C Series
The C series is used for postcards, folders and envelopes. Each C size envelope or folder will take the corresponding A size letter inside it without folding. For example, a C5 envelope is suitable for posting an A5 letter unfolded, or an A4 letter folded in half.
ISO International Standard Envelope Sizes
Size | Dimension | Takes letter… |
C3 | 324 x 458 | A3 unfolded |
B4 | 250 x 353 | A4 unfolded |
C4 | 229 x 324 | A4 unfolded |
B5 | 176 x 250 | A5 unfolded or A4 folded in half |
C5 | 162 x 229 | A5 unfolded or A4 folded in half |
B6/C4 | 324 x 176 | A4 folded in half lengthwise |
B6 | 125 x 176 | A6 unfolded or A5 folded in half |
C6 | 114 x 162 | A6 unfolded or A5 folded in half |
DL | 110 x 220 | A4 folded in thirds |
C7/6 | 81 x 162 | A5 folded in thirds |
C7 | 81 x 114 | A7 unfolded or A6 folded in half |