Envelope Sizes — C5, C6, DL, and #10

Envelope size is the inside rectangle that has to clear the insert, not the outside measurement of the flap. ISO 269 defines the C series so each envelope holds an A-series sheet, flat or folded. The numbers below are the standard sizes in millimeters, with inches rounded at 25.4 mm per inch. Markus Kuhn’s ISO paper notes include the C series and the DL business envelope (International standard paper sizes).

A chart of every series, including sizes too rare to spec by default, is on Paper Sizes. This page is the set people actually order.

ISO envelopes for A4

A4 is 210 × 297 mm. How you fold it picks the envelope.

Envelope	Millimeters	Inches (rounded)	What fits
C4	229 × 324	9.02 × 12.76	A4 flat, or A3 folded once
C5	162 × 229	6.38 × 9.02	A4 folded once (A5), or A5 flat
C6	114 × 162	4.49 × 6.38	A4 folded twice (A6), or A6 flat
DL	110 × 220	4.33 × 8.66	A4 folded in thirds
C6/C5	114 × 229	4.49 × 9.02	A4 folded in thirds, with more clearance than DL

C5 envelope size is the one for a booklet or a letter folded once. The insert is A5-shaped: 148 × 210 mm. C5 is 162 × 229 mm, so there is a few millimeters of play. A thick booklet still needs a check. The flap and the window eat into that play.

C6 envelope is smaller. The insert is A6-shaped: 105 × 148 mm. A greeting card or a thin folded letter belongs here. A 20-page document does not.

DL is the everyday business envelope outside North America. Fold A4 into three equal panels parallel to the short side and it drops in. DL is slightly tight for machine inserting. C6/C5, at 114 × 229 mm, is the roomier twin for the same fold. If a mail house complains that DL jams, specify C6/C5 instead of “a bigger DL.”

C4 is the flat mailer. Use it when the sheet must not be creased: a certificate, a drawing on A4, a poster folded only to A4 from A3. It does not go through a normal letter slot as happily as DL.

The sheet these envelopes assume is A4 Paper Size. If the insert is A3, you want C4 only after one fold, or a larger mailer. A3 itself is in A3 Paper Size.

US envelopes

US commercial envelopes are specified in inches. Two cover almost every letter.

Name	Inches	Millimeters	Typical insert
#10	4.125 × 9.5	104.8 × 241.3	US Letter folded in thirds
Monarch	3.875 × 7.5	98.4 × 190.5	A smaller personal or executive letter

#10 is the US default, the way DL is the ISO default. It holds US Letter folded in thirds (a panel about 8.5 × 3.67 in). It does not hold A4 folded in thirds as cleanly, because A4 is taller. If you mail from the US to a country on A4, either use a C4 or DL envelope that matches the insert, or accept a tighter fit and test one piece.

Monarch is narrower and shorter. It suits executive stationery and personal notes. It is not a #10 with a fancier flap. A Letter sheet folded in thirds is too wide for it.

These two are the American sizes worth putting in a template. Announcement and catalog envelopes exist in a long numbered list. Pick them from the printer’s stock table when the piece is a card or a booklet, not from memory.

How to spec the pair

Write both sizes on the job: “A4 insert, DL envelope” or “Letter insert, #10 envelope.” A window position is a third number. The window has to land on the address after the fold you actually designed. A DL window layout will not line up on a #10.

Leave clearance. ISO sizes already include a few millimeters so the insert slides in. A deckled edge, a thick cover, or a clip can erase that. Order one sample before you print the run.

The DL fold is easy to mis-measure. A4’s long side is 297 mm. Three equal panels are 99 mm each. The DL envelope is 110 mm on the short side, so each panel has about 11 mm of spare height, shared with the thickness of the fold. The other side of A4 is 210 mm, and DL is 220 mm long, so there is about 10 mm of spare width. A panel that is 105 mm because the fold was eyeballed will catch. Mark the two fold lines at 99 mm and 198 mm from the top of the sheet before you place a rule or a photograph across them.

A #10 fold is the same idea in inches. Letter’s 11-inch side divides into three panels of about 3.67 inches. The envelope is 4.125 inches tall. A design that ignores the crease will be sliced by it.

Bleed and full-bleed envelopes are a different object. The size in the table is the envelope’s face, not your artwork plus bleed. Ask the converter where the glue and the flap cover the print.

FAQ

What is a standard envelope size?

For an A4 letter folded in thirds, DL at 110 × 220 mm. For a US Letter sheet folded in thirds, #10 at 4.125 × 9.5 inches. “Standard” depends on the paper in the printer tray.

What are C5 envelope dimensions?

162 × 229 mm (6.38 × 9.02 in). It holds A4 folded once, or A5 flat.

What is a C6 envelope?

114 × 162 mm (4.49 × 6.38 in). It holds A4 folded twice, or A6 flat.

Is Monarch the same as #10?

No. Monarch is 3.875 × 7.5 inches. #10 is 4.125 × 9.5 inches. Monarch is for a smaller letter.

What to open next

Fold a blank sheet the way the reader will receive it, then slide it into the envelope you named. If it bows or catches the flap, change the envelope before you design the letterhead around a window. Series you are not mailing, such as B5 notebooks, are in B5 Paper Size. The rest of the chart is Paper Sizes.