Yes, you can use Forever stamps on a square envelope, but one ordinary Forever stamp is usually not enough. A square letter has an aspect ratio of 1.0, below USPS’s machinable range, so a letter-size square envelope is subject to the nonmachinable surcharge.
How Much Postage Does a Square Envelope Need?
For rates effective July 12, 2026, a stamped one-ounce First-Class Mail letter costs 82¢ and the nonmachinable surcharge is 49¢. A qualifying one-ounce square letter therefore needs $1.31 in postage.
| Square letter weight | Letter postage | Nonmachinable surcharge | Total | Ordinary Forever stamps |
|---|---|---|---|---|
| Up to 1 oz | $0.82 | $0.49 | $1.31 | 2 stamps ($1.64; overpays 33¢) |
| Over 1 oz through 2 oz | $1.11 | $0.49 | $1.60 | 2 stamps ($1.64; overpays 4¢) |
| Over 2 oz through 3 oz | $1.40 | $0.49 | $1.89 | 3 stamps ($2.46; use smaller-denomination postage to avoid overpaying) |
Can I Put Two Forever Stamps on a Square Envelope?
Yes. At current rates, two ordinary Forever stamps provide $1.64. That covers a letter-size square envelope weighing up to 2 ounces, assuming it has no other feature that changes its mail class. Two stamps do not automatically cover heavier, oversized, rigid, lumpy or unusually thick pieces.
Why Square Envelopes Cost More
USPS defines a letter as nonmachinable when its length-to-height ratio is below 1.3 or above 2.5. Because equal sides produce a 1.0 ratio, a true square is outside the machinable range. The extra charge reflects different handling; it is not caused by the design of the stamp.
Check the Envelope Before Mailing
- Minimum letter size: 5 inches long, 3½ inches high and 0.007 inch thick.
- Maximum letter size: 11½ inches long, 6⅛ inches high and ¼ inch thick.
- Maximum First-Class letter weight: 3½ ounces.
- Other nonmachinable features: rigidity, clasps, strings, buttons, uneven contents, plastic wrapping or an address parallel to the shorter dimension.
A piece outside letter dimensions or standards may be priced as a large envelope or parcel instead. Measure the sealed, filled mailpiece—not the empty envelope package. For standard and seasonal mail, use the greeting-card postage guide to check weight, shape and decoration together.
A Lower-Overpayment Option
Instead of two ordinary Forever stamps on a one-ounce square letter, you can combine one ordinary Forever stamp with 49¢ of valid additional postage, or use a current Butterfly Nonmachinable Surcharge stamp designed for this purpose. Always verify the value of older specialty stamps before assuming they match today’s surcharge. For the category rule and alternatives, see whether nonmachinable stamps are Forever stamps.
See the official USPS letter-size and nonmachinable standards and the current Notice 123 price list. For a different shape or uncertain contents, use the USPS Postage Calculator or ask at a Post Office.
Rates checked August 31, 2026. Amounts above use USPS prices effective July 12, 2026.