Do square envelopes cost extra?
Yes, in the US. A square envelope has a length-to-height ratio of 1.0. USPS letters must be between 1.3 and 2.5 to be machinable, so square letters pay the nonmachinable surcharge on top of normal postage.
How much?
The surcharge is listed in USPS DMM Notice 123; for First-Class Mail letters it was $0.49 when we checked. USPS sells "additional ounce" and nonmachinable-surcharge stamps, or you can add stamps to cover the total.
Alternatives that look similar
- A2 (4⅜ × 5¾): ratio 1.31, just machinable.
- A7 (5¼ × 7¼): ratio 1.38.
- Squares bigger than 6⅛″ (e.g. 6½ × 6½) are also over the letter height limit and are priced as large envelopes (flats).
Sources
- 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.