One ordinary Forever stamp covers a rectangular, machinable domestic First-Class Mail letter weighing no more than one ounce. The finished envelope must be at least 5 inches long by 3.5 inches high by 0.007 inch thick and no more than 11.5 inches long by 6.125 inches high by 0.25 inch thick. Size alone is not enough: weight, shape, rigidity and thickness also affect postage.
USPS Envelope Size Limits for One Forever Stamp
| Dimension | Minimum letter size | Maximum letter size |
|---|---|---|
| Length | 5 inches | 11.5 inches |
| Height | 3.5 inches | 6.125 inches |
| Thickness | 0.007 inch | 0.25 inch |
| Weight for one ordinary Forever stamp | No more than 1 ounce | |
USPS treats the side parallel to the address as the length. The piece must be rectangular. For machinability, its length divided by height should generally be from 1.3 through 2.5; a square envelope has a ratio of 1 and needs additional nonmachinable postage.
Common Envelope Examples
| Envelope | Typical size | Can one Forever stamp work? |
|---|---|---|
| No. 10 business envelope | 9.5 × 4.125 inches | Yes, if machinable and 1 oz. or less |
| A7 invitation envelope | 7.25 × 5.25 inches | Yes, if rectangular, flexible, uniformly thick and 1 oz. or less |
| 6 × 9 envelope | 9 × 6 inches | It fits letter dimensions, but weight, rigidity, thickness and contents still decide |
| Square greeting-card envelope | For example, 5 × 5 inches | Not with one stamp alone at the ordinary machinable price; square pieces are nonmachinable |
| 9 × 12 envelope | 12 × 9 inches | No; it exceeds letter dimensions and is generally priced as a large envelope or another category |
When One Forever Stamp Is Not Enough
- The finished letter weighs more than one ounce.
- The envelope is square, rigid, clasped, uneven, unusually shaped or otherwise nonmachinable.
- It exceeds the letter maximum in any dimension.
- It contains bulky items that make it a large envelope or package.
- It is going outside the United States.
How to Check an Envelope Before Mailing
- Seal and finish the envelope before measuring or weighing it.
- Measure length, height and the thickest point.
- Confirm it is rectangular, flexible and uniformly thick.
- Weigh it with all contents included.
- If it is near a limit or contains rigid material, have USPS verify the category and postage.
Use the postage calculator and decision guide, see when an envelope becomes a package, or review the current value of a Forever stamp.
Frequently Asked Questions
Does a large envelope need two Forever stamps?
Do not decide by stamp count alone. A piece exceeding letter dimensions is priced under the applicable large-envelope or package rules, which depend on weight and physical characteristics.
Can a 6 × 9 envelope use one Forever stamp?
It can if the finished piece remains rectangular, machinable, no more than 0.25 inch thick and no more than one ounce. Rigidity, uneven contents or extra weight changes the postage.
Can a square envelope use one Forever stamp?
An ordinary Forever stamp contributes its value, but a square letter has a nonmachinable characteristic and requires additional postage.
Standards and rates checked August 31, 2026: USPS Postal Explorer letter size requirements and Notice 123 price list. Rules and prices can change.