Paper Size Table
| Format | A series (mm) | A series (in) | B series (mm) | C series (mm) |
|---|---|---|---|---|
| 4A0 | 1682 × 2378 | 66.2 × 93.6 | — | — |
| 2A0 | 1189 × 1682 | 46.8 × 66.2 | — | — |
| A0 / B0 / C0 | 841 × 1189 | 33.1 × 46.8 | 1000 × 1414 | 917 × 1297 |
| A1 / B1 / C1 | 594 × 841 | 23.4 × 33.1 | 707 × 1000 | 648 × 917 |
| A2 / B2 / C2 | 420 × 594 | 16.5 × 23.4 | 500 × 707 | 458 × 648 |
| A3 / B3 / C3 | 297 × 420 | 11.7 × 16.5 | 353 × 500 | 324 × 458 |
| A4 / B4 / C4 | 210 × 297 | 8.3 × 11.7 | 250 × 353 | 229 × 324 |
| A5 / B5 / C5 | 148 × 210 | 5.8 × 8.3 | 176 × 250 | 162 × 229 |
| A6 / B6 / C6 | 105 × 148 | 4.1 × 5.8 | 125 × 176 | 114 × 162 |
| A7 / B7 / C7 | 74 × 105 | 2.9 × 4.1 | 88 × 125 | 81 × 114 |
| A8 / B8 / C8 | 52 × 74 | 2.0 × 2.9 | 62 × 88 | 57 × 81 |
| A9 / B9 / C9 | 37 × 52 | 1.5 × 2.0 | 44 × 62 | 40 × 57 |
| A10 / B10 / C10 | 26 × 37 | 1.0 × 1.5 | 31 × 44 | 28 × 40 |
The international paper system is one elegant number wearing a thousand envelope labels. Every size in the chart below is the previous size cut in half across its long edge: fold an A4 sheet in half and you hold two sheets of A5, fold again for A6 - and the ratio that makes this work is the square root of two, the one rectangle shape whose proportions survive folding unchanged. The A series is office paper proper, the B series fills the gaps for posters and books at roughly 1.4 times the area, and the C series sits between them as the envelope geometry: a C4 envelope takes flat A4 sheets, C5 takes A5, and the letter slides in without a fold. Every dimension is millimeters first because that is how the standard defines it; the inches column is the translation.
The two sizes everyone actually argues about are A4 (210 x 297 millimeters) and the North American Letter (216 x 279) - six millimeters wider and eighteen millimeters shorter, close enough to look interchangeable and different enough to jam a tight printer tray or miss a printed margin. The A system earns its mathematical reputation honestly: A0 is defined as exactly one square meter, each step halves the area, and that is where the famous paper-weight arithmetic falls out - an A4 sheet is one-sixteenth of A0, so an 80 gsm sheet (eighty grams per square meter) weighs eighty divided by sixteen: five grams a sheet. The chart carries the full ladder from the 4A0 plotter sheets down to the A10 stamp territory, all three series side by side.
How to use
- Pick your series first: A for documents and printing, B for when A feels too small or too formal (posters, book pages, passport covers lean B), C exclusively for envelopes - the C number matches the flat sheet it swallows (C4 holds A4).
- Go down the chart by halving: if A4 is too big, the next smaller sheet is A5 at exactly half the area; going up, A3 doubles it - the millimeter columns make the fold-and-compare logic visible, short edge of one row equals the long edge of the next.
- For weights and postage, use the halving arithmetic: sheets per square meter = 2 to the power of the format number (16 sheets of A4 per square meter), so sheet weight in grams = gsm divided by that count - 80 gsm A4 is 5 grams, the number postage and mailing-cost estimates are built on.
Frequently asked questions
Why is A4 paper exactly 210 x 297 millimeters?
Because of one design decision plus one constraint. The design decision: each format must halve into the next without changing shape, and the only rectangle whose sides keep the same ratio when folded in half is one with a ratio of the square root of two (about 1.414 to 1) - fold it across the long side and the folded short side becomes the new long side at exactly the same ratio. The constraint: A0 (the largest A size) is defined as one square meter of paper. Solve the two together - a 1:root-2 rectangle of area one square meter - and A0 lands at 841 x 1189 millimeters, rounded to the nearest millimeter. Every size below is that rectangle halved repeatedly: A1 is 594 x 841, down to A4 at 210 x 297. The system was standardized in 1975 as ISO 216, but the root-two idea is a eighteenth-century German proposal that Germany formalized in 1922 - the mathematics was always the selling point.
What is the difference between A4 and Letter, and which should I use?
A4 is 210 x 297 millimeters (8.3 x 11.7 inches); US Letter is 216 x 279 millimeters (8.5 x 11 inches). Letter is six millimeters wider and eighteen millimeters shorter - about two percent more area, differently shaped. Geography decides: A4 is the standard everywhere the metric system rules, from Europe through Asia to South America and Australia; Letter (with Legal and the ANSI sizes) governs the US, Canada, Mexico and parts of Central America. The practical frictions: an A4 page in a Letter tray clips the bottom margin or shifts the print; a Letter page in an A4 tray leaves uneven side margins; and two-sided filing mixes are the worst case. Printing cross-border documents, set the paper size explicitly in the print dialog rather than trusting the default - and if the document must survive both worlds, keep critical content 20 millimeters inside every edge so the difference never touches it.
How do C-series envelopes match A-series paper?
The C series is built as the geometric middle between A and B at each size, and its whole job is envelope duty: C4 (229 x 324 millimeters) swallows a flat, unfolded A4 sheet (210 x 297); C5 (162 x 229) takes a flat A5 or a folded A4; C6 (114 x 162) takes a flat A6 or a folded A5. The rule of thumb: the C number equals the A number it carries, and the few millimeters of clearance around every edge are the tolerance that keeps the insert from bending. Two office footnotes: DL envelopes (110 x 220) are not a C size at all but a separate standard sized for an A4 folded in thirds - it is the most common business envelope despite being off-system; and B4/C4 style flat mailers are what catalog printers use precisely because nothing gets folded.
How much does a sheet of paper weigh, and why does 80 gsm matter?
Paper weight is quoted in grams per square meter (gsm), and the A-series halving makes the per-sheet arithmetic one division. A4 is one-sixteenth of a square meter (A0 is exactly one square meter; each format number below halves the area), so a sheet of 80 gsm paper weighs 80 divided by 16 - five grams. That single number drives real decisions: a 500-sheet ream of 80 gsm A4 is about 2.5 kilograms, four sheets weigh twenty grams for postage estimates, and a printer specification of 80 to 120 gsm tells you the feed rollers handle three to six grams per sheet comfortably. Heavier paper scales linearly: 160 gsm A4 is ten grams a sheet, card stock at 250 gsm is over fifteen. When a postal limit says one hundred grams, the arithmetic says roughly twenty sheets of standard office paper plus the envelope - no scale required.