The A series paper sizes run from A0 (the largest, at 841 x 1189mm) down to A10 (the smallest, at 26 x 37mm), with each size being exactly half the area of the one before it. A4 (210 x 297mm) is the standard size for letters, printing and everyday documents worldwide, except in the US and Canada, which use Letter and Legal instead.
Here's the full breakdown most guides won't give you in one place, along with the practical stuff, like why your office printer sometimes rejects a "correct" A4 file, and why A3 isn't just "double A4" the way people assume.
Full A0 to A10 Size Chart (mm, cm, inches)
| Size | Millimeters | Centimeters | Inches |
|---|---|---|---|
| A0 | 841 x 1189 mm | 84.1 x 118.9 cm | 33.1 x 46.8 in |
| A1 | 594 x 841 mm | 59.4 x 84.1 cm | 23.4 x 33.1 in |
| A2 | 420 x 594 mm | 42.0 x 59.4 cm | 16.5 x 23.4 in |
| A3 | 297 x 420 mm | 29.7 x 42.0 cm | 11.7 x 16.5 in |
| A4 | 210 x 297 mm | 21.0 x 29.7 cm | 8.3 x 11.7 in |
| A5 | 148 x 210 mm | 14.8 x 21.0 cm | 5.8 x 8.3 in |
| A6 | 105 x 148 mm | 10.5 x 14.8 cm | 4.1 x 5.8 in |
| A7 | 74 x 105 mm | 7.4 x 10.5 cm | 2.9 x 4.1 in |
| A8 | 52 x 74 mm | 5.2 x 7.4 cm | 2.0 x 2.9 in |
| A9 | 37 x 52 mm | 3.7 x 5.2 cm | 1.5 x 2.0 in |
| A10 | 26 x 37 mm | 2.6 x 3.7 cm | 1.0 x 1.5 in |
Every dimension is rounded to the nearest millimeter, which is standard practice under ISO 216. That rounding matters more than most articles admit, and I'll get into why below.
The Math Behind the Sizes (And Why It Actually Matters)
Most sites tell you "each size is half the previous one" and stop there. What they skip is why that specific ratio was chosen, and what it means for you practically.
The A series uses an aspect ratio of 1:√2 (roughly 1:1.414). This isn't arbitrary. It's the only ratio where cutting a sheet in half, parallel to its shorter side, produces two smaller sheets with the exact same proportions as the original.
That's why an A3 sheet folded in half becomes two perfect A4 sheets, and why a photocopier set to "reduce" from A3 to A4 uses 71% scaling, not 50%. People get tripped up by this constantly: halving the paper size does NOT mean halving the dimensions. It means halving the area. Each dimension only shrinks by a factor of 0.707.
Practical fallout from this:
- Enlarging A4 to A3 on a copier requires 141% scaling, not 200%.
- If you design a poster at A2 and try to "double it" to A0 by eyeballing the dimensions, you'll distort the proportions unless you scale by exactly 200% in area terms (which means roughly 141% per linear dimension, twice).
- Design software that lets you type in exact percentages avoids this problem entirely. Eyeballing it in a physical print shop does not.
Where Each Size Actually Gets Used in the Real World
Charts elsewhere just label these "poster" or "notepad" without explaining the decision-making. Here's what I've actually seen each size used for on the ground:
- A0: Architectural drawings, construction blueprints, large trade-show graphics. Rarely printed at home; most desktop printers can't handle it, so it goes to a plotter or a print shop.
- A1: Flip chart pages, technical drawings, large presentation boards. A1 is the largest size some wide-format home printers can still manage.
- A2: Museum and gallery posters, calendars, some technical schematics. It's a size that looks impressive without needing the space A0 or A1 demands.
- A3: Tabloid newspapers in many countries, art portfolios, spreadsheets with wide columns, children's activity books. Most small offices own at least one A3-capable printer for exactly these jobs.
- A4: Letters, invoices, resumes, reports, standard printer paper nearly everywhere outside North America.
- A5: Notepads, greeting cards, flyers, half-size manuals.
- A6: Postcards, index cards, small flyers.
- A7: Small notepads, some voucher and coupon printing.
- A8, A9, A10: These almost never get printed as standalone sheets. You'll mostly encounter them as reference sizes in packaging design (tiny product tags, small labels) or as the theoretical bottom of the scale in printing spec sheets. A10 is roughly the size of a large postage stamp.
Paper Weight (GSM) Changes How a Size Feels, and That Trips People Up
This is the part almost nobody covers when writing about A-series sizes, and it causes real ordering mistakes. The size (A4, A3, etc.) tells you the sheet's dimensions. It tells you nothing about thickness.
Paper weight is measured in GSM (grams per square meter). Standard office paper sits around 80gsm. Cardstock starts around 200gsit and up. Here's the catch: because A0 has a much larger surface area than A4, an A0 sheet at 80gsm weighs dramatically more in total, even though the "gsm" number is identical.
I've seen people order "80gsm A0 posters" expecting something light and flimsy like standard printer paper, then get surprised when a single sheet arrives weighing several hundred grams and feeling stiff simply because of its size. If you're ordering large-format prints, ask about total sheet weight, not just gsm, if handling and shipping matter to you.
The A Series Has a History Most Charts Skip Entirely
The 1:√2 ratio wasn't invented for modern printing. A German physicist named Georg Christoph Lichtenberg described the mathematical advantage of this ratio in a letter back in 1786, over a century before any standard existed. It sat unused as a curiosity for decades.
The actual standard, DIN 476, was formalized in Germany in 1922. It later became the basis for ISO 216, the international standard most of the world still uses today. The US never fully adopted it, which is exactly why American Letter (8.5 x 11in) doesn't share the same proportions as A4, despite being a similar size on paper (literally).
That mismatch is the actual root cause of the most common cross-border printing headache: a Word document formatted for Letter size will not center or margin correctly when printed on A4 paper, and vice versa. It's not a bug. It's two incompatible standards colliding.
Common Mistakes People Make With A-Series Paper
- Assuming "half size" means half the length or width. It means half the area. Confusing the two leads to distorted scaling in Photoshop, Illustrator, and print shop software.
- Ordering A4 paper for a US printer without checking margin settings. Software defaults to whatever region your OS is set to, and it silently clips or shrinks pages if the physical loaded paper doesn't match.
- Assuming all "A3 printers" print true A3. Some consumer printers marketed for A3 only handle A3+ (329 x 483mm), a non-ISO size made specifically for borderless printing with bleed margins. If you need exact A3 dimensions for binding or framing, check the spec sheet, not just the marketing copy.
- Mixing up B-series and C-series sizes with A-series. B-series (used for books and passports) and C-series (used for envelopes designed to fit A-series sheets) follow similar logic but different base dimensions. An A4 sheet fits inside a C4 envelope with room to spare, which is intentional.
Quick Reference: Choosing a Size by Task
| Task | Best Size | Why |
|---|---|---|
| Everyday printing, letters, forms | A4 | Universal standard outside North America |
| Notebooks, journals, cards | A5 or A6 | Portable, still legible |
| Posters, presentation boards | A1 or A2 | Visible from a distance, manageable to transport |
| Architectural or engineering drawings | A0 | Enough surface area for detail at scale |
| Spreadsheets, wide tables, art prints | A3 | Extra width without going oversized |
Frequently Asked Question
Is A4 the same as Letter size?
No. A4 is 210 x 297mm while US Letter is 216 x 279mm. They're close but not interchangeable, and documents formatted for one will misalign when printed on the other.What size is A3 compared to A4?
A3 is exactly double the area of A4, but its individual dimensions are about 1.414 times larger, not double, because of the 1:√2 ratio.Can a normal home printer print A3?
Most standard home printers only handle up to A4. You need a wide-format or A3-capable printer, and even then, some models only support A3+ rather than true ISO A3.What is the smallest A-series paper size?
A10 is the smallest defined size in the standard, at 26 x 37mm, roughly the size of a large postage stamp. It's rarely used for actual printing.Why is it called "A" series and not something else?
ISO 216 also defines B and C series. A-series is for general printing and documents, B-series covers sizes falling between A sizes (used in books and passports), and C-series is built specifically for envelopes that fit A-series paper.Does A0 paper weigh more than A4 paper at the same gsm?
Yes. GSM measures weight per square meter, so a larger sheet at the same gsm rating will always weigh more in total simply due to its larger surface area.What's the difference between A3 and A3+?
A3+ (sometimes called Super A3) is a non-ISO size at 329 x 483mm, made for borderless printing with extra bleed margin. True A3 is 297 x 420mm exactly.