Envelope size chart
Standard US (A-style, commercial #) and ISO (C/DL) envelopes. All sizes are outside dimensions; leave about ⅛–¼″ clearance per side for the contents.
| Envelope | Size (in unless mm) | Typical contents | USPS class by size* |
|---|---|---|---|
| A1 | 3 5/8 × 5 1/8 | 3½×4⅞ RSVP cards | Letter |
| A2 | 4 3/8 × 5 3/4 | 4¼×5½ cards (quarter-fold Letter) | Letter |
| A6 | 4 3/4 × 6 1/2 | 4½×6¼ cards, 4×6 photos | Letter |
| A7 | 5 1/4 × 7 1/4 | 5×7 cards & invitations | Letter |
| A8 | 5 1/2 × 8 1/8 | 5¼×7⅞ cards | Letter |
| A9 | 5 3/4 × 8 3/4 | 5½×8½ cards (half-fold Letter) | Letter |
| A10 | 6 × 9 1/2 | 5¾×9 cards | Letter |
| #6¾ | 3 5/8 × 6 1/2 | personal checks | Letter |
| #9 | 3 7/8 × 8 7/8 | reply envelope (fits inside #10) | Letter |
| #10 | 4 1/8 × 9 1/2 | Letter paper folded in thirds | Letter |
| 6 × 9 | 6 × 9 | Letter paper folded in half, booklets | Letter |
| 9 × 12 | 9 × 12 | Letter paper unfolded | Large envelope (flat) |
| 10 × 13 | 10 × 13 | thick Letter sets, catalogs | Large envelope (flat) |
| DL | 110 × 220 mm | A4 folded in thirds | Letter |
| C6 | 114 × 162 mm | A6 / A4 folded twice | Letter |
| C5 | 162 × 229 mm | A5 / A4 folded once | Large envelope (flat) |
| C4 | 229 × 324 mm | A4 unfolded | Large envelope (flat) |
| 5 × 5 square | 5 × 5 | square cards | Letter – nonmachinable (surcharge) |
| 6 × 6 square | 6 × 6 | square cards | Letter – nonmachinable (surcharge) |
*Computed from USPS DMM 101 dimensions and the 1.3–2.5 aspect-ratio rule, assuming flexible, ≤¼″ thick, ≤3.5 oz, no clasps. Sizes are standard nominal outside dimensions; manufacturers vary slightly.
Reading the names
A-style (A2, A7…) are announcement envelopes for cards. Commercial numbers (#9, #10) are long business envelopes. C-series (C4, C5, C6) are ISO 269 sizes built to hold the matching A-series paper; DL holds A4 folded in thirds.
Sources
- Envelope Sizes & Dimensions chart (The Letter Pilot)
- Paper Papers – envelope sizes guide
- ISO 216 overview (Wikipedia)
- Paper size – North American sizes (Wikipedia)
- ISO 269 envelope sizes (Wikipedia)
- USPS DMM 101 – Physical standards for letters, flats, parcels
- USPS DMM Notice 123 – Price list (nonmachinable surcharge)
Checked 2026-10-21. Postal rules change; confirm with USPS before a large mailing.