ISO Envelope sizes

International standard ISO (International Organization for Standardization) 269 defines several standard envelope sizes, which are designed for use with ISO 216 standard paper sizes (the A series). The German standard DIN 678 defines a similar list of envelope formats.

This envelope series was designed to accommodate ISO A size papers. Metric measurements are the finished envelope size. Inch measurements are approximate (calculated at 25.4 mm = 1 inch and rounded off).

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.

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