The two sheets
| A4 | Letter | |
|---|---|---|
| Millimetres | 210 × 297 | 216 × 279 |
| Inches | 8.27 × 11.69 | 8.5 × 11 |
| Ratio, height to width | 1.414 (√2) | 1.294 |
| Standard | ISO 216 | ANSI/ASME Y14.1 (ANSI A) |
| Used in | Most of the world | United States, Canada, the Philippines; common in Mexico and parts of Latin America |
A4 is 6 millimetres narrower and 18 millimetres taller than Letter. The difference is small enough that either fits a standard folder and large enough that a layout designed for one leaves the wrong margins on the other.
The A series and the square root of two
The ISO 216 A series starts from a single definition: A0 has an area of one square metre, and every sheet has sides in the ratio 1 to √2. Cut a sheet in half across its long side and you get two sheets of the next size down with exactly the same proportions, which is why a page can be enlarged from A4 to A3 or reduced to A5 without changing shape. The dimensions look odd because they are rounded from that ratio, not chosen as round numbers.
| Size | Millimetres | Inches | Relation |
|---|---|---|---|
| A0 | 841 × 1189 | 33.11 × 46.81 | 1 square metre |
| A1 | 594 × 841 | 23.39 × 33.11 | Half of A0 |
| A2 | 420 × 594 | 16.54 × 23.39 | Half of A1 |
| A3 | 297 × 420 | 11.69 × 16.54 | Half of A2 |
| A4 | 210 × 297 | 8.27 × 11.69 | Half of A3 |
| A5 | 148 × 210 | 5.83 × 8.27 | Half of A4 |
| A6 | 105 × 148 | 4.13 × 5.83 | Half of A5 |
A4 is the office sheet, A3 the drawing and poster size, A5 the notebook, A6 the postcard. The B and C series from the same family sit between the A sizes; the C series is the one used for envelopes.
The North American sizes
| Size | Inches | Millimetres | Note |
|---|---|---|---|
| Letter | 8.5 × 11 | 216 × 279 | ANSI A; the office sheet |
| Legal | 8.5 × 14 | 216 × 356 | Traditional size, not in the ANSI series |
| Tabloid (Ledger) | 11 × 17 | 279 × 432 | ANSI B; two Letter sheets side by side |
Letter and Tabloid double in the same way the A series does, so a Tabloid sheet folds into two Letter pages, but the ratio changes with each step (1.294 for Letter, 1.545 for Tabloid) because the series was not built on √2. Legal is the odd one out: the same width as Letter and three inches taller, used for contracts and still a tray option on many printers.
Printing across the two
- Letter layout on A4 paper: at 100 percent the page is 6 millimetres too wide for the sheet, so about 3 millimetres are lost at each side, and 18 millimetres of A4 stay empty at the bottom.
- A4 layout on Letter paper: the width fits with 3 millimetres to spare per side, but the last 18 millimetres of the page fall off the bottom unless the printer scales.
- The fix: "fit to page" or "scale to fit" in the print dialog, which shrinks by about 6 percent in the worse direction. For documents that will be printed on both continents, set margins of at least 20 millimetres (0.8 inches) and nothing important in the bottom 20 millimetres, and the page prints at 100 percent on either sheet.
- PDFs carry the page size. A PDF made from an A4 document stays A4 wherever it is opened; the sheet it lands on is decided at print time. The JPG to PDF tool lets you pick A4 or Letter for the pages it creates, and the compress PDF tool leaves the page size untouched.
Pixels for a printed page
To fill a sheet with an image at 300 pixels per inch, A4 needs 2480 × 3508 pixels and Letter 2550 × 3300; at 150 pixels per inch, half of each. The PPI vs DPI guide explains the arithmetic, and the image resizer produces the exact pixel size for either sheet.
Frequently asked questions
What size is A4 in inches?
A4 is 210 by 297 millimetres, which is 8.27 by 11.69 inches. US Letter is 8.5 by 11 inches, 216 by 279 millimetres. So A4 is about a quarter of an inch narrower and three quarters of an inch taller than Letter.
Which countries use Letter instead of A4?
The United States, Canada and the Philippines primarily use the North American sizes, and Letter remains common in Mexico and several other countries in the Americas even where ISO sizes have been adopted officially. Almost everywhere else, A4 is the standard office sheet.
Why does A4 have such odd dimensions?
Because the A series is built from a ratio, not from round numbers. Every A sheet has sides in the ratio 1 to the square root of 2 (about 1.414), and A0 is defined as having an area of one square metre. Halving A0 gives A1 with the same proportions, and so on down; A4 is A0 folded in half four times. The ratio is what lets a document scale from A4 to A3 without changing shape.
Will a Letter document print on A4?
Yes, with margins to spare at the sides and a little lost at the bottom unless the printer scales it. Letter is wider and shorter than A4, so a Letter layout printed on A4 at 100 percent leaves a 3 millimetre gap on each side and runs 18 millimetres short at the bottom; the reverse direction clips 3 millimetres per side. Most print dialogs offer "fit to page" to avoid both.
What are the ANSI sizes?
The American National Standards Institute adopted ANSI/ASME Y14.1 in 1996, which names Letter as ANSI A and Tabloid as ANSI B, with larger sizes above them. Legal is not in that series; it is a traditional size of 8.5 by 14 inches.