Paper sizes
ISO 216 A-sizes have a 1:√2 ratio, so halving a sheet gives the next size. A0 is one square metre.
| Size | mm | inches | Envelope (unfolded / folded) |
|---|---|---|---|
| A3 | 297 × 420 | 11.7 × 16.5 | C3 / C4 (folded once) |
| A4 | 210 × 297 | 8.3 × 11.7 | C4 / C5 (half) / DL (thirds) |
| A5 | 148 × 210 | 5.8 × 8.3 | C5 / C6 |
| A6 | 105 × 148 | 4.1 × 5.8 | C6 |
| US Letter | 216 × 279 | 8.5 × 11 | 9×12 / 6×9 (half) / #10 (thirds) |
| US Legal | 216 × 356 | 8.5 × 14 | 10×15 / #10 (folded in fourths) |
| Tabloid (Ledger) | 279 × 432 | 11 × 17 | 12×18 / 9×12 (half) |
| Half Letter (Statement) | 140 × 216 | 5.5 × 8.5 | A9 / A2 (half) |
A4 vs Letter
A4 is about 6 mm narrower and 18 mm taller than US Letter. A4 folded in thirds is 99 mm tall, so it fits a DL envelope (110 mm) and, snugly, a #10 (4⅛″ = 105 mm).
Sources
- ISO 216 overview (Wikipedia)
- Paper size – North American sizes (Wikipedia)
- ISO 269 envelope sizes (Wikipedia)
Checked 2026-10-21. Postal rules change; confirm with USPS before a large mailing.