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Defined by ISO 269, this standard defines the size of envelopes. It closely follows the paper standard ISO 216 for paper sizes. For example, a sheet of A4 paper, folded three times, will fit snugly into a DL envelope. In addition, an A4 piece of paper wil lfit snugly into a C4 envelope.

This standard is most commonly used in offices worldwide.

Format Width x Height (mm) Width x Height (inches)
DL 220 x 110 mm 8.7 x 4.3 inches
B4 353 x 250 mm 13.9 x 9.8 inches
B5 250 x 176 mm 9.8 x 6.9 inches
B6 176 x 125 mm 6.9 x 4.9 inches
C3 458 x 324 mm 18 x 12.8 inches
C4 324 x 229 mm 12.8 x 9 inches
C5 229 x 162 mm 9 x 6.4 inches
C6/C5 229 x 114 mm 9 x 4.5 inches
C6 162 x 114 mm 6.4 x 4.5 inches
C7/C6 162 x 81 mm 6.4 x 3.2 inches
C7 114 x 81 mm 4.5 x 3.2 inches
E4 400 x 280 mm 15.7 x 11 inches