Envelope sizes and finder: C5, DL, A7 and #10

How to use the finder

1. Measure the card or sheet before folding and enter its width and height in inches or millimetres.
2. Choose how it will be folded. A letter is usually folded in three; a greeting card made from one sheet is folded in half.
3. The finder names the smallest envelope of the ISO and of the North American sizes that leaves at least 1/4 inch in both directions, and a smaller one if it would take the piece with less room.
4. For thick contents, several sheets or an enclosure, take the next larger envelope.

Without JavaScript the finder is unavailable; the charts below list every envelope with its size.

C series envelope chart

Sizes are the outer width by height of the envelope. The last column names the sheet that the envelope is made for.

DL and other long envelopes

Long envelopes hold a sheet that is folded in three parallel parts, the usual fold for a business letter.

North American envelopes

In the United States and Canada, envelopes are named by a code and sized in inches. The A sizes are announcement envelopes for cards and invitations; #10 is the business envelope for a letter folded in three.

The last column applies the size limits of the US Postal Service to the empty envelope: letters are 5 to 11 1/2 inches long and 3 1/2 to 6 1/8 inches high, and pieces up to 15 by 12 inches are flats. A letter whose length is less than 1.3 or more than 2.5 times its height, such as a square envelope, is nonmachinable and costs more.

How to choose an envelope

1. Decide how the sheet will be folded: not at all, once, twice or in three.
2. Find the row whose last column matches the sheet and the fold.
3. For more than a few sheets, a booklet or a card, take the next larger envelope. The chart assumes thin contents.

A C envelope is always a little larger than the A sheet with the same number. C4 is 229 mm by 324 mm, which leaves 19 mm in width and 27 mm in height around an A4 sheet of 210 mm by 297 mm.

How the sizes are derived

Each C size is the geometric mean of the A size and the B size with the same number. C0 is therefore 917 mm by 1297 mm, between A0 (841 mm by 1189 mm) and B0 (1000 mm by 1414 mm). The smaller sizes follow by halving the long side and rounding down to the whole millimetre.

This places the three series in a fixed order: a sheet of A4 fits into a C4 envelope, and a C4 envelope fits into a B4 envelope.

DL does not come from halving. It is 110 mm by 220 mm, made for one third of an A4 sheet, which is 99 mm by 210 mm.

The envelope standard ISO 269 was withdrawn in 2009 without a listed replacement. The sizes are still what stationers, printers and software use.

North American sizes are conventions of the envelope trade. A2, #10, 6 × 9 and 9 × 12 appear in the media names used by printers; A6, A7 and A9 are taken from an envelope retailer. The finder’s margin of 1/4 inch in each direction is that retailer’s advice, and the finder compares the folded piece with the inside of the envelope as if the paper had no thickness.

Worked example: a 5 × 7 card and a two page letter

A 5 × 7 in card, not folded: the A7 envelope is 5 1/4 by 7 1/4 in and leaves exactly 1/4 in in each direction. A6, at 4 3/4 in wide, is too narrow. Of the ISO sizes, C5 is the smallest that takes the card, with about 1 3/8 by 2 in to spare.

Two A4 sheets folded in three measure 99 mm by 210 mm. A DL envelope is 110 mm by 220 mm and leaves 11 mm in height and 10 mm in width, enough for a thin letter.

A US Letter sheet folded in three measures 8 1/2 by 3 2/3 in. A #10 envelope leaves about 1/2 by 1 in. DL takes it too but leaves only about 4 mm in length, so the finder shows DL as a tight fit and C6/C5 as the ISO size with enough room.

For a certificate that must not be folded, use C4. For a greeting card made from an A4 sheet folded once, which is A5 size, use C5.

Common mistakes

• Buying C5 for an unfolded A4 sheet. C5 holds A5, or A4 folded once.
• Confusing DL with C6/C5. C6/C5 is 114 mm by 229 mm, slightly larger than DL.
• Forgetting the thickness of the contents. A thick stack needs a larger envelope than the chart suggests.
• Assuming postal prices follow the envelope name. Postal operators set their own size and weight limits; check with yours.
• Choosing a square envelope for a square card without checking the postage. In the United States a square letter is nonmachinable and costs more.
• Measuring a folded card after folding and then choosing the fold in the finder as well. Enter the size of the flat sheet.

Sources and checks

The values in the chart are calculated from the definitions in the sources below. They are not copied from another chart. Last checked against the sources on September 29, 2026.

C sizes are calculated as the geometric mean of the A and B size with the same number, then halved and rounded down; the result was compared with the C0 to C10 values in PWG 5101.1 and matches.

ISO 269 was withdrawn in 2009 and its text was not read. The sizes remain in everyday use and are listed by printer and software vendors.

The column about what fits follows from the sheet sizes: an envelope is a few millimetres larger than the sheet with the same number. Thick contents need a larger envelope.

North American envelopes are named and sized in inches. A2, #10, 6 x 9 and 9 x 12 are listed in PWG 5101.1; A6, A7 and A9 are not, and their sizes are those of envelopes.com. Millimetres are conversions at 25.4 mm per inch.

The USPS column classifies the empty envelope by its size only. Thickness, weight and a stiff or square card can move a letter into a higher price class; check with the post office.

• ISO 269:1985, Correspondence envelopes: designation and sizes (ISO catalogue entry, withdrawn)

Used for: Title of the envelope standard and its withdrawal on 27 April 2009.

• PWG 5101.1-2023, Media Standardized Names 2.1 (IEEE-ISTO Printer Working Group)

Used for: Dimensions of North American, engineering, architectural, ISO C, envelope and JIS B sizes as used by printers and print software, including the A2, #10, 6 x 9 and 9 x 12 envelopes.

• Markus Kuhn, International standard paper sizes (University of Cambridge Computer Laboratory)

Used for: Rounding down to the millimetre after each halving, the tolerances of ISO 216, the C series as geometric mean of A and B, and which sheet fits which envelope.

• Microsoft Learn, Paper sizes (Windows print constants)

Used for: Second reference for Letter, Legal, Tabloid, Statement, Executive and JIS B sheet dimensions.

• Envelopes.com, envelope sizes

Used for: Dimensions of the A2, A6, A7 and A9 announcement envelopes, #10, 6 x 9 and 9 x 12, what each is used for, and the advice to choose an envelope 1/4 inch larger than the height and the width of the piece.

• USPS, Domestic Mail Manual 101, physical standards for retail letters, flats and parcels

Used for: Letters are 5 to 11 1/2 inches long and 3 1/2 to 6 1/8 inches high; a letter with a length to height ratio below 1.3 or above 2.5 is nonmachinable; flats are up to 15 by 12 inches.