C Series Envelope Sizes

The dimensions of the C series envelope sizes, as defined by ISO 216, are given in the table below in both millimetres and inches. The diagrams to the right show the size of each of the envelopes when compared to a sheet of A4 paper.

Table of Envelope Sizes From C0 to C10

Size	Height x Width (mm)	Height x Width (in)
C0	1297 x 917 mm	51.5 x 36.1 in
C1	917 x 648 mm	36.1 x 25.5 in
C2	648 x 458 mm	25.5 x 18.0 in
C3	458 x 324 mm	18.0 x 12.8 in
C4	324 x 229 mm	12.8 x 9.0 in
C5	229 x 162 mm	9.0 x 6.4 in
C6	162 x 114 mm	6.4 x 4.5 in
C7	114 x 81 mm	4.5 x. 3.2 in
C8	81 x 57 mm	3.2 x 2.2 in
C9	57 x 40 mm	2.2 x 1.6 in
C10	40 x 28 mm	1.6 x 1.1 in

C Series Envelope Size Tolerances:

Tolerances specified in ISO 216 for the C series envelope sizes are the same as for A & B series paper sizes and are as follows:

• ±1.5 mm (0.06 in) for dimensions up to 150 mm (5.9 in)
• ±2 mm (0.08 in) for lengths in the range 150 to 600 mm (5.9 to 23.6 in)
• ±3 mm (0.12 in) for any dimension above 600 mm (23.6 in)

C Series Envelope Size Definitions:

C envelopes sizes are defined as the geometric mean of the A and B sizes with the same number i.e. C4 dimensions are the geometric mean of A4 and B4. This produces a size between the two that makes an envelope 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.

It should be noted that C format envelopes also have an aspect ratio of 1:root2 and because of this an A4 sheet folded parallel to its shortest sides will fit in a C5 envelope and folded twice will fit a C6 envelope.

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.