C-Series Envelope Sizes: Complete ISO 269 Guide from C0 to C10

The C-series envelope sizes follow the ISO 269 standard, specifically designed to hold A-series paper. A C4 envelope perfectly fits an unfolded A4 sheet, C5 fits A4 folded once, and C6 fits A4 folded twice. This elegant system makes choosing the right envelope straightforward.

Here's everything you need to know about C-series envelope dimensions.

The C-Series System

C-series dimensions fall exactly between A-series and B-series sizes. This mathematical relationship ensures:

• A C-size envelope fits the same number A-size paper flat
• C4 envelope → A4 paper unfolded
• C5 envelope → A4 paper folded once (or A5 flat)
• C6 envelope → A4 paper folded twice (or A6 flat)

Complete C-Series Reference

Dimensions at a Glance

C4 - A4 Envelope (229 × 324 mm)

C4 is the standard envelope for sending A4 documents flat, without folding. Essential for professional correspondence where document presentation matters.

Common applications:

• Business documents
• Legal papers
• Certificates
• Photographs
• Contracts

C4 Design Considerations

When designing C4 envelopes:

• Address window placement (if used)
• Return address area
• Postage area (top right)
• Branding elements

C5 - Half A4 Envelope (162 × 229 mm)

C5 is the most popular business envelope size. It fits A4 folded once or A5 sheets flat.

Common applications:

• Business letters
• Invoices
• Marketing materials
• Greeting cards
• A5 booklets

C5 Variants

C5 with Window: Standard business envelope with address window C5 Pocket: Opens on the short edge for easier insertion

C6 - Quarter A4 Envelope (114 × 162 mm)

C6 fits A4 folded twice, A5 folded once, or A6 cards flat. Popular for personal correspondence and invitations.

Common applications:

• Personal letters
• Greeting cards
• Invitations
• RSVPs
• A6 postcards

C6/C5 (C65) Envelope (114 × 229 mm)

This in-between size comfortably fits A4 tri-folded letters and thicker inserts. It’s sometimes marketed as C65. Note that it’s slightly larger than the true DL standard (110 × 220 mm), giving your contents a bit more breathing room.

C3 - A3 Envelope (324 × 458 mm)

C3 holds A3 documents flat, used for larger prints and documents.

Common applications:

• Large documents
• Architectural drawings
• Art prints
• Certificates
• Posters

C2 - A2 Envelope (458 × 648 mm)

C2 fits A2 sheets flat, used for oversized documents.

C1 and C0 - Large Format

These oversized envelopes hold A1 and A0 sheets respectively. Typically custom-ordered.

C1 (648 × 917 mm)

C0 (917 × 1297 mm)

C7 and Smaller

Small C-sizes are used for specialized applications.

C7 - A7 Envelope (81 × 114 mm)

Small cards and notes.

C8 (57 × 81 mm)

Very small cards, tags.

Paper and Envelope Matching

Quick reference for matching papers to envelopes:

DL Envelope Format (110 × 220 mm)

While not strictly C-series, the DL (Dimension Lengthwise) format is extremely common. It's slightly narrower than C6/C5, so choose DL when you need a snug fit and C6/C5 when you want extra wiggle room.

DL dimensions:

• 110 × 220 mm
• 4.33 × 8.66 inches
• 1299 × 2598 pixels at 300 DPI

Fits A4 paper folded into thirds (the standard business letter fold).

Envelope Design Elements

Standard Layout

┌──────────────────────────────────────┐
│ Return Address          [Postage]    │
│ Company Name                         │
│ Street Address                       │
│ City, State ZIP                      │
│                                      │
│                                      │
│         ┌─────────────┐              │
│         │   Window    │   OR         │
│         │  (Address)  │   Recipient  │
│         └─────────────┘   Address    │
│                                      │
└──────────────────────────────────────┘

Window Envelope Placement

Standard window positions:

• Left window: 20mm from left, 15mm from bottom
• Window size: Approximately 45 × 90mm for C5

Postage Area

Reserve top-right corner (approximately 40 × 40mm) for stamps or franking.

Printing Specifications

Bleed

For professional envelope printing:

• Standard bleed: 3mm on all sides
• Extend designs past trim lines

Safe Zone

Keep critical text/elements:

• 5mm from edges
• Away from fold lines (for pocket styles)
• Clear of seal areas

Color

• CMYK for offset printing
• Consider envelope color (white, manila, colored)
• Test readability on envelope stock

Envelope Styles

Wallet (Banker) Style

Flap on long edge. Standard for business envelopes.

Pocket Style

Flap on short edge. Easier document insertion.

Window Envelopes

Transparent panel showing recipient address. Common sizes:

• C5 window: 45 × 90mm
• C6 window: 35 × 70mm
• DL window: 35 × 90mm

Self-Seal vs Gummed

• Self-seal: Peel-and-stick closure
• Gummed: Requires moisture to seal

International Mailing

C-series envelopes work internationally, but note:

• Postal regulations vary by country
• Maximum thickness limits apply
• Weight affects postage

Standard Postal Limits

Most countries allow:

• C6/C5 or DL: Standard letter rate (usually up to 20g)
• C5: May require large letter rate
• C4: Large letter or packet rate

Custom Envelope Design

When designing custom envelopes:

Branding Elements

• Logo placement (usually top-left)
• Brand colors
• Tagline or website

Print Areas

• Front: Maximum branding opportunity
• Back: Additional information, closures
• Flap: Often branded for presentation

Technical Specifications

Paper Weight

Common envelope paper weights:

• 80-90 gsm: Standard
• 100-120 gsm: Premium
• 120+ gsm: Heavy duty

Opacity

Higher gsm generally means better opacity. Important for:

• Confidential documents
• Presentation quality
• Preventing show-through

Regional Variations

Europe

C-series widely used alongside A-series paper.

North America

Less common; US envelope sizes (like #10) dominate. However, C-series used for international correspondence.

Asia Pacific

C-series used, particularly C5 and C6 for business.

Common Mistakes

1. Wrong paper-envelope match: A4 in C5 requires folding
2. Ignoring window position: Address misaligned with window
3. Postage area conflicts: Design covers stamp area
4. Forgetting fold lines: Design elements on creases
5. Insufficient margins: Text too close to edges
6. Wrong orientation: Portrait vs landscape confusion

Summary

The C-series provides a logical envelope system that perfectly complements A-series paper. When in doubt, remember: the C-number matches the A-number it holds flat.

Most used C-sizes:

• C4 (229 × 324 mm): A4 flat
• C5 (162 × 229 mm): A4 folded once, A5 flat
• C6 (114 × 162 mm): A4 folded twice, A6 flat
• DL (110 × 220 mm): A4 tri-folded

Choose the right envelope for your paper, and your mailings will look professional every time.