The sizes
| Size | Millimetres | Inches |
|---|---|---|
| A3 | 297 × 420 | 11.7 × 16.5 |
| A4 | 210 × 297 | 8.3 × 11.7 |
| A5 | 148 × 210 | 5.8 × 8.3 |
| A6 | 105 × 148 | 4.1 × 5.8 |
Why they halve so neatly
The ideal A-series rectangle has sides in the ratio 1 : √2 (about 1 : 1.414). Fold one in half across its long side and you get the next size down, with approximately the same shape after millimeter rounding. That’s why an A4 page can be shrunk onto A5 without cropping or gaps.
US Letter is a different shape, so it doesn’t scale onto A sizes cleanly. See US Letter and A4.