ISO 269 defines envelope formats designed around ISO 216 A-series paper. The C series is the practical everyday set: a C4 envelope fits an unfolded A4 sheet, C5 fits an A5 sheet or folded A4, and DL fits a third-fold A4 letter.
Metric measurements are finished envelope sizes. Inch values are rounded conversions, so use millimetres for production artwork. Envelope templates also define flap shape, seams, window positions and the printable area; none of those are implied by the size name alone.
ISO Envelope Sizes
| Name | Size in millimeters | Size in inches | Usage and comments |
|---|---|---|---|
| C0 | 917 mm x 1297 mm | 36.12 in x 51.6 in | For flat A0 sheet. |
| C1 | 648 mm x 917 mm | 25.50 in x 36.12 in | For flat A1 sheet. |
| C2 | 458 mm x 648 mm | 18 in x 25.50 in | For flat A2 sheet, A1 folded in half. |
| C3 | 324 mm x 458 mm | 12.75 in x 18 in | For flat A3 sheet, A2 folded in half. |
| C4 | 229 mm x 324 mm | 9 in x 12.75 in | For flat A4 sheet, A3 folded in half. Common. |
| C5 | 162 mm x 229 mm | 6.38 in x 9 in | For flat A5 sheet, A4 folded in half. Common. |
| C6 | 114 mm x 162 mm | 4.5 in x 6.38 in | For A5 folded in half. Common. |
| C7/6 | 81 mm x 162 mm | 3.19 in x 6.38 in | For A5 sheet folded in thirds. Uncommon. |
| C7 | 81 mm x 114 mm | 3.19 in x 4.5 in | For A5 folded in quarters. Uncommon. |
| DL | 110 mm x 220 mm | 4.32 in x 8.69 in | For A4 sheet folded in thirds, A5 in half. Common. |
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.