Envelope sizes in the US have two naming systems: numbers for business envelopes (#10 is the standard letter envelope) and A-sizes for cards and invitations. The rest of the world uses the ISO sizes DL and C4 to C6. Sizes are written height × width, so a #10 envelope is 4⅛ in tall and 9½ in wide.
The chart
| Envelope | Size | In mm | Holds | US postage class |
|---|---|---|---|---|
| #10 | 4⅛ × 9½ in | 105 × 241 mm | Business letters: a Letter sheet folded in three | Letter |
| #9 | 3⅞ × 8⅞ in | 98 × 225 mm | Reply envelopes that fit inside a #10 | Letter |
| #6¾ | 3⅝ × 6½ in | 92 × 165 mm | Personal letters, bills and checks | Letter |
| Monarch (#7¾) | 3⅞ × 7½ in | 98 × 190 mm | Monarch stationery (7¼ × 10½ in) folded in three | Letter |
| A2 | 4⅜ × 5¾ in | 111 × 146 mm | 4¼ × 5½ in cards (a Letter sheet folded in quarters) | Letter |
| A6 | 4¾ × 6½ in | 121 × 165 mm | 4 × 6 in photo cards and invitations | Letter |
| A8 | 5½ × 8⅛ in | 140 × 206 mm | 5¼ × 7¾ in cards | Letter |
| A7 | 5¼ × 7¼ in | 133 × 184 mm | 5 × 7 in invitations and greeting cards | Letter |
| A9 | 5¾ × 8¾ in | 146 × 222 mm | 5½ × 8½ in cards and half-Letter flyers | Letter |
| A10 | 6 × 9½ in | 152 × 241 mm | 6 × 9 in cards and brochures | Letter |
| #11 | 4½ × 10⅜ in | 114 × 264 mm | Thicker business mailings | Letter |
| #12 | 4¾ × 11 in | 121 × 279 mm | Bulky letters and folded forms | Letter |
| #14 | 5 × 11½ in | 127 × 292 mm | Legal-size sheets folded in three | Letter |
| 6 × 9 in catalog or booklet | 6 × 9 in | 152 × 229 mm | A Letter sheet folded in half | Letter |
| 9 × 12 in catalog | 9 × 12 in | 229 × 305 mm | Unfolded Letter sheets, documents | Large envelope (flat) |
| 10 × 13 in catalog | 10 × 13 in | 254 × 330 mm | Unfolded Letter sheets with a few extra pages | Large envelope (flat) |
| DL | 110 × 220 mm | 110 × 220 mm | An A4 sheet folded in three | Letter |
| C6 | 114 × 162 mm | 114 × 162 mm | A6 cards, or A4 folded in quarters | Letter |
| C5 | 162 × 229 mm | 162 × 229 mm | A5 sheets, or A4 folded in half | Large envelope (flat) |
| C4 | 229 × 324 mm | 229 × 324 mm | Unfolded A4 sheets | Large envelope (flat) |
US postage class from the Postal Service's size rules for letters (DMM 101): 3½ to 6⅛ in high and 5 to 11½ in long, up to ¼ in thick and 3.5 oz. Larger envelopes are mailed as flats, which cost more.
Business envelopes: #6¾, #9, #10 and up
The #10 is the envelope for a Letter sheet folded in three: bills, letters, invoices and checks. A #9 is slightly smaller, so it can go inside a #10 as a reply envelope. The #6¾ is the small envelope for personal letters, bills and checks. The larger #11, #12 and #14 take thicker mailings and Legal-size sheets.
Card and invitation envelopes: the A-sizes
A-size envelopes are about ¼ in bigger than the card that goes in them:
- A2 (4⅜ × 5¾ in) for 4¼ × 5½ in cards, a Letter sheet folded in quarters.
- A6 (4¾ × 6½ in) for 4 × 6 in photo cards and 4½ × 6¼ in cards.
- A7 (5¼ × 7¼ in) for 5 × 7 in invitations and greeting cards: the most popular size for weddings.
- A9 (5¾ × 8¾ in) for 5½ × 8½ in cards, a Letter sheet folded in half.
Catalog and booklet envelopes
6 × 9 envelopes take a Letter sheet folded once or a small booklet, and still count as letters for postage if they are no thicker than ¼ in. 9 × 12 and 10 × 13 take documents flat without folding; they are mailed as large envelopes (flats).
International sizes: DL, C6, C5 and C4
ISO envelopes are made to fit A-series paper. DL (110 × 220 mm) takes an A4 sheet folded in three, like a US #10. C6 (114 × 162 mm) takes A6 cards or A4 folded in quarters, C5 (162 × 229 mm) takes A5 or A4 folded once, and C4 (229 × 324 mm) takes A4 flat.
Size rules for US postage
- Letter size: at least 3½ × 5 in and at most 6⅛ × 11½ in, up to ¼ in thick and 3.5 oz.
- Shape: rectangular, with the length between 1.3 and 2.5 times the height. Square envelopes and other letters outside that range are nonmachinable and pay a surcharge, as do rigid letters and envelopes with clasps or strings.
- Address direction: the address must run parallel to the long side.
- Larger envelopes are mailed as flats, which cost more than letters.
For printing the addresses, see how to address an envelope.