One Forever stamp covers up to one ounce of First-Class Mail in a standard rectangular envelope, which works out to roughly four sheets of regular 8.5-by-11-inch paper plus the envelope. That answers the practical question behind how much weight a Forever stamp covers: one ounce, full stop, as long as the envelope is the right shape. The stamp is currently worth $0.78, and its value floats with the First-Class 1-ounce rate, so a stamp bought years ago still pays the current first-ounce price.1USPS. First-Class Mail
The One-Ounce Rule Comes With a Shape Rule
The one-ounce coverage only applies if your envelope qualifies as a standard letter. It has to be rectangular, made of paper, and fall within these dimensions:2USPS Postal Explorer. Sizes for Letters
- Minimum: 5 inches long, 3.5 inches high, and 0.007 inches thick
- Maximum: 11.5 inches long, 6.125 inches high, and 0.25 inches thick
Meet both the weight limit and the size limits and a single Forever stamp is enough. Miss either one and you owe more, sometimes a lot more.
When Your Letter Weighs More Than an Ounce
Past one ounce, each additional ounce adds $0.29 in postage. First-Class letters can weigh up to 3.5 ounces before USPS reclassifies them as large envelopes.1USPS. First-Class Mail The full ladder:3USPS. How to Send a Letter or Postcard: Domestic
- 1 ounce: $0.78
- 2 ounces: $1.07
- 3 ounces: $1.36
- 3.5 ounces: $1.65
USPS sells a $0.29 Additional Ounce stamp designed to sit next to a Forever stamp.4USPS. Additional Postage You can also stack Forever stamps: two of them add up to $1.56, which covers anything through 3 ounces with a small overpayment. USPS won’t refund the difference, so precision saves money if you mail often.
A rough way to estimate weight without a scale: one sheet of standard 20-pound printer paper weighs about 0.16 ounces. Five sheets plus an envelope will push you past the one-ounce line. That’s where most accidental underpayments happen.
Shape Can Add Postage Even Under an Ounce
An envelope that can’t run through automated sorting equipment carries a $0.49 nonmachinable surcharge on top of regular postage, regardless of weight.5USPS Postal Explorer. Notice 123 Price List, July 2025 A nonmachinable 1-ounce letter needs $1.27 in postage, not $0.78.1USPS. First-Class Mail
Mail gets flagged as nonmachinable when it’s square instead of rectangular, when it’s rigid or lumpy, or when it has clasps, strings, or buttons.1USPS. First-Class Mail Wedding invitations in square envelopes are the classic trap. So are greeting cards with a pen or key tucked inside. If the envelope feels bumpy when you run your hand over it, assume the surcharge applies.
Postcards, Large Envelopes, and International Mail
These sit outside the Forever-stamp-covers-an-ounce rule, and it’s worth knowing where the boundaries fall.
A standard postcard costs $0.61 to mail.6USPS Postal Explorer. Notice 123 Price List, Effective January 18, 2026 A Forever stamp works on a postcard but overpays by $0.17, which USPS won’t refund. Postcards must be between 3.5 by 5 inches and 4.25 by 6 inches and no thicker than 0.016 inches to qualify for that rate.7USPS Postal Explorer. Sizes for Postcards
Large envelopes, also called flats, are anything past the letter size limits but within 15 by 12 by 0.75 inches. They start at $1.63 for the first ounce and can weigh up to 13 ounces.6USPS Postal Explorer. Notice 123 Price List, Effective January 18, 2026 One Forever stamp isn’t enough. Two Forever stamps ($1.56) still fall $0.07 short of the base large-envelope rate.
International 1-ounce letters and postcards cost $1.70, and USPS sells a Global Forever stamp at that price.8USPS. How to Send a Letter or Postcard: International A domestic Forever stamp on international mail is not enough on its own; you’d need to combine stamps to reach $1.70, and no clean combination of standard denominations hits that number exactly.
What Happens if You Underpay
Short postage doesn’t just delay a letter. USPS handles the situation in one of two ways:9USPS Postal Explorer. Domestic Mail Manual 507 Mailer Services
- Mail with no postage is marked “Returned for Postage” and sent back to the return address without a delivery attempt.
- Mail with insufficient postage is marked with the amount owed and delivered to the recipient, who pays the difference. Nonmachinable First-Class letters that are short get returned to the sender instead.
Mail with no return address and no postage becomes dead mail.
Getting the Postage Right Before You Mail
Weigh the envelope. A kitchen scale is usually close enough, and a small postal scale removes the guesswork on anything near the one-ounce line. Then check the shape: rectangular, flexible, within the letter dimensions, no lumps. If it clears both tests and weighs an ounce or less, one Forever stamp is exactly right. If it fails either, add postage or bring it to a post office counter, where staff will weigh and measure it at no cost.