DPI. A4 is 210 mm wide. Divide by 25.4 to convert to inches: 210 / 25.4 = 8.2677 inches. Multiply by the DPI: 8.2677 × 300 = 2480.31 pixels. Round to the nearest whole pixel: 2480 pixels. The height works the same way, since 297 mm / 25.4 × 300 = 3508 pixels, giving the standard A4 print size of 2480 by 3508 pixels at 300 DPI. The same A4 sheet at 72 DPI, a classic screen value, is only 595 by 842 pixels.
That difference is why designers set canvas sizes in pixels but think in paper sizes, and why a print shop will ask for 300 DPI artwork when they want crisp output. The chart lets you switch DPI values without re-measuring the sheet, so you can prepare both print-ready and screen-ready exports from the same reference.