Introduction
International envelope sizes follow the ISO 269 standard, and the four you meet most are DL, C4, C5, and C6. If your business mails to the UK, EU, or Australia, these sizes are the default — not the US #10. This guide gives you the exact dimensions, the mm-to-inch conversion, and a direct comparison against US standards so you can spec envelopes for overseas campaigns without guessing.
For the full master reference, our envelope size chart collects every common format in one table. Here we focus on the ISO series, how each size nests inside the next, and how they map to the US envelopes your US-based team may already use, such as the number 10 envelope size.
ISO 269: The International Standard
ISO 269 defines the C series of envelopes so that a folded sheet of A-series paper drops in cleanly. The logic is geometric: each size is half the previous one.
| Standard | Region | Series |
|---|
| ISO 269 | International | C (C4, C5, C6, DL) |
| US custom | United States | #10, #9, etc. |
| ISO 216 | Paper (not envelope) | A series |
DL, C4, C5, C6 Defined
| Size | Dimensions (mm) | Dimensions (inch, approx) | Fits |
|---|
| DL | 220 × 110 | 8.66 × 4.33 | A4 folded twice / A5 folded once |
| C4 | 324 × 229 | 12.75 × 9.02 | Unfolded A4 |
| C5 | 229 × 162 | 9.02 × 6.38 | A4 folded once / A5 |
| C6 | 162 × 114 | 6.38 × 4.49 | A4 folded twice / A6 |
How ISO Sizes Derive
| Relation | Math |
|---|
| C5 = half C4 | Fold C4 once |
| C6 = half C5 | Fold C5 once |
| DL ≈ 1/3 A4 | Fold A4 twice |
DL Envelope Size
The DL is the workhorse of European business mail — the equivalent role of the US #10.
| Attribute | DL Value |
|---|
| Width | 220 mm (8.66 in) |
| Height | 110 mm (4.33 in) |
| Common use | Invoices, statements, letters |
| Fits | A4 tri-fold, A5 folded |
Because the DL envelope size is so common overseas, most UK/EU mailing houses standardize window positions to it.
C4 Envelope Size
| Attribute | C4 Value |
|---|
| Width | 324 mm (12.75 in) |
| Height | 229 mm (9.02 in) |
| Common use | Unfolded A4 documents |
| Fits | Full A4, magazines (small) |
C5 Envelope Size
The C5 envelope size is the international middle ground — a flat A4 folded once, or an A5 inserted directly.
| Attribute | C5 Value |
|---|
| Width | 229 mm (9.02 in) |
| Height | 162 mm (6.38 in) |
| Common use | Booklets, A5 mailers |
| Fits | A4 folded once, A5 flat |
C6 Envelope Size
| Attribute | C6 Value |
|---|
| Width | 162 mm (6.38 in) |
| Height | 114 mm (4.49 in) |
| Common use | Small invites, RSVP |
| Fits | A6, A4 folded twice |
International vs US Standards: Conversion Table
Mapping ISO sizes to US formats helps a single artwork spec serve both markets.
| ISO Size | Nearest US | US Inches (approx) | Note |
|---|
| DL | #10 (close) | 9.50 × 4.13 | Similar but not equal |
| C4 | 9×12 / 10×13 | 9×12 | Unfolded A4 |
| C5 | 6×9 (close) | 6×9 | A4 folded once |
| C6 | A2 (close) | 4.38×5.75 | Small invite |
DL vs Number 10
| Feature | DL | #10 |
|---|
| mm | 220 × 110 | 241 × 105 |
| inch | 8.66 × 4.33 | 9.50 × 4.13 |
| Paper | A4 tri-fold | Letter folded |
C5 vs US 6×9
| Feature | C5 | 6×9 |
|---|
| mm | 229 × 162 | 229 × 152 |
| Difference | Slightly wider | US standard |
C4 vs US Large
| Feature | C4 | 9×12 |
|---|
| mm | 324 × 229 | 229 × 305 |
| Note | Unfolded A4 | US flat doc |
C6 vs US A2
| Feature | C6 | A2 |
|---|
| mm | 162 × 114 | 111 × 146 |
| Note | Not identical | Invite size |
Fits-Inside Nesting
One of the neatest ISO properties is nesting — a smaller envelope fits inside the next size with the insert.
| Outer | Holds Inner |
|---|
| C4 | C5, C6, DL |
| C5 | C6, DL |
| C6 | (smallest) |
Which Size for Which Mail
| Mail Type | Best ISO Size |
|---|
| Invoice/letter | DL |
| Flat A4 contract | C4 |
| Booklet/A5 | C5 |
| Invitation/RSVP | C6 |
| Small promo | DL or C6 |
For event and wedding work, our invitation envelope sizes guide expands on C6 and decorative formats. For company stationery, see corporate envelope size.
Regional Notes: UK / EU / AU
| Region | Default Size | Note |
|---|
| UK | DL | Standard business |
| EU | DL / C5 | Varies by country |
| Australia | DL | C4/C5 for flats |
Common Documents and Matching Envelopes
| Document | ISO Envelope |
|---|
| A4 letter | DL (tri-fold) |
| A4 flat | C4 |
| A5 leaflet | C5 |
| A6 card | C6 |
| Greeting card | C6 / C5 |
Window Envelope Sizes International
| Size | Window Typical Position |
|---|
| DL window | Front, lower-right |
| C5 window | Front, lower area |
| C4 window | Rare, large doc |
Weight and Postage Implications
| Size | Empty Weight (gsm stock, approx) |
|---|
| DL | 5–8 g |
| C5 | 8–12 g |
| C4 | 12–20 g |
| C6 | 4–7 g |
Postage bands differ by country, so confirm local rates; the table shows typical empty weights only.
Printing and Artwork Safe Area
| Size | Safe Margin (mm, approx) |
|---|
| DL | 10–15 from edges |
| C5 | 12–15 |
| C4 | 15 |
| C6 | 10 |
Quick Selection Decision
| If You Mail… | Choose |
|---|
| A4 tri-fold letter | DL |
| Unfolded A4 | C4 |
| A5 booklet | C5 |
| Card/invite | C6 |
| US-only | #10 (see US guide) |
FAQ
What is the DL envelope size in inches? DL is 220 × 110 mm, or about 8.66 × 4.33 inches — the standard European business envelope, close to but not the same as a US #10.
Is C5 bigger than DL? Yes. C5 (229 × 162 mm) holds an A4 sheet folded once, while DL holds an A4 sheet folded twice; C5 is the larger of the two.
What is the difference between international and US envelope sizes? International sizes follow ISO 269 (DL, C4, C5, C6) based on A-series paper; US sizes like #10 are inch-based and not directly interchangeable.
Can I use a #10 envelope for A4 paper? Not flat. A4 is larger than Letter; a #10 holds Letter folded. For A4 use DL (tri-fold) or C4 (flat).
Which ISO size fits an unfolded A4? C4 (324 × 229 mm) fits an unfolded A4 sheet; C5 fits A4 folded once.
Are C6 and A2 the same? They are close but not identical — C6 is 162 × 114 mm and US A2 is about 111 × 146 mm. Use the correct one per market.
Conclusion
International envelope sizes are governed by ISO 269, and DL, C4, C5, and C6 cover virtually all UK, EU, and Australian mailing needs. DL is your overseas #10, C5 your A5/booklet size, C4 your flat A4, and C6 your small invite — and each nests neatly inside the next. Keep the mm-to-inch conversions above handy so one artwork spec serves both markets. For the complete master list, use our envelope size chart; for US-only specs, the number 10 envelope size page.
Mailing overseas and need ISO-size envelopes printed? Request a quote with your target market and we will match DL, C4, C5, or C6 to your contents.