A single First-Class Forever stamp ($0.78 in 2026) will mail a 5×7 envelope only if it weighs an ounce or less and is thin, flat, and flexible. In practice, most 5×7 envelopes need more than one stamp because the things people put in them — wedding invitations, greeting cards with inserts, photo prints — trigger a $0.49 non-machinable surcharge, push the weight past one ounce, or both. So the honest answer to how many stamps for a 5×7 envelope is: one if you’re lucky, two or more if you’re mailing anything with heft or stiffness.
When One Forever Stamp Is Enough
A 5×7 envelope fits within USPS letter dimensions. Letters must be rectangular, at least 3.5 × 5 × 0.007 inches and no larger than 6.125 × 11.5 × 0.25 inches, and weigh 3.5 ounces or less.1United States Postal Service. Physical Standards for Letters A 5×7 oriented the standard way also passes the aspect-ratio test USPS applies to letters.2USPS. Quick Service Guide 201
So one Forever stamp covers your 5×7 when all of these are true: the envelope weighs one ounce or less with everything inside, it’s no thicker than a quarter inch, it bends easily, its contents are evenly distributed, and the address runs parallel to the long (7-inch) edge. A thin greeting card on regular cardstock with nothing tucked inside usually clears the bar.
The Non-Machinable Surcharge Is Where Most 5×7s Get Caught
If the envelope can’t run through automated sorting equipment, USPS adds a $0.49 non-machinable surcharge on top of regular postage.3Postal Explorer (USPS). January 2026 Price Change – Notice 123 Draft At the one-ounce letter rate, that means $0.78 + $0.49 = $1.27, which works out to two Forever stamps (a slight overpayment) or a Forever plus a $0.49 stamp.
The surcharge applies whenever a letter has any of these features:
- Rigid contents, like thick cardstock invitations, magnets, or photo prints that keep the envelope from bending around sorting rollers.
- Uneven thickness from coins, keys, ribbon, or wax seals. Loose small objects can actually make a paper envelope unmailable, not just surcharged.
- Clasps, strings, or buttons — anything beyond a standard gummed flap.
- A non-paper exterior, such as plastic, cellophane, or fabric wrapping.
- An address running parallel to the short (5-inch) edge instead of the long edge.
- A square shape. A true 5×7 is fine, but a 5×5 or similar is not.
Wedding invitation suites almost always trigger this. A rigid cardstock invite plus a vellum overlay, a reply card, and an inner envelope will be either stiff, uneven, or both. Budget for the surcharge on every piece rather than hoping some slip through.
Postage by Weight
A Forever stamp covers one ounce. Each additional ounce costs $0.29 in 2026.4USPS: Postal Explorer. Notice 123 – January 2026 Price Change The stamped-letter rates:
- 1 oz: $0.78
- 2 oz: $1.07
- 3 oz: $1.36
- 3.5 oz: $1.65
Add the $0.49 surcharge if the piece is non-machinable. A two-ounce non-machinable 5×7 comes to $1.56. Weigh the finished envelope with the card, all inserts, and the stamps already stuck on, because a fraction over an ounce boundary bumps you into the next tier.
When a 5×7 Isn’t a Letter Anymore
If the assembled envelope is thicker than a quarter inch, USPS reclassifies it as a large envelope (flat), and flat pricing starts at $1.63 for the first ounce.5USPS. DMM 101 Physical Standards for Retail Letters, Flats, and Parcels6Postal Explorer (USPS). Notice 123 – January 2026 Price Change Each additional ounce adds roughly $0.27 to $0.30. Flats also have to bend; a thick, rigid 5×7 that won’t flex gets reclassified again as a parcel, which costs more still. Measure the assembled thickness before buying stamps for a stuffed invitation suite.
Mailing a 5×7 Internationally
One Global Forever stamp ($1.70) covers a one-ounce letter to any country, and international letters can weigh up to 3.5 ounces before they shift to large-envelope pricing.7USPS. First-Class Mail International The same $0.49 non-machinable surcharge applies to international letters with rigid or uneven contents, and non-rectangular pieces face additional charges.3Postal Explorer (USPS). January 2026 Price Change – Notice 123 Draft
Getting the Number of Stamps Right
The safest method for a batch is to bring one fully assembled envelope to the post office counter. The clerk weighs and measures it, tells you the exact postage, and you stamp the rest at home to match.
Doing it yourself: weigh the sealed envelope on a scale accurate to a tenth of an ounce, measure the thickest point where inserts overlap, and try to bend it. If you have to force the bend, assume it’s non-machinable. The USPS Retail Postage Price Calculator confirms the total once you have weight and dimensions.8United States Postal Service (USPS). Retail Postage Price Calculator
You can combine stamps of any denomination to hit an odd amount like $1.27 or $1.56. The total face value just needs to equal or exceed the required postage. If it falls short, the envelope comes back to the sender with a notice — or, if there’s no return address, arrives postage-due to the recipient, which is not the impression most people want their invitation to make.