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.
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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