A3 paper measures 297 × 420 mm (11.7 × 16.5 inches), making it exactly twice the area of A4 paper.
Understanding A3 comes down to one elegant ISO 216 rule: every A-series size is exactly half the area of the one above it, and all share the same 1:√2 aspect ratio. So while A3 looks like “a bigger A4,” it isn’t scaled up arbitrarily — it’s a precisely defined step that lets paper fold and cut into consistent sizes. The practical takeaway: one A3 sheet, cut in half along its long edge, gives you two perfect A4 sheets — a fact that drives how printers, copiers, and layout software treat these formats.
The Exact A3 and A4 Dimensions
ISO 216 defines paper sizes in millimeters — inches-based approximations only get you close. Here are the official dimensions:
- A3: 297 × 420 mm (11.7 × 16.5 in) — the larger of the two
- A4: 210 × 297 mm (8.27 × 11.69 in) — the standard stationery size in most countries
The 1:√2 aspect ratio on both is what makes the folding trick work. Fold or cut an A3 sheet in half parallel to its short side, and the two resulting pieces are exactly A4 size; two A4 pages side by side make one A3 layout. The U.S. caveat: most Americans encounter Letter (8.5 × 11 inches) and Tabloid (11 × 17 inches) more often. A3 is close to Tabloid but not identical — Tabloid is wider and shorter — so knowing the exact numbers matters when buying paper or setting up a printer.
How A3 and A4 Differ in Practice
A3’s larger surface area is its real advantage. Spreadsheets, architectural drawings, posters, presentations, and tabloid-style layouts all benefit from the extra space without non-standard aspect ratios. A4, by contrast, is the default for letters, forms, and general document printing across most of the world. Key practical consequences:
- Printing: An A4 document will not automatically center or scale on A3 without adjusting page setup; printers assuming U.S. Letter sizes often need manual paper-size selection for either format.
- Photocopying: An A3-capable copier can reduce two A4 pages onto one A3 sheet or enlarge a single A4 page to A3 — but you must select the ratio explicitly; the machine won’t guess it.
- Layout software: Setting page size to A3 before creating two A4 panels beats scaling an A4 file afterward, which risks blurry text and misaligned margins.
Common Mistake: Treating A3 as a “Bigger A4”
The most frequent error is assuming A3 is simply an enlarged A4. It isn’t — the area relationship is exact: A3 is twice A4, not 50% bigger. That precision is the whole point of ISO 216, which is why dimensions are defined in millimeters rather than rounded inches. Two other mistakes trip people up:
- Mixing measurement systems: Comparing a 297 mm figure to an 11-inch letter measurement without converting properly leads to ordering the wrong paper or expecting files to fit that they won’t.
- Confusing A3 with A3+: A3+ is a marketing size used by some photo printers — it’s larger than standard ISO A3. If a spec sheet says “A3+,” the sheet is wider than 297 × 420 mm.
| Paper Size | Dimensions (mm) | Typical Use |
|---|---|---|
| A3 | 297 × 420 | Spreadsheets, drawings, posters |
| A4 | 210 × 297 | Letters, forms, general printing |
| U.S. Letter | 216 × 279 | Standard U.S. documents |
| U.S. Tabloid | 279 × 432 | Bigger layouts, often confused with A3 |
If you’re shopping for paper, knowing what you need before you buy saves a wasted trip — a roundup of the best A3 printer paper can help you match the right sheet to the job.
The One Rule That Ties It Together
The entire A series runs on a single idea: each size is half the area of the next larger one, and every size shares the same aspect ratio. That’s why A3 is two A4 sheets, why A4 is two A5 sheets, and why the system scales cleanly from a tiny card up to a large poster. The ISO 216 standard (downloadable as a public PDF from the ITeh standards archive) spells out these relationships in full, and the same math is laid out in the Cambridge Computer Laboratory’s ISO paper-size reference.
The bottom line: A3 is exactly twice the area of A4, at 297 × 420 mm, and the two sizes are designed to be compatible through simple folding or cutting. Master that relationship, and you’ll never order the wrong sheet again.
FAQs
Is A3 paper bigger than A4?
Yes. A3 measures 297 × 420 mm, giving it exactly double the surface area of A4’s 210 × 297 mm. An A3 sheet is also much larger than U.S. Letter, and it’s close to — but not identical to — U.S. Tabloid size, which is wider and shorter.
Can I print A4 documents on A3 paper?
Yes, but you’ll need to set the paper size and scaling in your printer settings or layout software. Most printers won’t automatically fit an A4 document perfectly onto A3; you’ll choose between centering it with margins or scaling it up to fill the sheet.
How do I make two A4 pages from one A3 sheet?
Fold or cut the A3 sheet in half parallel to its long edge — the same direction as the 420 mm dimension. Each resulting half is exactly A4 size. The same relationship lets you create an A3 layout by combining two A4 panels during document setup.
References & Sources
- ISO 216:2007. “ISO 216:2007 Writing paper and certain classes of printed matter — Trimmed sizes — A and B series.” Official standard defining A-series dimensions and the 1:√2 aspect ratio.
- Cambridge Computer Laboratory. “ISO 216 paper sizes.” Clear reference for A-series dimensions and the folding relationship between sizes.