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Which envelope suits your mailing?

For an A4 letter, DL (110 × 220 mm) and C6/5 (114 × 229 mm) are the usual choices: folded twice in parallel, the sheet fits both. C5 takes A5 or A4 folded once, C4 takes unfolded A4. C5 stays in the smallest letter class in AT, CZ and SK; in DE it falls under Großbrief or Dialogpost Groß.

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Choose the envelope for a mailing by what goes inside it first: how big is the letter once folded, what else is enclosed, and should the address show through a window? The post then decides whether the item stays in the smallest letter class. This page sorts the common sizes by both, for Austria, Germany, Czechia and Slovakia.

Envelope sizes and what fits inside

The sizes come from the standards for envelopes: DIN 678-1 (1998 edition) is current, while the international standard ISO 269 has been withdrawn. [3] [4]

The dimension tables are only published in the paid standard texts. The dimensions on this page therefore rest on two independent secondary sources that agree with each other. [1] [2]

Common envelope sizes with dimensions and suitable contents
SizeDimensionsFits insideNote
DL110 × 220 mmA4 folded twice in parallel (one third of A4, 99 × 210 mm); DL leafletsAccording to one of the two secondary sources, slightly too small for reliable machine insertion. [2]
C6/5 (also C6/C5)114 × 229 mmSame as DL: A4 folded twice in parallel, with more room to spareIntroduced in DIN 678 as an alternative to DL for machine insertion. [2]
C6114 × 162 mmA6; A5 folded once; A4 cross-folded twiceAn A4 letter has to be cross-folded to fit.
C5162 × 229 mmA5 unfolded; A4 folded onceTakes A5 inserts and booklets without further folding.
C4229 × 324 mmA4 unfoldedFalls outside the smallest postal letter class in all four countries (see below).
⅓ A4DL110 × 220 mm⅓ A4C6/5114 × 229 mmA5C5162 × 229 mmA4C4229 × 324 mmA4folded twice in parallel= 99 × 210 mm, for DL and C6/5
The four usual mailing envelopes at the same scale, with suitable contents in grey. Bottom right: an A4 sheet folded twice in parallel measures 99 × 210 mm and fits DL and C6/5.

For bulky items there is the B series; each B envelope takes the C envelope with the same number: B6 (125 × 176 mm), B5 (176 × 250 mm) and B4 (250 × 353 mm). E4 (280 × 400 mm) takes a B4 envelope. [1]

The size name alone is rarely enough for a brief: also say how the letter is folded and which inserts go in with it. The differences between folds are explained in our guide to fold types.

Window envelopes: where the address window sits

Under DIN 680, a window envelope in DL, C6/5 or C6 has a 45 × 90 mm window, 20 mm from the left edge and 15 mm from the bottom edge. DIN 680 is also only available for a fee; size and position rest on the same two secondary sources. For C5 and C4 the position depends on the form (A or B). For C4 the sources contradict each other, so we give no millimetre values for it. [1] [2] [5]

The posts set their own requirements for the window:

  • Austria: minimum size 35 × 90 mm; no windows on the front other than the address window. An address carrier in a window envelope must be at least 90% of the envelope's length and width. [7]
  • Germany: normally 90 × 45 mm; at least 40 mm from the top edge, at least 15 mm from the other edges, and a clear zone of at least 10 mm around the window. [6]
  • Czechia: window in the lower left or lower right of the address side, at least 15 mm from the side edge, 27 mm from the bottom and 40 mm from the top; at least 35 × 70 mm, at most 43 × 105 mm (height × length). [12] [14]
  • Slovakia: we found no window dimensions in Slovenská pošta's terms; envelopes should be white or a light pastel colour. [15]

For Czechia, the DIN window does not simply work: under DIN 680 it sits 15 mm above the bottom edge, while Česká pošta requires at least 27 mm. The window height of 45 mm also exceeds the 43 mm that Česká pošta gives as its maximum. For mailings to Czechia, check the address zone against Česká pošta's rules before ordering envelopes. [12]

Postal size limits in AT, DE, CZ and SK

As of 23 October 2026; the posts' current terms are authoritative. The table shows whether an envelope fits the smallest letter class; thickness and weight also decide.

Do the envelope sizes fit the smallest postal letter class?
CountrySmallest letter classDL and C6/5C5C4
Austria [8][9]Brief S and M, Info.Mail Werbung Classic: up to 235 × 162 × 5 mm; Brief S up to 20 g, Brief M up to 75 g, Info.Mail Classic up to 50 gfitsfits (162 × 229 mm is within the limit)larger class: Päckchen S up to 353 × 250 × 30 mm or Info.Mail Werbung Plus up to 324 × 229 × 24 mm
Germany [10][11]Standardbrief and Dialogpost Standard: length up to 235 mm, width up to 125 mm, thickness up to 5 mm; Standardbrief up to 20 g, Dialogpost Standard up to 50 gfitslarger class: Großbrief or Dialogpost Groß, up to 353 × 250 mmlarger class: Großbrief or Dialogpost Groß
Czechia [12][13][14]obyčejné psaní – standard: up to 231 × 164 × 5 mm, 2.5 to 50 gfitsfits; Česká pošta explicitly lists 162 × 229 mm as permittedlarger class: obyčejné psaní or Obchodní psaní up to 353 × 250 × 20 mm
Slovakia [15]štandardná listová zásielka: up to 235 × 165 × 5 mm, up to 50 gfitsfitslarger class: listová zásielka plochého tvaru up to 350 × 250 × 30 mm

For machine-processable items, three Česká pošta documents give different widths: up to 120 mm with 162 × 229 mm as an exception, up to 162 mm, and up to 164 mm. C5 is permitted under all three. [12] [13] [14]

Whether a mailing also stays in the smallest class by weight depends on the paper, the number of sheets and the inserts. The paper weight calculator works out the weight and matches it to each country's postal weight classes.

Envelope paper: what the posts require

For the envelope itself, the posts in Germany and Austria set a minimum paper weight; for Czechia and Slovakia we found none.

  • Austria: at least 70 g/m², with the flaps glued right up to the outer edge. [7]
  • Germany: Standardbrief and Kompaktbrief at least 70 g/m², with the flap firmly glued along its full length. Großbrief and Maxibrief: up to C5 at least 70 g/m², larger than C5 at least 100 g/m²; envelopes with an unfilmed window at least 120 g/m². [6]
  • Czechia: no minimum paper weight found. For Obchodní psaní, envelopes must be sealed along the full length of the flap, and the window film must be colourless. [14]
  • Slovakia: no minimum paper weight given; the envelope paper must support machine processing. [15]

How to read paper weights in general is explained in our guide to paper weight.

Sources and context

All sources checked on 23 October 2026. The texts of DIN 678-1 and DIN 680 are paid; dimensions and window position rest on secondary sources 1 and 2, which agree. The postal details come from the posts' own terms; we deliberately leave out prices. Text and drawing are our own work.

  1. Wikipedia (German): Briefumschlag — Secondary source: sizes under DIN 678-1 and window position under DIN 680.
  2. Markus Kuhn, University of Cambridge: International standard paper sizes — Secondary source: envelope sizes, suitable contents and the origin of C6/5.
  3. DIN: DIN 678-1:1998-01 Envelopes – Part 1: Sizes — Status of the standard (current).
  4. NSAI: ISO 269:1985 Correspondence envelopes — Status of the standard (withdrawn).
  5. DIN Media: DIN 680:2008-02 Window envelopes — Status of the standard (current).
  6. Deutsche Post: Automationsfähige Briefsendungen (January 2026) — Window in section 3.1.2, envelope paper in 4.1 and 6.1.
  7. Austrian Post: Voraussetzungen für die Maschinenfähigkeit (valid from 1 April 2020) — Window, address carrier and envelope paper (section 2.3.1).
  8. Austrian Post: AGB Brief National (valid from 1 August 2026) — Sizes and weights of Brief S, Brief M and Päckchen S (point 3.5.1).
  9. Austrian Post: product and price list Info.Mail Werbung (valid from 1 January 2026) — Classic and Plus sizes.
  10. Deutsche Post: Briefe in Deutschland — Sizes and weights of Standard, Kompakt, Groß and Maxi letters.
  11. Deutsche Post: Dialogpost — Standard and Groß sizes.
  12. Česká pošta: Provedení a úprava adresní strany obálek a psaní — Window position and machine-processable envelope sizes.
  13. Česká pošta: Poštovní podmínky, Základní poštovní služby (version of 1 October 2026) — Sizes of obyčejné psaní (article 11).
  14. Česká pošta: Poštovní podmínky, Ostatní služby (version of 1 October 2026) — Obchodní psaní: sizes, window and sealing.
  15. Slovenská pošta: Poštové podmienky, Listová zásielka (effective from 1 January 2026) — Sizes, envelope colour and envelope paper.
  16. Wikipedia (Czech): ISO 269 — Secondary source: Czech size names and adoption as ČSN ISO 269.