A Forever stamp needs extra postage whenever the finished mailpiece costs more than the current one-ounce, machinable domestic First-Class Mail letter rate. Common triggers are weight above one ounce, square or rigid shape, large-envelope dimensions, international destination and paid add-on services. “Forever” protects the stamp’s base value from rate increases; it does not cover every mailing charge.
When Extra Postage Is Required
| Mailing condition | Why one Forever stamp is not enough | What to do |
|---|---|---|
| Letter over 1 ounce | Additional-ounce postage applies | Weigh the sealed letter and pay the difference |
| Square, rigid, lumpy or uneven letter | It may be nonmachinable | Add the current surcharge after confirming letter eligibility |
| Large envelope or flat | It uses a different price table | Calculate by category, dimensions and weight |
| International mail | Destination and international rules apply | Use the international rate, not the domestic base rate |
| Certified Mail or another add-on | The service has a separate fee | Pay postage plus the selected service |
| Package | Package prices depend on weight, dimensions and zone | Use a shipping label or current package calculation |
Current Domestic Letter Examples
At rates effective July 12, 2026, one ordinary Forever stamp has an $0.82 postal value and each additional ounce for a single-piece letter costs $0.29.
| Machinable letter weight | Required postage | Lower-overpayment combination |
|---|---|---|
| Up to 1 ounce | $0.82 | 1 Forever stamp |
| Over 1 through 2 ounces | $1.11 | 1 Forever stamp + $0.29 |
| Over 2 through 3 ounces | $1.40 | 1 Forever stamp + $0.58 |
| Over 3 through 3.5 ounces | $1.69 | 2 Forever stamps + $0.05 |
You may use multiple Forever stamps if their combined value meets the required amount, but whole stamps can overpay. More stamps also do not turn a flat or package into a letter.
Shape Can Matter as Much as Weight
A one-ounce envelope can still need extra postage when it is square, too rigid, unusually shaped, uneven in thickness or otherwise outside machinable-letter standards. Large or thick pieces may move into another mail category rather than merely receiving a surcharge. Measure the sealed piece, not an empty envelope.
International and Extra-Service Mail
An ordinary Forever stamp contributes its current domestic value toward international postage, but does not by itself cover the international rate. Likewise, Certified Mail, insurance and other services add their own prices and preparation requirements. Some stamped packages must be presented at a retail counter under USPS acceptance rules.
Four-Step Check Before Mailing
- Classify the finished item as a postcard, letter, large envelope or package.
- Choose domestic or international destination.
- Weigh and measure the sealed item; note square, rigid or uneven features.
- Calculate the total, subtract the value already affixed and add only the shortfall.
Browse Additional Ounce stamp options when an eligible letter needs another ounce of postage. Use the site’s Forever stamp calculator and decision guide, then verify unusual mailpieces with the current USPS calculator or a postal counter.
Frequently Asked Questions
Do Forever stamps cover extra postage?
They contribute their current postal value, but any amount above the one-ounce domestic letter rate still must be paid.
Do I add postage after a Forever stamp?
Only when the finished mailpiece requires more than the stamp’s current value or uses a service with an additional fee.
Can I use a second Forever stamp instead of smaller postage?
Yes if the combined value is enough, but it may overpay and does not change the mailpiece category.
Does a square one-ounce envelope need more postage?
Usually it requires the applicable nonmachinable surcharge if it otherwise qualifies as a letter.
Official sources: USPS Notice 123, letter standards, and the Retail Postage Price Calculator. Rates checked September 1, 2026 and effective July 12, 2026.
For the core value, validity, weight, and usage rules, see How Do Forever Stamps Work?.