Introduction
The C-series envelope sizes — C4, C5, C6, and the DL format — are the international standard, used across the UK, Europe, and Australia. They're sized to hold the matching A-series paper (A4, A5, and folded variants), which is the standard paper size outside North America.
For businesses mailing internationally, understanding C-series sizes matters. This guide covers the dimensions, what each holds, and the common size-comparison questions.
The C-Series at a Glance
The C-series is keyed to ISO A-series paper.
| Size | Dimensions | Holds |
|---|---|---|
| C4 | 229 mm × 324 mm | A4 flat |
| C5 | 162 mm × 229 mm | A4 folded in half |
| C6 | 114 mm × 162 mm | A4 folded twice |
| DL | 110 mm × 220 mm | A4 folded in thirds |
The C-series envelopes nest with A-series paper:
| A-paper | C-envelope |
|---|---|
| A4 flat | C4 |
| A4 half-fold | C5 |
| A4 quarter | C6 |
| A4 tri-fold | DL |
C5 vs C4
The most common comparison — which one for an A4 document.
| Factor | C5 | C4 |
|---|---|---|
| Dimensions | 162 × 229 mm | 229 × 324 mm |
| Holds | A4 folded in half | A4 flat |
| Folds | One fold | None |
The choice is whether you fold the A4:
| If you… | Choose |
|---|---|
| Fold A4 in half | C5 |
| Keep A4 flat | C4 |
| Fold A4 in thirds | DL |
DL Envelope Size
The DL format is the standard business letter envelope internationally.
| Trait | DL detail |
|---|---|
| Dimensions | 110 × 220 mm |
| Holds | A4 in thirds |
| Equivalent | Like US #10 |
DL is the international counterpart to the #10 — the standard business letter:
| Use | DL |
|---|---|
| Business letter | Yes, standard |
| Invoice | Common |
| A4 tri-fold | Exact fit |
C-Series Dimensions in Detail
The full C-series reference:
| Size | mm | Approx inches | Fits |
|---|---|---|---|
| C4 | 229 × 324 | 9 × 12¾ | A4 flat |
| C5 | 162 × 229 | 6.4 × 9 | A4 half |
| C6 | 114 × 162 | 4.5 × 6.4 | A4 quarter |
| DL | 110 × 220 | 4.3 × 8.7 | A4 thirds |
These correspond to the US sizes roughly:
| C-series | Rough US equivalent |
|---|---|
| C4 | 9×12 |
| C5 | 6×9 |
| C6 | A6 |
| DL | #10 |
When to Use C-Series
C-series envelopes matter when mailing internationally or to A-paper regions.
| Scenario | C-series needed? |
|---|---|
| Mail to UK/EU/AU | Yes |
| Mail within US | No (use US sizes) |
| International business | Yes |
| Global direct mail | Yes |
For businesses with international clients, knowing C-sizes avoids misfit mail:
| Need | C-size |
|---|---|
| A4 document flat | C4 |
| A4 letter | C5 or DL |
| A5 booklet | C5 or C6 |
Choosing the Right C-Size
A quick decision guide:
| If you mail… | Choose |
|---|---|
| A4 flat | C4 |
| A4 half-fold | C5 |
| A4 tri-fold | DL |
| A5 card | C5 or C6 |
| Small note | C6 |
For the broader international picture, see our international envelope sizes guide.
Want the printable version of this table with every format side by side? See the envelope size chart, which links each C-series format to its own page: C4, C5, B6 and DL.
Conclusion
The C-series envelope sizes — C4, C5, C6, and DL — are the international standard, keyed to A-series paper: C4 for flat A4, C5 for half-folded A4, C6 for quarter-folded, and DL for the standard business letter. For businesses mailing to the UK, EU, or Australia, matching the C-size to the A-paper is essential. For the full international context, see our international envelope sizes guide.
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