Pick any paper size and get its exact dimensions in millimeters, centimeters, and inches, plus how it relates to the sizes around it. The A series is built on one elegant trick: every size is the previous one folded in half, and this page shows the whole ladder.
The A series is the rare piece of everyday design that is actually a piece of mathematics. One ratio, one anchor, and the whole system unfolds: every size is the last one folded in half, nothing is wasted at the print shop, and the weight of any sheet can be computed in your head. The US sizes are older and stubbornly unrelated, so this page speaks both languages and translates between them.
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How the A series works
Every A sheet has sides in the ratio 1 to √2 (about 1 to 1.414). That ratio has a unique property: cut the sheet in half across its long side and both halves keep the same ratio. So the series is just repeated folding. A0 is defined as one square meter of area with that shape, and each step down halves the area: A1 is half a square meter, A4 is a sixteenth. The number in the name counts the folds from A0.
The formula
aspect ratio = 1 : √2
A0 area = 1 m², and each step halves the area
A(n+1) = A(n) folded across the long side
Dimensions are defined in millimeters, rounded down at each halving, which is why A4 is 210 x 297 rather than 210.2 x 297.3. Inches here are the millimeter values divided by 25.4.
Worked example
Take A5. It is A4 (210 x 297 mm) folded in half across the long side: the 297 mm side halves to 148 (rounded down from 148.5), and the 210 mm side carries over as the new long side. So A5 is 148 x 210 mm, which is 5.83 x 8.27 inches. Its area is 1/32 of a square meter, so 32 A5 sheets cut from one A0, and a sheet of 80 gsm A5 paper weighs 80 ÷ 32 = 2.5 grams.
The full chart
| Size | Millimeters | Inches | Common use |
|---|
| A0 | 841 x 1189 | 33.11 x 46.81 | Technical drawings, posters |
| A1 | 594 x 841 | 23.39 x 33.11 | Flip charts, posters |
| A2 | 420 x 594 | 16.54 x 23.39 | Art prints, small posters |
| A3 | 297 x 420 | 11.69 x 16.54 | Spreadsheets, diagrams |
| A4 | 210 x 297 | 8.27 x 11.69 | Documents (worldwide standard) |
| A5 | 148 x 210 | 5.83 x 8.27 | Books, flyers, notepads |
| A6 | 105 x 148 | 4.13 x 5.83 | Postcards |
| A7 | 74 x 105 | 2.91 x 4.13 | Sticky notes |
| A8 | 52 x 74 | 2.05 x 2.91 | Business cards (near) |
| A9 | 37 x 52 | 1.46 x 2.05 | Tickets, labels |
| A10 | 26 x 37 | 1.02 x 1.46 | Stamps, tiny labels |
| US Letter | 216 x 279 | 8.5 x 11 | Documents (US standard) |
| US Legal | 216 x 356 | 8.5 x 14 | Contracts |
| US Tabloid | 279 x 432 | 11 x 17 | Two Letters side by side |
The gsm trick
Paper weight is quoted in grams per square meter, and A0 is one square meter, so the weight of any A sheet is the gsm divided by two to the power of the size number. An A4 sheet of standard 80 gsm office paper: 80 ÷ 16 = 5 grams. This is genuinely useful at the post office. A letter with four A4 sheets (20 g) plus a 5 g envelope sits at 25 g, comfortably under the common 50 g postage step, and you knew it without a scale. US paper, sized in inches and sold by the confusing "pound" system, cannot do this trick, which is a small daily tax on an entire continent.
Frequently asked questions
What size is A4 paper?
A4 measures 210 by 297 millimeters, which is 21 by 29.7 centimeters or 8.27 by 11.69 inches. It is the standard document size everywhere except the United States, Canada, and a few other countries, which use US Letter instead. Letter is slightly wider and shorter: 8.5 by 11 inches.
What is the difference between A4 and US Letter?
A4 is 210 by 297 mm; Letter is 216 by 279 mm. So Letter is 6 mm wider and 18 mm shorter. The practical consequence is that documents formatted for one can clip or reflow on the other, which is why PDF exports ask. If you design for both, keep content inside the overlap: 210 mm wide, 279 mm tall.
How does the A series work?
Fold any A size in half across its long side and you get the next size exactly: A4 folded is A5, and two A4s side by side make an A3. This works because every sheet has the same width-to-height ratio, the square root of 2 (about 1:1.414), which is the only ratio that survives halving. The series starts from A0, defined as one square meter of area.
What paper size is used for books and flyers?
A5 (148 by 210 mm, about 5.8 by 8.3 inches) is the workhorse: trade paperbacks, flyers, and notepads. A6 (105 by 148 mm) is the standard postcard. A3 doubles A4 for posters and spreadsheets, and A2 through A0 are drawing and poster sizes. If you know A4, you can derive every one of them by doubling or halving.
What are US Legal and Tabloid sizes?
Legal is 8.5 by 14 inches (216 by 356 mm), the same width as Letter but 3 inches longer, historically used for contracts. Tabloid, also called Ledger, is 11 by 17 inches (279 by 432 mm), which is exactly two Letter sheets side by side, the US counterpart to the A3-from-two-A4s trick.
Why is A0 one square meter?
It makes paper weight arithmetic trivial. Paper is sold by grams per square meter (gsm), so a full A0 sheet of 80 gsm paper weighs exactly 80 grams, an A1 weighs 40, and an A4 weighs 5. Sixteen A4 sheets cut from one A0 sheet, 16 times 5 is 80, and the books balance. The US system offers no such kindness.