Three Forever stamps are enough for a 9×12 envelope holding a few sheets of paper. At $0.78 each, three stamps come to $2.34, which covers the large-envelope rate up to 3 ounces. Heavier envelopes need more, because USPS treats any 9×12 as a “large envelope” (a flat), and flats cost more than letters.
Why One Stamp Is Never Enough
A 9×12 exceeds the maximum size for a letter (6-1/8 inches high, 11-1/2 inches long), so it falls into the large envelope category no matter how little it weighs or what’s inside.1United States Postal Service. Postal Explorer – Sizes for Large Envelopes and Flats The 1-ounce flat rate is $1.63, roughly double the $0.78 letter rate that a single Forever stamp covers.2United States Postal Service. U.S. Postal Service Recommends New Prices for July 2025 One stamp will always come up short.
Stamps Needed by Weight
Forever stamps are worth $0.78 apiece, and postage has to meet or exceed the required amount. Round down and the envelope comes back. These figures use the rates that took effect July 13, 2025:3United States Postal Service. Notice 123 – July 2025 Price Change
- Up to 1 oz ($1.63): 3 stamps
- Up to 2 oz ($1.90): 3 stamps
- Up to 3 oz ($2.17): 3 stamps
- Up to 4 oz ($2.44): 4 stamps
- Up to 5 oz ($2.72): 4 stamps
- Up to 6 oz ($3.00): 4 stamps
- Up to 7 oz ($3.28): 5 stamps
Three stamps handles most everyday mailings. Past 3 ounces you start overpaying by real amounts, so if you mail heavy documents often, paying the exact rate at the counter or through printed postage will save money.
Weigh It Before You Stamp It
A kitchen scale is fine. Weigh the envelope sealed, with any cardboard stiffeners inside. An empty 9×12 manila envelope weighs about half an ounce, and standard 20-pound copy paper runs 4 to 5 sheets per ounce, so five sheets plus the envelope lands near 1.5 ounces.
USPS rounds any fraction of an ounce up to the next whole ounce.4United States Postal Service. Domestic Mail Manual P013 Rate Application and Computation An envelope at 1.2 ounces is charged as 2 ounces; one at 3.1 ounces is charged as 4 ounces. That rounding is where most miscalculations happen. A 3.05-ounce envelope owes 4-ounce postage ($2.44), not 3-ounce postage ($2.17), and that bump moves you from three stamps to four.
When the Contents Change the Category
Weight isn’t the only factor. A 9×12 fits within the large-envelope size limits (up to 12 inches by 15 inches by 3/4 inch thick), but what’s inside can push it out.1United States Postal Service. Postal Explorer – Sizes for Large Envelopes and Flats If the envelope is thicker than 3/4 inch, too rigid to bend, or lumpy and uneven, USPS reclassifies it as a parcel. Parcel rates are substantially higher, and the envelope will likely come back for more postage if you only stamped it as a flat. Binder clips, USB drives, keys, and thick booklets are common culprits.5United States Postal Service. Publication 25 – Nonmachinable Criteria
Clasps, string closures, and plastic exteriors can also trigger processing issues. If you’re sending anything beyond flat paper, take it to the counter and let a clerk weigh and classify it.
What Happens If You Underpay
Short-paying postage on a 9×12 can mean your mail never arrives. If there’s no postage at all, the envelope gets marked “Returned for Postage” and goes back to the return address without a delivery attempt. If there’s some postage but not enough, USPS notes the shortage and attempts delivery with that amount due; the recipient can pay and accept it or refuse it. Refused or undeliverable shortpaid mail returns with a “Returned for Additional Postage” notice, and you can add postage, cross out the notice, and remail.6United States Postal Service. Domestic Mail Manual P011 Payment
An underpaid envelope with no return address is treated as dead mail. It won’t reach the recipient and won’t come back. Always put a return address on it.