The most common envelope sizes are C-sizes and DL. In this post you can find some interesting information about the envelope sizes and the exact measurements.
C envelopes are a size that will neatly hold the A series paper of the same size, thus a C4 envelope is perfect for an A4 sheet of paper unfolded.
| Size | Width x Height (mm) | Width x Height (in) |
| C0 | 917 x 1297 mm | 36.1 x 51.5 in |
| C1 | 648 x 917 mm | 25.5 x 36.1 in |
| C2 | 458 x 648 mm | 18.0 x 25.5 in |
| C3 | 324 x 458 mm | 12.8 x 18.0 in |
| C4 | 229 x 324 mm | 9.0 x 12.8 in |
| C5 | 162 x 229 mm | 6.4 x 9.0 in |
| C6 | 114 x 162 mm | 4.5 x 6.4 in |
| C7 | 81 x 114 mm | 3.2 x 4.5 in |
| C8 | 57 x 81 mm | 2.2 x 3.2 in |
| C9 | 40 x 57 mm | 1.6 x 2.2 in |
| C10 | 28 x 40 mm | 1.1 x 1.6 in |
DL Envelope Size – The Business Envelope Extension
One of the most widely used business envelopes, the DL format does not fall under the C series sizes as it has a different aspect ratio. This envelope originated in Germany in the 1920’s and was known as DIN Lang, but DL is now more commonly expanded to ‘Dimension Lengthwise’. This size is defined in the ISO standards for envelope sizes, as the standard would have been remiss in omitting the most commonly used business envelope size.
The dimensions of DL are 110 x 220 mm (4⅓” x 8⅔”) and as such the DL envelope will hold an A4 sheet of paper folded into 3 equal sections parallel to its shortest sides.
Despite complaints from manufacturers of automatic enveloping machines that it is slightly too small for reliable enveloping and the introduction of a C6/5 envelope at 114 x 229 mm, the original DL size continues to be most commonly used.
The following diagrams show C4, C5 and C6 envelopes compared to A4 paper size (the envelope being shown in brown with the paper shown as grey) as can be seen in the first diagram the C4 envelope can contain an A4 sheet, the C5 envelope can contain an A4 sheet folded in half (an A5 sheet) and the C6 envelope can contain an A4 sheet folded in half twice (an A6 sheet). This is the reason that you will sometimes see these envelope sizes being referred to as A4 envelope size, A5 envelope size and A6 envelope size.
Source: http://www.papersizes.org/
