Envelope Sizes
ISO paper sizes (plus rounded inch values)
| Format | Envelope usage | |
|---|---|---|
| Size | mm x mm | Suitable for... |
| DL | 110 × 220 | 1/3 A4 |
| C7/C6 | 81 x 162 | 1/3 A5 |
| C6 | 114 × 162 | A6 (or A4 folded in half twice) |
| C6/C5 | 114 × 229 | 1/3 A4 |
| C5 | 162 × 229 | A5 (or A4 folded in half once) |
| C4 | 229 × 324 | A4 |
| C3 | 324 × 458 | A3 |
| B6 | 125 × 176 | C6 |
| B5 | 176 × 250 | C5 |
| B4 | 250 × 353 | C4 |
| E3 | 280 × 400 | B4 |
| Format | C series (envelopes) | |
|---|---|---|
| Size | mm × mm | in × in |
| C0 | 917 × 1297 | 36.1 × 51.1 |
| C1 | 648 × 917 | 25.5 × 36.1 |
| C2 | 458 × 648 | 18.0 × 25.5 |
| C3 | 324 × 458 | 12.8 × 18.0 |
| C4 | 228 × 324 | 9.0 × 12.8 |
| C5 | 162 × 229 | 6.4 × 9.0 |
| C6 | 114 × 162 | 4.5 × 6.4 |
| C7 | 81 × 114.9 | 3.2 × 4.5 |
| C8 | 57 × 81 | 2.2 × 3.2 |
| C9 | 40 × 57 | 1.6 × 2.2 |
| C10 | 28 × 40 | 1.1 × 1.6 |
The C series is used only for envelopes and is defined in ISO 269. The area of C series sheets is the geometric mean of the areas of the A and B series sheets of the same number; for instance, the area of a C4 sheet is the geometric mean of the areas of an A4 sheet and a B4 sheet. This means that C4 is slightly larger than A4, and B4 slightly larger than C4. The practical usage of this is that a letter written on A4 paper fits inside a C4 envelope, and a C4 envelope fits inside a B4 envelope.