Envelopes come in many types and sizes but a classification scheme exists that will help you navigate your way through the choices. The table below shows the standard ISO sizes of envelopes and their size-relationship to each other and to standard paper sizes; please note that DL is not actually an ISO it is a DIN (see note under table).
|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.|
|C5||162 x 229||6 3/8 x 9||For flat A5 sheet, A4 sheet folded in half.|
|C6||114 x 162||4 1/2 x 6 3/8||For A5 sheet folded in half, A4 sheet folded in quarters.|
|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.|
|DL*||110 x 220||4 5/16 x 8 11/16||For A4 sheet folded in thirds, A5 sheet folded in half
* 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.
Below we have outlined the most commonly used types of envelopes within this scheme and given some extra information about their usage.
This is for an A4 document without any folding. A C4 envelope measures 32.4cm width and 22.9cm in height. C4 envelopes are used mainly for important letters and brochures that cannot or should not be folded.
This is for an A4 document that is folded in half. A C5 envelope measures 22.9cm in width and 16.2cm in height. C5 envelopes are used mainly for A4 documents that are more than two pages long, and also quite commonly for mail shots and smaller brochures that cannot be folded. The need for only a single fold helps makes letters look more professional, but without being too large for common use.
This is for an A4 sized document that is folded over twice to create 3 roughly equal sections. A DL envelope measures 22cm in width and 11cm in height. DL envelopes are generally used for single page letters and similar (usually short) correspondence; however the need to fold each letter twice creates creases which can sometimes look unprofessional if there are more than two pages.