Core answer: A4 = 210×297 mm, A3 = 297×420 mm (two A4 sheets), A5 = 148×210 mm (A4 folded in half). The pattern: each fold of an A-series sheet adds 1 to the number, and the aspect ratio stays √2:1 — so A4 enlarged 141% is exactly A3.
Full A-series table
| Size | Dimensions (mm) | Typical use |
|---|---|---|
| A0 | 841×1189 | Engineering drawings, posters |
| A1 | 594×841 | Display boards, charts |
| A2 | 420×594 | Banners, drawings |
| A3 | 297×420 | Exam papers, reports, small posters |
| A4 | 210×297 | The office standard |
| A5 | 148×210 | Notebooks, manuals |
| A6 | 105×148 | Postcards, notes |
| A7 | 74×105 | Cards |
| A8 | 52×74 | Business-card size |
Rule: adjacent sizes relate by folding/joining — the area doubles or halves.
B and C series (envelopes)
| Size | Dimensions (mm) | Use |
|---|---|---|
| B4 | 250×353 | Exam sheets, magazines |
| B5 | 176×250 | Notebooks, books |
| B6 | 125×176 | Pocket books |
| C4 | 229×324 | Holds A4 unfolded |
| C5 | 162×229 | Holds A5 / A4 folded once |
| DL | 110×220 | Standard business envelope for A4 trifolded |
Why the aspect ratio is √2
A-series long side : short side = √2:1 ≈ 1.414:1. The magic: folding in half preserves the ratio (a (√2/2):1 rectangle rotated is again 1:√2).
Practical payoffs:
- A4 content shrinks to A5 (71%) or enlarges to A3 (141%) with zero distortion
- Booklet printing (A4 folded and bound into A5) scales margins proportionally
Chinese book formats: 16mo / 32mo
Chinese publishing still uses "kai" (开) formats — the count of sheets cut from a full sheet (787×1092 mm). After trim losses, these do not equal A-series:
| Format | Trimmed size (approx, mm) | Closest A/B size |
|---|---|---|
| 16 kai | 185×260 | Slightly narrower than A4 |
| Large 16 kai | 210×285 | Close to A4 |
| 32 kai | 130×185 | Near A5, narrower |
| Large 32 kai | 145×210 | Near A5 |
| 64 kai | 92×126 | Near A7 |
Photo paper and frame sizes
| Common name | Dimensions (mm) | Note |
|---|---|---|
| 5-inch (3R) | 89×127 | Wallet photo |
| 6-inch (4R) | 102×152 | Home standard |
| 7-inch (5R) | 127×178 | Desk frame |
| 8-inch | 152×203 | Close to A5 |
| 10-inch | 203×254 | Wall hanging |
| 12-inch | 254×305 | Enlargements |
Photo "inches" measure the long edge in inches — a completely different track from A-series; never mix them.
Print settings and bleed
- Borderless printing: make the file 3 mm larger than the paper (bleed) — e.g., an A4 design at 216×303 mm, trimmed to hide white edges
- Scaling cheat sheet: A4→A3 use 141%, A4→A5 use 71%, A3→A4 use 71% (1/√2 ≈ 70.7%)
- Booklets: A4 duplex + "2 pages per sheet + booklet layout" — fold for an A5 manual
Common mistakes and myths
- "A4 is 21×30" — the exact value is 210×297 mm; rounding to 21×30 breaks imposition alignment.
- "16 kai equals A4" — 16 kai (185×260) is narrower and shorter; textbooks use 16 kai, offices use A4. Print school PDFs with "fit to page" or edges get cut.
- "B5 is bigger than A4" — B5 (176×250) is clearly smaller; within B-series only B4 (250×353) sits between A4 and A3.
- "A 6-inch photo equals A6" — 6-inch photo is 102×152 mm, A6 is 105×148 mm: close but not equal. Measure before buying frames.
Use the [Paper Size Calculator](/c/convert/paper-size) for any size and scaling ratio, and the [PPI Calculator](/c/convert/ppi) for print resolution.