A single First-Class Forever stamp covers an envelope weighing up to one ounce, which works out to about four or five sheets of standard copy paper inside a normal business or card-sized envelope. That’s the ceiling for how much an envelope can weigh with one stamp, and it assumes the envelope is a standard rectangular shape that USPS machines can process. Go over an ounce, use an oddly shaped envelope, or stuff something lumpy inside, and one stamp won’t get your letter delivered.
What One Ounce Actually Holds
A standard #10 envelope with four sheets of 20-pound copy paper weighs around 0.8 ounces. Five sheets brings the total to roughly 0.95 ounces, still just under the cutoff. Six sheets almost always tips you over.
USPS rounds weight up to the next ounce, so a letter clocking in at 1.1 ounces is billed as a two-ounce letter. If your envelope feels borderline, weigh the whole thing sealed rather than guessing from the paper count. Heavier card stock, photos, or a folded return envelope add weight faster than most people expect. A kitchen scale is accurate enough for this.
The Forever stamp is priced at whatever the current one-ounce First-Class rate is, which was $0.78 as of July 2025, and USPS confirmed no stamp price increase took effect in January 2026.1USPS. 2026 Postage Price Change Stamps you bought years ago at lower prices still work with no extra postage.2USPS. First-Class Mail and Postage
Size and Shape Matter As Much As Weight
Weight isn’t the only thing that decides whether one stamp is enough. The envelope has to be rectangular and fall within specific dimensions to qualify for the letter rate:3Postal Explorer. Sizes for Letters
- Height between 3.5 and 6.125 inches
- Length between 5 and 11.5 inches
- Thickness between 0.007 and 0.25 inches
Anything larger becomes a “flat,” which starts at $1.63 for the first ounce. A 9-by-12 manila envelope full of documents falls into this category, and one Forever stamp won’t cover it no matter how little the contents weigh. Standard business envelopes and most greeting-card envelopes qualify for the letter rate without issue.
When the Non-Machinable Surcharge Applies
Even an envelope under one ounce and within the size limits can cost more than one stamp if USPS sorting machines can’t handle it. The non-machinable surcharge is an extra $0.49, bringing a one-ounce non-machinable letter to $1.27.4United States Postal Service. July 2025 Price Change Notice 123 A single Forever stamp doesn’t cover that.
USPS considers a letter non-machinable if it has any of these traits:5United States Postal Service. 3-6 Nonmachinable Criteria
- Square shape, or more precisely, a length-to-height ratio below 1.3 or above 2.5
- Too rigid to bend around a curved sorting belt
- Clasps, strings, buttons, or other protruding closures
- Uneven thickness from items like keys, pens, or loose coins inside
Square greeting cards are the most common trap. People buy one stamp, drop the card in the mailbox, and it comes back marked for additional postage. If you’re mailing a square card or anything lumpy, add the surcharge.
When One Stamp Isn’t Enough
For letters over one ounce, each additional ounce costs $0.29, up to a maximum letter weight of 3.5 ounces. USPS sells an “Additional Ounce” stamp at that denomination, so a Forever stamp plus one Additional Ounce stamp covers a two-ounce letter for $1.07.4United States Postal Service. July 2025 Price Change Notice 123 Any combination of stamps that adds up to the right amount works too.6USPS. How to Send a Letter or Postcard – Domestic
The full stamped letter rate table:4United States Postal Service. July 2025 Price Change Notice 123
- 1 ounce: $0.78
- 2 ounces: $1.07
- 3 ounces: $1.36
- 3.5 ounces: $1.65
Past 3.5 ounces, a First-Class letter is reclassified as a flat or a package, both at higher rates.2USPS. First-Class Mail and Postage
Postcards are the one case where less postage works. A regular Forever stamp is more than a postcard needs. The domestic postcard stamp is $0.61, but the card has to be no larger than 4.25 by 6 inches and no thicker than 0.016 inches.7Postal Explorer. Sizes for Postcards Anything bigger is charged at the letter rate.
For international mail, a domestic Forever stamp isn’t enough. The Global Forever stamp costs $1.70 and covers a one-ounce letter to any country, with a special allowance of two ounces to Canada.8FAQ | USPS. Postage Stamps – The Basics
What Happens If You Underpay
A letter with insufficient postage doesn’t vanish. If it has a return address, USPS sends it back stamped “Returned for Additional Postage,” and you can add postage and remail it. If it slips through and reaches the destination post office, the carrier may deliver it marked “Postage Due,” and the recipient has to pay the difference to accept it. Most people refuse, and the letter comes back to you anyway.
Without a return address, an underpaid letter that no one accepts becomes dead mail. USPS opens it to search for a return address inside, and if none exists, the contents are destroyed or auctioned depending on what’s in them.
The safe move when you’re near the one-ounce line is to add an Additional Ounce stamp. Spending an extra $0.29 costs less than a delayed bill, a missed deadline, or a letter that never arrives.