Envelope Sizes Guide
Standard envelope dimensions for US and international mail.
| Envelope | Inches | mm | Fits |
|---|---|---|---|
| #10 (Business) | 4.125 x 9.5 | 105 x 241 | Letter folded in thirds |
| #9 | 3.875 x 8.875 | 98 x 225 | Letter folded in thirds |
| #6 3/4 | 3.625 x 6.5 | 92 x 165 | Personal letters |
| A2 (Invitation) | 4.375 x 5.75 | 111 x 146 | RSVP cards, invitations |
| A7 (Invitation) | 5.25 x 7.25 | 133 x 184 | 5x7 cards |
| 6x9 | 6 x 9 | 152 x 229 | Half-fold letter |
| 9x12 | 9 x 12 | 229 x 305 | Unfolded letter/A4 |
| C4 (ISO) | 9.02 x 12.76 | 229 x 324 | Unfolded A4 |
| C5 (ISO) | 6.38 x 9.02 | 162 x 229 | A4 folded once |
| C6 (ISO) | 4.49 x 6.38 | 114 x 162 | A4 folded twice |
| DL (ISO) | 4.33 x 8.66 | 110 x 220 | A4 folded in thirds |
Choosing the Right Envelope
For standard business correspondence in the US, the #10 envelope is the standard choice. For international mail, the DL envelope is most common. For unfolded documents, use 9x12 (US) or C4 (international). Invitation envelopes (A2, A7) are sized for greeting cards and invitations.
US Commercial Envelope Sizes
US envelope sizes use a numbering system where higher numbers generally indicate larger envelopes. The most important sizes for business and personal use are:
- #10 (4.125 × 9.5 in) — the definitive US business envelope. Holds a standard US Letter page folded in thirds. Used for invoices, contracts, statements, and general correspondence. Window versions are common for billing inserts.
- #9 (3.875 × 8.875 in) — slightly smaller than #10, fits inside a #10 for reply envelopes in billing packages.
- #6¾ (3.625 × 6.5 in) — a personal-size envelope for greeting cards, checks, and personal letters. Holds a Letter page folded in quarters.
- 6×9 in — fits a half-folded Letter page. Common for catalogs, booklets, and promotional mailers.
- 9×12 in (Catalog) — holds a US Letter or A4 sheet flat and unfolded. Essential when you cannot fold the contents.
ISO C-Series Envelope Sizes
The ISO 216 C-series envelopes are designed specifically to match A-series paper. Each C envelope holds the corresponding A-size paper:
- C6 (114 × 162 mm) — holds an A4 sheet folded twice (into quarters). The standard European personal letter envelope.
- C5 (162 × 229 mm) — holds an A4 sheet folded once in half, or an A5 sheet flat. Common for formal invitations and documents in Europe.
- C4 (229 × 324 mm) — holds an A4 sheet flat and unfolded. The international equivalent of the US 9×12 catalog envelope.
- DL (110 × 220 mm) — holds an A4 sheet folded in thirds. This is the most widely used business envelope outside North America, equivalent to the US #10 in function.
Invitation Envelope Sizes
Invitation envelopes follow a separate A-size system (not to be confused with ISO A-series paper):
- A2 (4.375 × 5.75 in) — fits a 4.25 × 5.5 inch card. Standard for RSVP cards, thank-you notes, and small invitations.
- A7 (5.25 × 7.25 in) — fits a 5 × 7 inch card. The most popular size for wedding invitations and greeting cards in the US.
- A9 (5.75 × 8.75 in) — fits a 5.5 × 8.5 inch card. Used for larger formal invitations and announcements.
Envelope Paper Weight and Finish
Envelope paper weight is measured in grams per square metre (gsm) or pounds (lb text). Standard mailing envelopes use 75–90 gsm paper. Premium envelopes for invitations and formal correspondence typically use 100–120 gsm. Heavier stock (120+ gsm) is used for luxury mailers and padded envelopes. Finish options include white wove (smooth, classic), kraft (brown, recycled), and metallic or linen textures for specialty use.
Frequently Asked Questions
What size envelope fits a standard letter?
A US Letter page (8.5 × 11 in) folded in thirds fits a #10 envelope. Folded in half it fits a 6×9 envelope. Flat and unfolded it needs a 9×12 catalog envelope.
What envelope fits A4 paper?
A4 paper folded in thirds fits a DL envelope. Folded in half it fits a C5 envelope. Flat it fits a C4 envelope.
What is a window envelope?
A window envelope has a transparent opening, usually on the left side of the front face, through which an address printed on the enclosed document shows through. This eliminates the need to print or write the address separately on the envelope.