Envelope Sizes
C envelope sizes are the ISO 216 set made to hold A-series sheets. C4 fits an unfolded A4, C5 fits it folded once, and DL fits it folded in three.
C0 measures 917 millimetres wide and 1297 millimetres tall.
Dimensions of All Envelope Sizes
The Envelope Sizes run from C0 at 917 × 1297 mm to C8 at 57 × 81 mm. Each size, with its typical use:
| Size | mm | cm | inches | Holds flat | Common use |
|---|---|---|---|---|---|
| C0 | 917 × 1297 | 91.7 × 129.7 | 36.1 × 51.06 | A0 | flat delivery of A0 posters and drawings |
| C1 | 648 × 917 | 64.8 × 91.7 | 25.51 × 36.1 | A1 | flat delivery of A1 plans and posters |
| C2 | 458 × 648 | 45.8 × 64.8 | 18.03 × 25.51 | A2 | flat delivery of A2 documents and prints |
| C3 | 324 × 458 | 32.4 × 45.8 | 12.76 × 18.03 | A3 | flat delivery of A3 documents |
| C4 | 229 × 324 | 22.9 × 32.4 | 9.02 × 12.76 | A4 | posting A4 documents unfolded |
| C5 | 162 × 229 | 16.2 × 22.9 | 6.38 × 9.02 | A5, or A4 folded once | posting A4 letters folded once, or A5 flat |
| DL | 110 × 220 | 11.0 × 22.0 | 4.33 × 8.66 | A4 folded in three | posting A4 business letters folded in three |
| C6 | 114 × 162 | 11.4 × 16.2 | 4.49 × 6.38 | A6, or A5 folded once | posting A6 cards, or A4 folded twice |
| C7/6 | 81 × 162 | 8.1 × 16.2 | 3.19 × 6.38 | A5 folded in three | posting A5 letters folded in three |
| C7 | 81 × 114 | 8.1 × 11.4 | 3.19 × 4.49 | A7, or A6 folded once | posting A7 cards and small inserts |
| C8 | 57 × 81 | 5.7 × 8.1 | 2.24 × 3.19 | A8 | small enclosures and coin envelopes |
Dimensions are portrait (short edge first), rounded to the nearest millimetre exactly as ISO 216 defines them. Centimetre, inch and pixel values are derived by formula, not separate standards.
Which Envelope Size Fits Which Sheet
Each C size is the geometric mean of the A size and the B size with the same number, which makes it just big enough to hold that A sheet flat. C4 (229 × 324 mm) fits an A4 page unfolded; C5 (162 × 229 mm) fits A4 folded once or A5 flat; C6 (114 × 162 mm) fits an A6 card or A4 folded twice. DL, 110 × 220 mm, sits outside the series and is the standard business envelope for an A4 letter folded in three. The full fold-to-envelope table is further down this page.
The Common US Envelope Sizes
North American envelopes are numbered rather than lettered. The #10 is the day-to-day business envelope at 4.125 × 9.5 in, sized for a US Letter sheet folded in three, the way DL is sized for A4. A #9 (3.875 × 8.875 in) is the reply envelope that slips inside a #10. For documents that must not be folded, the 6 × 9 in booklet envelope holds a half-folded Letter sheet and the 9 × 12 in catalogue envelope holds Letter flat. These are commercial conventions, not an ANSI standard, so dimensions shift a millimetre or two between makers.
Frequently Asked Questions
What are the standard envelope sizes?
Internationally, the ISO C series: C4, C5, C6, C7 and the DL business envelope. In the United States, the numbered series led by the #10, plus 6 × 9 in and 9 × 12 in for documents sent flat.
What envelope fits an A4 sheet?
A C4 envelope holds A4 unfolded. Folded once it fits C5; folded in three it fits DL, the standard business envelope.
What is a DL envelope?
DL is 110 × 220 mm, sized for an A4 letter folded in three. It is the most produced envelope size in the world and the default for statements and correspondence.
What size is a #10 envelope?
4.125 by 9.5 inches, or about 105 × 241 mm. It holds a US Letter sheet folded in three and is the North American equivalent of DL.
Are C5 and DL interchangeable?
Both take an A4 letter. C5 takes it folded once and looks more formal; DL takes it folded in three and is more compact. A C5 sheet will not fit a DL envelope.
✓ Member dimensions per ISO 216:2007. Centimetre, inch and pixel values derived by formula. Last verified . Editorial Process →