One Forever stamp covers a standard First-Class letter weighing up to 1 ounce. To figure out how many stamps to mail by weight beyond that, you add one $0.29 Additional Ounce stamp for each extra ounce (or fraction of an ounce), up to a 3.5-ounce maximum for letters. The exact count depends on whether you’re sending a postcard, a standard letter, a large envelope, or something heavier, and the shape of the mailpiece can bump you into a higher price tier regardless of weight.
Standard Letters
A First-Class letter costs $0.78 for the first ounce and $0.29 for each additional ounce or fraction, capped at 3.5 ounces.1Postal Explorer. Notice 123 – Price List Effective January 18, 2026 In stamps:
- Up to 1 oz: 1 Forever stamp ($0.78)
- Up to 2 oz: 1 Forever + 1 Additional Ounce stamp, or a single Two-Ounce stamp ($1.07)
- Up to 3 oz: 1 Forever + 2 Additional Ounce stamps, or a single Three-Ounce stamp ($1.36)
- Up to 3.5 oz: 1 Forever + 3 Additional Ounce stamps ($1.65)
If regular Forever stamps are all you have on hand, two of them ($1.56) cover anything up to 3 ounces and three ($2.34) cover the heaviest letters, but you’ll overpay compared to using Additional Ounce stamps for the extras.
A Forever stamp always covers the current 1-ounce letter rate, even if prices go up after you bought it.2United States Postal Service. USPS Adjusts Prices Additional Ounce stamps work the same way, always paying whatever the current additional-ounce rate is.
Postcards
A standard postcard costs $0.61.1Postal Explorer. Notice 123 – Price List Effective January 18, 2026 To qualify, the card must be rectangular and fit these dimensions:3Postal Explorer. Sizes for Postcards
- Height: 3-1/2 to 4-1/4 inches
- Length: 5 to 6 inches
- Thickness: 0.007 to 0.016 inches
A postcard-rate stamp is the right choice. A Forever stamp will get the card there, but you’ll overpay by $0.17. Anything outside the size limits is charged as a letter.
Large Envelopes (Flats)
Large envelopes, which USPS calls flats, run on a different scale: $1.63 for the first ounce and $0.27 for each additional ounce, up to 13 ounces. Above 13 ounces, the item moves into Priority Mail pricing.1Postal Explorer. Notice 123 – Price List Effective January 18, 20264Postal Explorer. First-Class Mail Common weights:
- 1 oz: $1.63
- 2 oz: $1.90
- 4 oz: $2.44
- 8 oz: $3.56
- 13 oz: $5.04
Stamps don’t map cleanly onto these prices. Three Forever stamps ($2.34) cover roughly the first 3 ounces of a flat, but past that, buying exact postage at a post office counter, a self-service kiosk, or through USPS.com is cheaper and cleaner than layering Forever stamps.
To count as a flat rather than a package, the envelope must stay within:5Postal Explorer. Sizes for Large Envelopes and Flats
- Height: up to 12 inches
- Length: up to 15 inches
- Thickness: up to 3/4 inch
Go over any of those and it’s a package, which costs more.
Shape and Surcharges That Change the Count
Weight sets the baseline, but shape and size can add stamps you didn’t plan for.
Letter Dimensions
For the basic letter rate, the envelope must be rectangular and within:6Postal Explorer. Sizes for Letters
- Height: 3-1/2 to 6-1/8 inches
- Length: 5 to 11-1/2 inches
- Thickness: 0.007 to 1/4 inch
A letter thicker than 1/4 inch but under the flat limits gets reclassified as a large envelope. Bigger than that, and it’s a package. Each jump pushes you into a higher price tier no matter what the scale says.
Nonmachinable Surcharge
Square envelopes, rigid ones, lumpy ones, and any envelope that isn’t uniformly thick can’t run through USPS sorting equipment. Those get a $0.49 nonmachinable surcharge added to regular postage.7Postal Explorer. Notice 123 Effective January 18, 2026 A 1-ounce square wedding invitation or a card with a wax seal costs $1.27 to mail, not $0.78. In stamps, that’s a Forever plus enough additional postage to cover the surcharge.
International Letters
A Global Forever stamp costs $1.70 and covers a 1-ounce letter to any country.1Postal Explorer. Notice 123 – Price List Effective January 18, 2026 The 3.5-ounce cap matches domestic letters, but the per-ounce rate jumps around by bracket:8Postal Explorer. Country Price Groups and Weight Limits
- 1 oz: $1.70
- 2 oz: $2.00
- 3 oz: $2.70
- 3.5 oz: $3.40
Because the increments aren’t uniform ($0.30 to add the second ounce, then $0.70 for the third), assembling exact international postage from Forever stamps alone gets awkward past 1 ounce. Buy the exact amount at the counter or online.
When Stamps Stop Making Sense
First-Class Mail tops out at 13 ounces. Above that, you’re into Priority Mail or USPS Ground Advantage, both priced by weight, dimensions, and distance.
A Priority Mail Flat Rate Envelope costs $11.95 no matter how heavy the contents, as long as the envelope closes. USPS gives the envelopes away at any post office. USPS Ground Advantage starts at $7.30 for items up to 4 ounces, though the final price depends on zone.1Postal Explorer. Notice 123 – Price List Effective January 18, 2026 Ground Advantage takes two to five business days; Priority Mail runs one to three. For packages, USPS also considers dimensional weight, so a large but light box can be priced on its size rather than what the scale shows.
If You Underpay
Mail with too little postage doesn’t just arrive late. It gets stamped “Returned for Postage” and sent back to the return address with no delivery attempt. No return address, or a return address that matches the delivery address, and USPS treats it as dead mail.
Round up when you’re close. A 1.1-ounce letter is charged as a 2-ounce letter because USPS rounds up to the next whole ounce for First-Class letters and flats.9USPS. How to Prepare and Send a Package Coming up a few cents short is enough to bounce a piece back.