An additional ounce stamp is a 29¢ stamp you add to a letter that already carries a Forever stamp when the letter weighs more than one ounce. A single Forever stamp covers the first ounce of a standard rectangular envelope, and every extra ounce after that, or any fraction of one, needs its own additional ounce stamp, up to the 3.5-ounce limit for First-Class letters.1USPS. First-Class Mail and Postage
How the Postage Adds Up
Additional ounce stamps ride alongside a primary stamp; they’re not valid on their own. A 1.3-ounce letter takes one Forever stamp ($0.78) plus one additional ounce stamp ($0.29), for $1.07 total. A 2.4-ounce letter takes the Forever stamp plus two additional ounce stamps, or $1.36, because any fraction of an ounce counts as a full ounce for postage.1USPS. First-Class Mail and Postage
If you regularly send heavier letters, USPS also sells combination stamps that bundle the first ounce and extra ounces together. A $1.07 “First-Class 2-oz” stamp covers a two-ounce letter outright, and a $1.36 “First-Class 3-oz” stamp handles up to three ounces without stacking anything.2USPS. Additional Ounces – The Postal Store
When You Actually Need One
The question is simple: does the letter weigh more than one ounce? A standard #10 business envelope with four or five sheets of regular copy paper usually comes in just under an ounce, so one Forever stamp is enough. Add a sixth or seventh sheet, thicker paper, a greeting card with inserts, or a small enclosure like a photo or gift card, and you’ll likely cross the threshold.
No scale at home? Five sheets of standard 20-pound copy paper plus the envelope weigh about 0.95 ounces. One more sheet, or a heavier envelope, pushes you over. When in doubt, a postal clerk will weigh your letter for free.
The 3.5-Ounce Ceiling
Additional ounce stamps only work on standard-sized letters up to 3.5 ounces. Past that weight, the piece is reclassified as a “large envelope” (a flat), which starts at $1.63 for the first ounce; a four-ounce flat runs $2.44.3Postal Explorer. Notice 123 – January 2026 Price Change Both letters and flats cap out at 13 ounces for First-Class Mail; anything heavier ships as a package.4Postal Explorer. Retail Mail Letters, Cards, Flats, and Parcels
Size matters too, not just weight. An envelope taller than 6⅛ inches, longer than 11½ inches, or thicker than ¼ inch is treated as a flat regardless of what it weighs.1USPS. First-Class Mail and Postage
Square Envelopes Cost More
Wedding invitations, specialty cards, and any envelope with a clasp, string, or button trigger a $0.49 non-machinable surcharge because they can’t run through USPS sorting equipment. Square envelopes trigger it too.3Postal Explorer. Notice 123 – January 2026 Price Change The surcharge applies even when the letter weighs less than an ounce.
So a one-ounce square envelope costs $1.27 (Forever stamp plus surcharge). If that square envelope weighs 1.5 ounces, add an additional ounce stamp on top, bringing the total to $1.56.1USPS. First-Class Mail and Postage Weigh a fully assembled invitation, with card, RSVP envelope, inserts, and outer envelope, before you buy postage for a batch. Cardstock and lined envelopes cross one ounce faster than most people expect.
International Letters Work Differently
The 29¢ per extra ounce rule is domestic only. A one-ounce international letter costs $1.70, which a single Global Forever stamp covers.5USPS. How to Send a Letter or Postcard – International Beyond one ounce, international rates depend on weight and destination: a two-ounce letter to Canada is $2.00, while the same letter to Europe is $3.40.3Postal Explorer. Notice 123 – January 2026 Price Change You can combine domestic additional ounce stamps to reach the right total, but for anything over an ounce it’s easier to have the post office weigh and price it.
If You Skip the Extra Stamp
Underpaid mail has a few possible fates, and none of them are fast:
- Returned to sender is the most common outcome for short-paid First-Class letters. The envelope comes back stamped “Returned for Additional Postage,” and you’ll need a fresh envelope with correct postage to try again.6USPS. Return to Sender Mail
- Sometimes USPS delivers the letter and charges the recipient the missing amount, which is awkward for professional correspondence or bill payments.
- Mail with no postage at all goes back to the sender without a delivery attempt, or becomes dead mail if there’s no return address.
USPS gives no fixed timeline for returns, so a short-paid letter can sit in limbo for days. If the mail is time-sensitive, 29 cents is cheap insurance.
How to Place the Stamps
Put additional ounce stamps in the upper-right corner of the envelope, next to the Forever stamp. Sorting machines scan that corner for postage, so keeping stamps together in one area helps them read correctly. Don’t let any stamp cover the delivery address, return address, or barcodes, and press the edges down. Stamps that peel up are a common cause of processing delays.
Where to Buy Them
You can order additional ounce stamps, two-ounce combo stamps, and three-ounce combo stamps directly from the USPS website and have them shipped to your home.2USPS. Additional Ounces – The Postal Store Any local post office sells them at the counter, and the clerk can weigh your mail and tell you what postage it needs. Many grocery stores, pharmacies, and office supply stores stock them too, usually in booklets or sheets. Buying from USPS or a post office guarantees face-value pricing; third-party sellers sometimes add a markup.