This envelope series was designed to accommodate ISO A size papers; Metric measurements are the finished envelope size; Inch measurements areapproximate (calculated at 25.4 mm = 1 inch and rounded off).
| Name | MM | Inches | Usage |
| C0 |
917 x 1297 |
36 1/8 x 51 1/16 |
For flat A0 sheet |
| C1 |
648 x 917 |
25 1/2 x 36 1/8 |
For flat A1 sheet |
| C2 |
458 x 648 |
18 x 25 1/2 |
For flat A2 sheet, A1 sheet folded in half |
| C3 |
324 x 458 |
12 3/4 x 18 |
For flat A3 sheet, A2 sheet folded in half |
| C4 |
229 x 324 |
9 x 12 3/4 |
For flat A4 sheet, A3 sheet folded in half; View sheet folding example |
| C5 |
162 x 229 |
6 3/8 x 9 |
For flat A5 sheet, A4 sheet folded in half; View sheet folding example |
| C6 |
114 x 162 |
4 1/2 x 6 3/8 |
For A5 sheet folded in half, A4 sheet folded in quarters; View sheet folding example |
| C7/6 |
81 x 162 |
3 3/16 x 6 3/8 |
For A5 sheet folded in thirds |
| C7 |
81 x 114 |
3 3/16 x 4 1/2 |
For A5 sheet folded in quarters; View sheet folding example |
| DL* |
110 x 220 |
4 5/16 x 8 11/16 |
For A4 sheet folded in thirds, A5 sheet folded in half
lengthwise; View sheet folding example |
* Although DL is not part of the ISO C series, it is a very widely used standard size. DL, probably at one time the abbreviation of DIN Lang (Deutsche Industrie Norm, Long), is now identified as "Dimension Lengthwise" by ISO 269.