ISO 269 · Worldwide except North America · 11 sizes
ISO C series
The ISO C series is the envelope range for A-series paper. C4 (229 × 324 mm) takes A4 flat, C5 (162 × 229 mm) takes A4 folded once, and C6 (114 × 162 mm) takes A4 folded twice. Each C size is the geometric mean of the same-numbered A and B sizes.
From C10 (28 × 40 mm) to C0 (917 × 1,297 mm).
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Every size in the series
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| Size | Width × Height (mm) | Centimetres | Inches | Points | Pixels @300 dpi | Ratio |
|---|---|---|---|---|---|---|
| C0 | 917 × 1,297 mm | 91.7 × 129.7 cm | 36.1 × 51.06 in | 2,599 × 3,677 pt | 10,831 × 15,319 px | 1:√2 |
| C1 | 648 × 917 mm | 64.8 × 91.7 cm | 25.51 × 36.1 in | 1,837 × 2,599 pt | 7,654 × 10,831 px | 1:√2 |
| C2 | 458 × 648 mm | 45.8 × 64.8 cm | 18.03 × 25.51 in | 1,298 × 1,837 pt | 5,409 × 7,654 px | 1:√2 |
| C3 | 324 × 458 mm | 32.4 × 45.8 cm | 12.76 × 18.03 in | 918 × 1,298 pt | 3,827 × 5,409 px | 1:√2 |
| C4 A4 envelope | 229 × 324 mm | 22.9 × 32.4 cm | 9.02 × 12.76 in | 649 × 918 pt | 2,705 × 3,827 px | 1:√2 |
| C5 A5 envelope | 162 × 229 mm | 16.2 × 22.9 cm | 6.38 × 9.02 in | 459 × 649 pt | 1,913 × 2,705 px | 1:√2 |
| C6 A6 envelope | 114 × 162 mm | 11.4 × 16.2 cm | 4.49 × 6.38 in | 323 × 459 pt | 1,346 × 1,913 px | 1:√2 |
| C7 | 81 × 114 mm | 8.1 × 11.4 cm | 3.19 × 4.49 in | 230 × 323 pt | 957 × 1,346 px | 1:√2 |
| C8 | 57 × 81 mm | 5.7 × 8.1 cm | 2.24 × 3.19 in | 162 × 230 pt | 673 × 957 px | 1:√2 |
| C9 | 40 × 57 mm | 4 × 5.7 cm | 1.57 × 2.24 in | 113 × 162 pt | 472 × 673 px | 1:1.43 |
| C10 | 28 × 40 mm | 2.8 × 4 cm | 1.1 × 1.57 in | 79 × 113 pt | 331 × 472 px | 1:1.43 |
Have a measurement instead of a name? Paste it into the paper size finder — it reads 210 x 297, 8.5 x 11 in or 24x36 and names the sheet.
About this series
The C series is defined in ISO 269 (1985) and exists for one reason: to wrap A-series paper with a comfortable margin. Every office and stationer outside North America stocks C4, C5 and C6 envelopes, and printers use the same numbers for folders and document wallets.
Each C size is the geometric mean of the A and B sizes with the same number, which makes it about 9 percent larger than the A sheet in each direction. C0 is 917 × 1297 mm and the series halves down to C10 at 28 × 40 mm. The extra room is enough for a few sheets plus the thickness of a fold.
Three sizes cover nearly every mailing. C4 (229 × 324 mm, 9.02 × 12.76 in) holds A4 unfolded, which is what you want for certificates, contracts and anything that must arrive flat. C5 (162 × 229 mm) holds A5 flat or A4 folded in half. C6 (114 × 162 mm) holds A6 flat or A4 folded into quarters, and is the standard greeting-card envelope. The DL envelope (110 × 220 mm) is not a C size but belongs to the same family and takes A4 folded into thirds.
The relationship runs both ways. A B envelope of the same number will hold a C envelope, so a B5 envelope (176 × 250 mm) is the right outer sleeve for a C5. In North America the closest matches are the No. 10 (104.8 × 241.3 mm) for DL and the 9 × 12 in catalog envelope for C4.
A practical tip: leave at least 3 mm each way inside the envelope for smooth machine insertion. If a pack is thicker than about 5 mm, order C4 or C5 envelopes with a gusset rather than forcing a standard wallet.
Questions about this series
What size envelope fits A4 paper?
C4 (229 × 324 mm) fits A4 flat. C5 (162 × 229 mm) fits A4 folded once. DL (110 × 220 mm) fits A4 folded into thirds. C6 (114 × 162 mm) fits A4 folded into quarters.
What are the ISO C series envelope sizes?
C0 917 × 1297 mm, C1 648 × 917, C2 458 × 648, C3 324 × 458, C4 229 × 324, C5 162 × 229, C6 114 × 162, C7 81 × 114, C8 57 × 81, C9 40 × 57 and C10 28 × 40 mm.
Is C5 bigger than A5?
Yes, by about 9 percent. C5 is 162 × 229 mm and A5 is 148 × 210 mm, so an A5 sheet or an A4 sheet folded in half slides in with 14 mm of spare width and 19 mm of spare height.
What is the difference between C6 and DL envelopes?
C6 is 114 × 162 mm and holds A4 folded twice (into A6). DL is 110 × 220 mm and holds A4 folded into thirds. DL is the usual business envelope; C6 is the usual card envelope.