The C series is usually used for envelopes and is defined in ISO 269. The area of C series sheets is the geometric mean of the areas of the A and B series sheets of the same number; for instance, the area of a C4 sheet is the geometric mean of the areas of an A4 sheet and a B4 sheet. This means that C4 is slightly larger than A4, and slightly smaller than B4. The practical usage of this is that a letter written on A4 paper fits inside a C4 envelope, and both A4 and C4 paper fits inside a B4 envelope.
Some envelope formats with mixed sides from adjacent sizes (and thus an approximate aspect ratio of 2:1) are also defined in national adaptations of the ISO standard, e.g. DIN C6/C5 is 114 mm × 229 mm where the common side to C5 and C6 is 162 mm.
C Series Paper Sizes From C0 to C10
| Format | Width × Height (in) | Width × Height (mm) |
|---|---|---|
| C0 | 36.1 × 51.1 in | 917 × 1297 mm |
| C1 | 25.5 × 36.1 in | 648 × 917 mm |
| C2 | 18 × 25.5 in | 458 × 648 mm |
| C3 | 12.8 × 18 in | 324 × 458 mm |
| C4 | 9 × 12.8 in | 229 × 324 mm |
| C5 | 6.4 × 9 in | 162 × 229 mm |
| C6 | 4.5 × 6.4 in | 114 × 162 mm |
| C7 | 3.2 × 4.5 in | 81 × 114 mm |
| C8 | 2.2 × 3.2 in | 57 × 81 mm |
| C9 | 1.6 × 2.2 in | 40 × 57 mm |
| C10 | 1.1 × 1.6 in | 28 × 40 mm |
The dimensions of the C series sizes are defined by the ISO 269 paper size standard. The C series is most commonly used for envelopes. The area of C series paper is the geometric mean of the areas of the A and B series paper of the same number. For example, C4 has a bigger area than A4, but smaller area than B4. Therefore an A4 piece of paper will fit into a C4 envelope. The aspect ratio of C series envelopes is 1:√2, and this means that an A4 sheet of paper when folded in half, parallel to its smaller sides, will fit nicely into a C5 envelope. When it is folded twice, it will fit into a C6 envelope, and so on.
ISO paper sizes in portrait view (with rounded inch values)
| Format | A series | B series | C series |
|---|---|---|---|
| Size | mm × mm in × in |
mm × mm in × in |
mm × mm in × in |
| 0 | 841 × 1189 mm 33.1 × 46.8 in |
1000 × 1414 mm 39.4 × 55.7 in |
917 × 1297 mm 36.1 × 51.1 in |
| 1 | 594 × 841 mm 23.4 × 33.1 in |
707 × 1000 mm 27.8 × 39.4 in |
648 × 917 mm 25.5 × 36.1 in |
| 2 | 420 × 594 mm 16.5 × 23.4 in |
500 × 707 mm 19.7 × 27.8 in |
458 × 648 mm 18.0 × 25.5 in |
| 3 | 297 × 420 mm 11.7 × 16.5 in |
353 × 500 mm 13.9 × 19.7 in |
324 × 458 mm 12.8 × 18.0 in |
| 4 | 210 × 297 mm 8.27 × 11.7 in |
250 × 353 mm 9.84 × 13.9 in |
229 × 324 mm 9.02 × 12.8 in |
| 5 | 148 × 210 mm 5.83 × 8.27 in |
176 × 250 mm 6.93 × 9.84 in |
162 × 229 mm 6.38 × 9.02 in |
| 6 | 105 × 148 mm 4.13 × 5.83 in |
125 × 176 mm 4.92 × 6.93 in |
114 × 162 mm 4.49 × 6.38 in |
| 7 | 74 × 105 mm 2.91 × 4.13 in |
88 × 125 mm 3.46 × 4.92 in |
81 × 114 mm 3.19 × 4.49 in |
| 8 | 52 × 74 mm 2.05 × 2.91 in |
62 × 88 mm 2.44 × 3.46 in |
57 × 81 mm 2.24 × 3.19 in |
| 9 | 37 × 52 mm 1.46 × 2.05 in |
44 × 62 mm 1.73 × 2.44 in |
40 × 57 mm 1.57 × 2.24 in |
| 10 | 26 × 37 mm 1.02 × 1.46 in |
31 × 44 mm 1.22 × 1.73 in |
28 × 40 mm 1.10 × 1.57 in |
| A series | B series | C series | US sizes | US Envelope |
| International Envelope | Photography Paper | Canadian | Japanese | Books |
| Newspaper | Chinese | Billboard | Imperial | Colombian |
| French | Raw | Transitional |
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