The dimensions of the C series envelope sizes, as defined by ISO 216, are given in the table below right in both millimetres and inches (cm measurements can be obtained by dividing the mm value by 10). The diagram below shows the size of each of the envelopes when compared to a sheet of A4 paper. US & North American Envelope Sizes are not covered by ISO 216 and are covered on this page.

 

To obtain sizes in centimetres, convert mm values to cm by dividing by 10 and in feet by dividing inch values by 12. More units here.

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.

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.

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.

DL+ and DL++

DL+, at 114 x 229 mm, is another name for the C6/C5 envelope and DL++ is slightly bigger again at 114 x 235 mm

B4 and B6 Envelope Sizes

Although initially specified as paper sizes both B4 and B6 have come to be used as envelope sizes in various countries around the world.

Note: Some sites list B4 envelopes at 352mm

Other Envelope Sizes C5+, C5++, C7/C6 and A3

These envelope sizes are available from some suppliers and we have listed them here for completeness

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