A4 is 210 × 297 mm; US Letter is 8.5 × 11 inches (215.9 × 279.4 mm). Every A size is half the one before it — fold A3 in half and you get A4.
A series (ISO 216)
| Size | Millimetres | Inches |
|---|
| A0 | 841 × 1189 | 33.11 × 46.81 |
| A1 | 594 × 841 | 23.39 × 33.11 |
| A2 | 420 × 594 | 16.54 × 23.39 |
| A3 | 297 × 420 | 11.69 × 16.54 |
| A4 | 210 × 297 | 8.27 × 11.69 |
| A5 | 148 × 210 | 5.83 × 8.27 |
| A6 | 105 × 148 | 4.13 × 5.83 |
| A7 | 74 × 105 | 2.91 × 4.13 |
| A8 | 52 × 74 | 2.05 × 2.91 |
B series (ISO 216)
| Size | Millimetres | Inches |
|---|
| B0 | 1000 × 1414 | 39.37 × 55.67 |
| B1 | 707 × 1000 | 27.83 × 39.37 |
| B2 | 500 × 707 | 19.69 × 27.83 |
| B3 | 353 × 500 | 13.9 × 19.69 |
| B4 | 250 × 353 | 9.84 × 13.9 |
| B5 | 176 × 250 | 6.93 × 9.84 |
| B6 | 125 × 176 | 4.92 × 6.93 |
Envelopes (ISO 269)
| Size | Millimetres | Inches |
|---|
| C4 | 229 × 324 | 9.02 × 12.76 |
| C5 | 162 × 229 | 6.38 × 9.02 |
| C6 | 114 × 162 | 4.49 × 6.38 |
| DL | 110 × 220 | 4.33 × 8.66 |
North America
| Size | Millimetres | Inches |
|---|
| Letter | 215.9 × 279.4 | 8.5 × 11 |
| Legal | 215.9 × 355.6 | 8.5 × 14 |
| Tabloid (Ledger) | 279.4 × 431.8 | 11 × 17 |
| Executive | 184.15 × 266.7 | 7.25 × 10.5 |
| Half Letter (Statement) | 139.7 × 215.9 | 5.5 × 8.5 |
A sizes keep the same shape (a ratio of 1 to √2) when halved, and A0 has an area of one square metre. Inches are rounded to two decimals (1 inch = 25.4 mm exactly).
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