Quick answer: The C series is a family of envelope sizes proportioned to hold corresponding A-series sheets. C4 is 229 × 324 mm for A4 flat; C5 is 162 × 229 mm for A5 or A4 folded once; C6 is 114 × 162 mm for A6 or A4 folded twice.
C sizes sit geometrically between corresponding A and B sizes, giving an A sheet enough clearance to slide into an envelope. The size names remain widely used by envelope manufacturers and postal users.
Measurement note: dimensions are stated width × height in portrait orientation. Turn the sheet through 90 degrees for landscape use. Millimetres are the controlling values for ISO sizes; inch figures are rounded conversions.
C0 to C10 size chart
| Size | Dimensions | Inches | Typical relationship |
|---|---|---|---|
| C0 | 917 × 1297 mm | 36.10 × 51.06 in | Holds A0 flat |
| C1 | 648 × 917 mm | 25.51 × 36.10 in | Holds A1 flat |
| C2 | 458 × 648 mm | 18.03 × 25.51 in | Holds A2 flat |
| C3 | 324 × 458 mm | 12.76 × 18.03 in | Holds A3 flat |
| C4 | 229 × 324 mm | 9.02 × 12.76 in | Holds A4 flat |
| C5 | 162 × 229 mm | 6.38 × 9.02 in | Holds A5 flat |
| C6 | 114 × 162 mm | 4.49 × 6.38 in | Holds A6 flat |
| C7 | 81 × 114 mm | 3.19 × 4.49 in | Small envelope format |
| C8 | 57 × 81 mm | 2.24 × 3.19 in | Small envelope format |
| C9 | 40 × 57 mm | 1.57 × 2.24 in | Small envelope format |
| C10 | 28 × 40 mm | 1.10 × 1.57 in | Small envelope format |
Which envelope fits which paper?
| Paper or card | Envelope | How it fits |
|---|---|---|
| A4 | C4 | Unfolded |
| A4 | C5 | Folded once to A5 |
| A4 | C6 | Folded twice to A6 |
| A5 | C5 | Unfolded |
| A5 | C6 | Folded once to A6 |
| A6 card | C6 | Unfolded with clearance |
C sizes and ISO status
C envelope dimensions were specified in ISO 269. The ISO catalogue now lists ISO 269:1985 as withdrawn. The withdrawal does not make familiar C4, C5 and C6 products disappear, but a regulated or contractual job should rely on the current specification required by the buyer, postal operator or manufacturer rather than assuming the withdrawn document is controlling.
Envelope fit is not only about width and height
An envelope must accommodate paper thickness, card weight, folds, inserts and manufacturing tolerances. A heavy folded invitation with multiple inserts may need a larger envelope or a gusset even when the nominal flat dimensions appear to fit. Window position, sealing method and automated inserting equipment can add further requirements.
Postal checks
Envelope size does not by itself determine postage. In the UK, check Royal Mail’s current letter and large-letter limits. In the US, check USPS dimensions, thickness, aspect ratio and machinability. The dedicated postcard guide covers card mail in more detail.
Frequently asked questions
Is C4 bigger than A4?
Yes. C4 is 229 × 324 mm, providing clearance around an A4 sheet measuring 210 × 297 mm.
Can A4 fit in C6?
Yes, when A4 is folded twice to A6 size. The fold pattern and contents may affect thickness.
Is DL a C-series size?
DL is a separate commonly used envelope designation. It measures 110 × 220 mm and is designed for A4 folded into thirds.
Test the complete mailed item
Postal success for C-Series Paper Sizes Explained depends on the assembled item, not the nominal paper size. Build a dummy using the intended card, folds, inserts, adhesive and envelope. Weigh it, measure its maximum thickness without compressing it, and check whether embellishments create rigid or uneven areas. A piece that fits while empty may bow, split a seam or move into another price category once assembled.
Before producing a campaign quantity of C-Series Paper Sizes Explained, take the finished dummy to the postal operator or mailing house. Bulk services can impose rules for address position, clear zones, barcodes, return addresses, tabs and machinability that do not apply to a single handwritten item.
Content and accessibility checks
On C-Series Paper Sizes Explained, use a clear hierarchy for the recipient name, date, address, response deadline or call to action. Avoid placing essential information over folds, textured backgrounds or metallic finishes. Address panels should have strong contrast. A QR code should supplement, not replace, a readable URL or essential instruction; test it on the final printed stock under ordinary indoor light.
How to specify the size without ambiguity
For C-Series Paper Sizes Explained, a reliable print specification states more than the familiar format name. Give the finished width and height, measurement unit, orientation, number of pages or panels, paper weight, surface, colour process, bleed, finishing and quantity. This prevents regional names and printer defaults from silently changing this particular job.
- Finished size: write width × height and identify portrait or landscape.
- Artwork size: distinguish the trimmed page from any larger bleed canvas or parent sheet.
- Stock: specify grammage or basis weight, coating, colour and recycled or certified requirements.
- Finishing: state folds, scoring, binding, drilling, corner rounding, lamination or mounting.
- Tolerance: ask the supplier what cutting, folding and registration variation should be expected.
When ordering C-Series Paper Sizes Explained, a short name alone leaves important decisions unstated. A specification such as “finished size as stated above; portrait; 3 mm bleed; 170 gsm uncoated white stock; full colour both sides; scored where shown” is harder to misinterpret. When the format has regional variants, include the governing standard or exact dimensions every time.
Make a full-size physical proof
Screens do not communicate the physical scale of C-Series Paper Sizes Explained reliably. Before a large order, print a representative page at 100%, trim it to the stated size and use it as the finished item will be used. Measure one horizontal and one vertical feature with a ruler. Fold it, place it in its envelope or holder where relevant, view it from the intended distance and ask someone unfamiliar with the design to find the main information.
A proof of C-Series Paper Sizes Explained should check more than spelling. Look for text that is technically present but difficult to read, lines that sit too close to the trim, images whose important details enter a fold or binding area, barcodes or QR codes that do not scan, and stocks that crack or show through. For colour-critical work, a calibrated contract proof or press proof is more informative than an office print.
Sources and standards
The measurements and production guidance in this guide were checked against the following standards bodies, postal operators and print-platform specifications. Standards can be revised or withdrawn, so check the linked record when a contractual or regulated specification matters.