C7 – ISO paper sizes C series formats

81 × 114mm | 3.2 × 4.5in

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 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 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.

ISO 216 specifies international standard (ISO) paper sizes used in most countries in the world today, although not in Canada, the United States, Mexico, Colombia, or the Dominican Republic. The standard defines the “A” and “B” series of paper sizes, including A4, the most commonly available paper size worldwide. Two supplementary standards, ISO 217 and ISO 269, define related paper sizes; the ISO 269 “C” series is commonly listed alongside the A and B sizes.

All ISO 216, ISO 217 and ISO 269 paper sizes (except some envelopes) have the same aspect ratio, √2:1, within rounding to millimetres. This ratio has the unique property that when cut or folded in half widthways, the halves also have the same aspect ratio. Each ISO paper size is one half of the area of the next larger size in the same series.

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.

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