Size comparison
A3 vs A4
A3 (297 × 420 mm) is exactly twice the area of A4 (210 × 297 mm). Two A4 sheets tile side by side into one A3, and both share the ISO 216 ratio of 1 : √2 — so designs scale between them at exactly 141.4 % (up) or 70.7 % (down) without cropping.
Both sheets drawn to the same scale. The dashed line is the A3 half-cut that produces two A4 sheets.
Side-by-side dimensions
| Property | A3 | A4 |
|---|---|---|
| Millimetres | 297 × 420 | 210 × 297 |
| Centimetres | 29.7 × 42.0 | 21.0 × 29.7 |
| Inches | 11.69 × 16.54 | 8.27 × 11.69 |
| Pixels @ 300 DPI | 3508 × 4961 | 2480 × 3508 |
| Pixels @ 150 DPI | 1754 × 2480 | 1240 × 1754 |
| Area | 1247.4 cm² | 623.7 cm² |
| Aspect ratio | 1 : √2 (≈ 1 : 1.414) — identical for both | |
| Reference page | Full A3 guide | Full A4 guide |
Choose A3 when…
- It hangs on a wall and is read from 1–2 m: posters, signage
- The layout needs a double-page spread or a large table
- Technical drawings need room for detail and annotations
- Headlines must be visible across a room or corridor
- Children are drawing or painting — bigger canvas, fewer limits
Choose A4 when…
- It is a document: reports, letters, CVs, contracts
- Recipients will print it themselves on a home printer
- It gets filed in folders, binders or filing cabinets
- It travels — A3 folds awkwardly, A4 fits every bag
- Cost and universality matter: A4 is everywhere, A3 is not
Scaling between A3 and A4
Print dialogs handle this natively: choose "Scale to paper size → A3" when enlarging, or print an A3 file with "Fit" onto A4 to reduce. Two checks after scaling: raster images lose effective DPI when enlarged (300 DPI at A4 becomes ~212 DPI at A3 — fine for posters, borderline for close inspection), and bleed remains an absolute 3 mm regardless of scale — re-export rather than scaling a bled file.
Frequently asked questions
Which is bigger, A3 or A4?
A3 is bigger. A3 measures 297 × 420 mm; A4 measures 210 × 297 mm. A3 has exactly twice the area of A4 — two A4 sheets side by side make one A3.
Is A4 half of A3?
Yes, exactly. Cutting an A3 sheet in half along its long edge produces two A4 sheets. The short edge of A3 (297 mm) equals the long edge of A4.
Can I print an A4 document at A3?
Yes — scale up by 141.4 % (√2). Because both share the 1:√2 aspect ratio, nothing crops or distorts. Check raster images though: a photo that was sharp at A4/300 DPI becomes effectively 212 DPI at A3, which may show softness up close.
Should my poster be A3 or A4?
A4 for notice boards read at arm's length and for anything people take away. A3 for wall posters read from 1–2 m, event signage and layouts that need bigger headlines. If the poster competes for attention in a corridor or shop window, A3 is the minimum that works.
Do home printers support A3?
Most home printers max out at A4. A3 needs a wide-format printer (common in offices and copy shops). Alternatively, tile an A3 design across two A4 sheets, or order online — A3 prints are usually the cheapest large-format option.
What is the scale factor between A3 and A4?
A4 → A3 is ×1.414 (√2, i.e. 141.4 % in a print dialog). A3 → A4 is ×0.707 (70.7 %). These same factors apply between any two adjacent A-series sizes.