How Many Forever Stamps Do You Need by Mail Type?

One Forever stamp, currently worth $0.78, covers a standard rectangular letter weighing up to 1 ounce. Anything heavier, larger, oddly shaped, or headed overseas needs more. How many Forever stamps you need by mail type depends on weight, envelope size, and destination, and the counts below cover every common case.

Standard Letters Up to 1 Ounce

A single Forever stamp is enough for a rectangular envelope weighing 1 ounce or less. To qualify for the letter rate, the envelope must be at least 3.5 inches tall, 5 inches long, and 0.007 inches thick, and no larger than 6.125 inches tall, 11.5 inches long, and 0.25 inches thick.1United States Postal Service. Business Mail 101 – Sizes for Letters

Most #10 business envelopes and A2 invitation envelopes fit. Weight is where people slip up. A standard sheet of copy paper weighs about 0.16 ounces, so roughly six sheets plus an envelope will push past 1 ounce. USPS rounds weight up to the next ounce, so a letter at 1.1 ounces is charged at the 2-ounce rate.

Heavier Letters: 2 to 3.5 Ounces

Each additional ounce on a First-Class letter costs $0.29.2U.S. Postal Service. U.S. Postal Service Recommends New Prices for July Using only Forever stamps, round up:

  • 2 ounces ($1.07): Two Forever stamps ($1.56), or one Forever stamp plus one 29-cent Additional Ounce stamp for the exact amount.
  • 3 ounces ($1.36): Two Forever stamps ($1.56), or one Forever plus two Additional Ounce stamps.
  • 3.5 ounces ($1.65), the heaviest a letter can be: Three Forever stamps ($2.34), or two Forever plus one Additional Ounce stamp ($1.85) for a closer match.3United States Postal Service. First-Class Mail Information

If you rarely send heavy mail, two Forever stamps handle anything up to 3.5 ounces with a little overpayment. Keeping a strip of Additional Ounce stamps on hand saves money when you mail heavier letters often.

Large Envelopes (Flats)

Oversized envelopes that exceed letter dimensions are called flats. Manila envelopes, 9-by-12 portfolios, and magazine-sized mailers fall here. A flat can measure up to 12 by 15 by 0.75 inches and weigh up to 13 ounces.3United States Postal Service. First-Class Mail Information

Postage starts higher and climbs by weight.4United States Postal Service. Notice 123 – Price List (July 2025) Using only Forever stamps:

  • 1 ounce ($1.63): Three Forever stamps. Two stamps ($1.56) fall $0.07 short.
  • 2 ounces ($1.90): Three Forever stamps.
  • 4 ounces ($2.44): Four Forever stamps. Three ($2.34) is not enough.
  • 8 ounces ($3.56): Five Forever stamps.
  • 13 ounces ($5.04): Seven Forever stamps.

Because flat rates don’t divide evenly by $0.78, you almost always overpay when using only Forever stamps. For flats you mail regularly, buying exact postage at the counter or printing it from postcalc.usps.com trims the waste.

Non-Machinable Letters

Some envelopes meet letter dimensions but can’t run through sorting machines. Square envelopes, rigid cards, envelopes with clasps, and pieces with uneven thickness (like a card mailed with a coin inside) trigger a non-machinable surcharge of $0.49 on top of the letter rate.4United States Postal Service. Notice 123 – Price List (July 2025)

A 1-ounce non-machinable letter costs $1.27. Two Forever stamps ($1.56) cover it. This comes up most often with wedding invitations, square holiday cards, and anything stiff enough that it won’t bend.

International Letters

A 1-ounce letter or postcard to any country costs $1.70.2U.S. Postal Service. U.S. Postal Service Recommends New Prices for July The simplest option is one Global Forever stamp, sold at that exact price.

If you only have domestic Forever stamps, two ($1.56) are $0.14 short. Three ($2.34) cover the international rate but overpay by $0.64. So three domestic Forever stamps will get an international letter delivered when you don’t have a Global Forever on hand.5USPS. Mailing and Shipping Prices

Postcards

A standard postcard costs $0.61, less than a Forever stamp.5USPS. Mailing and Shipping Prices A Forever stamp works, but you overpay by $0.17 each time. Postcard-rate stamps are sold at the exact price. To qualify for the postcard rate, the card must be rectangular, between 3.5 by 5 inches and 4.25 by 6 inches, and between 0.007 and 0.016 inches thick.

Quick Reference by Mail Type

Using only Forever stamps at $0.78 each, rounded up so postage is always sufficient:

  • Standard 1-ounce letter: 1 stamp.
  • 2-ounce letter: 2 stamps (overpays $0.49).
  • 3-ounce letter: 2 stamps (overpays $0.20).
  • 3.5-ounce letter: 3 stamps (overpays $0.69).
  • Non-machinable 1-ounce letter: 2 stamps (overpays $0.29).
  • 1-ounce large envelope: 3 stamps (overpays $0.71).
  • 4-ounce large envelope: 4 stamps (overpays $0.68).
  • 8-ounce large envelope: 5 stamps.
  • International 1-ounce letter: 3 stamps, or 1 Global Forever stamp at exact price.
  • Postcard: 1 stamp (overpays $0.17), or 1 postcard stamp at exact price.

Checking Weight Before You Mail

A kitchen food scale is enough to keep you out of trouble at home. Any post office counter will weigh mail for free. USPS also runs an online postage calculator at postcalc.usps.com where you enter weight, dimensions, and destination to see the exact rate. When in doubt, add a stamp.

What Happens If You Use Too Few Stamps

Underpaid mail doesn’t just disappear.6USPS. How Is Undeliverable and Misdelivered Mail Handled If your letter has some postage but not enough, USPS typically delivers it marked “postage due,” and the recipient has to pay the shortage in cash before receiving it. Awkward for a birthday card to your grandmother.

If a letter has no postage, or the recipient refuses to pay what’s owed, USPS stamps it “Returned for Additional Postage” and sends it back to the return address. Without a return address, the piece becomes dead mail. An extra stamp is cheaper than either outcome.