Choosing the wrong envelope can ruin a mailing, a wedding invitation, or a product sample. This guide covers the ISO C series, the DL envelope, North American formats, specialty envelopes, and how to match an envelope to its contents.
The ISO C Series (ISO 269)
The C series is the international standard for envelopes. Each size is designed to hold the matching A size flat — a C4 holds an A4 sheet unfolded, a C5 holds an A5 sheet, and so on.
| Format | mm | cm | inches | Holds |
|---|---|---|---|---|
| C0 | 917 × 1297 | 91.7 × 129.7 | 36.1 × 51.1 | A0 |
| C1 | 648 × 917 | 64.8 × 91.7 | 25.5 × 36.1 | A1 |
| C2 | 458 × 648 | 45.8 × 64.8 | 18.0 × 25.5 | A2 |
| C3 | 324 × 458 | 32.4 × 45.8 | 12.8 × 18.0 | A3 |
| C4 | 229 × 324 | 22.9 × 32.4 | 9.0 × 12.8 | A4 |
| C5 | 162 × 229 | 16.2 × 22.9 | 6.4 × 9.0 | A5 / A4 folded once |
| C6 | 114 × 162 | 11.4 × 16.2 | 4.5 × 6.4 | A6 / A4 folded twice |
| C7 | 81 × 114 | 8.1 × 11.4 | 3.2 × 4.5 | A7 |
| C8 | 57 × 81 | 5.7 × 8.1 | 2.2 × 3.2 | A8 |
| C9 | 40 × 57 | 4.0 × 5.7 | 1.6 × 2.2 | A9 |
| C10 | 28 × 40 | 2.8 × 4.0 | 1.1 × 1.6 | A10 |
The DL Envelope
The DL envelope (110 × 220 mm, 4.33 × 8.66 in) is not part of the C series, but it is the most common business envelope in Europe. It holds an A4 sheet folded in thirds (a standard letter fold). It is the European equivalent of the US #10 envelope.
| Property | Value |
|---|---|
| Size | 110 × 220 mm |
| Inches | 4.33 × 8.66 in |
| Holds | A4 folded in thirds |
| Use | Invoices, letters, mailings |
North American Envelope Sizes
The US uses a numbering system for business envelopes and a letter system for announcement and invitation envelopes.
| Name | inches | mm | Use |
|---|---|---|---|
| #10 (Business) | 4.125 × 9.5 | 104.8 × 241.3 | Standard business letters |
| #9 | 3.875 × 8.875 | 98.4 × 225.4 | Personal letters, reply mail |
| #8⅝ | 3.625 × 8.625 | 92.1 × 219.1 | Return envelopes |
| #7¾ | 3.875 × 7.5 | 98.4 × 190.5 | Small business mail |
| #6¾ | 3.625 × 6.5 | 92.1 × 165.1 | Personal correspondence |
| #5½ (Bar) | 3.125 × 5.5 | 79.4 × 139.7 | Notes, invitations |
| #6¼ | 3.5 × 6 | 88.9 × 152.4 | Notes |
Announcement and Invitation Envelopes (US)
| Name | inches | mm | Use |
|---|---|---|---|
| A2 | 4.375 × 5.75 | 111 × 146 | Invitations, notes |
| A6 | 4.75 × 6.5 | 121 × 165 | Photo cards, invitations |
| A7 | 5.25 × 7.25 | 133 × 184 | Wedding invitations |
| A8 | 5.5 × 8.125 | 140 × 206 | Cards, invitations |
| A9 | 5.75 × 8.75 | 146 × 222 | Cards, letters |
| A10 | 6 × 9.5 | 152 × 241 | Cards, letters |
| #6¼ | 3.5 × 6 | 88.9 × 152.4 | RSVP cards |
| #6⅞ | 3.625 × 6.875 | 92.1 × 174.6 | RSVP, replies |
Square Envelopes
Square envelopes are used for invitations, greeting cards, and small gifts. They usually require extra postage because they are not machine-readable by some postal services.
| Size (inches) | mm | Use |
|---|---|---|
| 4 × 4 | 102 × 102 | Gift cards, notes |
| 5 × 5 | 127 × 127 | Small invitations |
| 5.5 × 5.5 | 140 × 140 | Invitations |
| 6 × 6 | 152 × 152 | Greeting cards |
| 6.5 × 6.5 | 165 × 165 | Invitations |
| 7.5 × 7.5 | 190 × 190 | Wedding invitations |
| 8.5 × 8.5 | 216 × 216 | Large invitations |
Booklet and Catalog Envelopes
Booklet envelopes have the opening on the long edge, which allows thick contents to slide in easily. They are used for catalogs, brochures, and magazines.
| Name | inches | mm | Use |
|---|---|---|---|
| Booklet 6 × 9 | 6 × 9 | 152 × 229 | Brochures, cards |
| Booklet 6.5 × 9.5 | 6.5 × 9.5 | 165 × 241 | Brochures |
| Booklet 7 × 10 | 7 × 10 | 178 × 254 | Catalogs |
| Booklet 9 × 12 | 9 × 12 | 229 × 305 | Catalogs, magazines |
| Booklet 10 × 13 | 10 × 13 | 254 × 330 | Large catalogs |
Window and Specialty Envelopes
Window envelopes have a transparent window that shows the recipient's address printed on the contents. This saves time and reduces addressing errors.
- Single window — one window for the address
- Double window — address and return address
- Full-face window — a large window that shows the whole document
- Security tint — patterned interior that hides contents
- Self-seal — peel-and-stick closure
- Peel-and-seal — same as self-seal, common in offices
Matching Envelope to Contents
| Contents | Recommended envelope |
|---|---|
| A4 unfolded | C4 |
| A4 folded once (A5) | C5 |
| A4 folded twice (A6) | C6 |
| A4 folded in thirds | DL |
| US Letter folded in thirds | #10 (Business) |
| US Letter unfolded | 9 × 12 booklet |
| 5 × 7 invitation | A7 |
| 4 × 6 photo | A6 |
| 3.5 × 5 photo | A2 |
Envelope Paper Weight
Envelopes are usually made from 80–120 gsm paper. Heavier envelopes (120 gsm+) feel more premium but cost more to mail.
| GSM | Use |
|---|---|
| 80–90 gsm | Everyday mail |
| 100–110 gsm | Business correspondence |
| 120–140 gsm | Premium, invitations |
| 160+ gsm | Luxury, cards |
Postal Regulations and Surcharges
- Square envelopes often require extra postage
- Rigid or thick envelopes may be classified as parcels
- Window envelopes must have the window positioned to avoid the address area
- Minimum size for machine-readable mail: 90 × 140 mm
- Maximum size for standard letters varies by country
Frequently Asked Questions
What size is a C4 envelope?
229 × 324 mm — holds an unfolded A4 sheet.
What is a DL envelope?
110 × 220 mm — holds an A4 sheet folded in thirds.
What size is a #10 envelope?
4.125 × 9.5 inches (104.8 × 241.3 mm) — standard US business envelope.
What envelope fits a 5 × 7 card?
A7 (5.25 × 7.25 in) or A6 (4.75 × 6.5 in) depending on the card.
What envelope fits an A4 sheet without folding?
C4.
Do square envelopes cost more to mail?
Often yes — many postal services charge a surcharge.
What is a booklet envelope?
An envelope with the opening on the long edge, used for catalogs and brochures.
What is a window envelope?
An envelope with a transparent window showing the address printed on the contents.
Conclusion
Envelope sizes follow two main systems: the ISO C series (international) and the North American numbering system. Choose C4 for unfolded A4, C5 for folded A4, DL for letters folded in thirds, and #10 for US business mail. Match the envelope to the contents and check postal regulations before mailing at scale.