International standard sizes

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International standard ISO 269 defines several standard envelope sizes, which are designed for use with ISO 216 standard paper sizes:

Format	Dimensions (mm)	Suitable for content format
DL	110 × 220	1/3 A4
C7/C6	81 x 162	1/3 A5
C6	114 × 162	A6 (A4 folded twice)
C6/C5	114 × 229	1/3 A4
C5	162 × 229	A5 (A4 folded once)
C4	229 × 324	A4
C3	324 × 458	A3
B6	125 × 176	C6
B5	176 × 250	C5
B4	250 × 353	C4
E4	280 × 400	B4

The German standard DIN 678 defines a similar list of envelope formats.

North American sizes

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There are dozens of sizes of envelopes available. Not all are used for posting mail, but for such things as former pay packets or putting a gift card or a key in. US and presumably Canadian postal regulations differ from those of the rest of the world; although envelopes are still deliverable worldwide by the regulations of the Universal Postal Union, the sorting machines will not accept the European sizes. This is not as much a difference as usually thought, for the location of sending address and return address differ between Germany and France, for example.

Most envelope sizes in this system have names, not designations. The designations under the system "A2", etc., do not correspond to DIN paper sizes.

Format	Dimensions (in.)
A2	4 3/8 × 5 3/4
A6	4 3/4 × 6 1/2
A7	5 1/4 × 7 1/4
No. 10	4 1/8 × 9 1/2

American envelope sizes

My Design Primer - standard envelope sizes

Standards | Stationery