Postage is decided by weight, shape and size — and shape is the one people never check. Here are the actual numbers.
To qualify as a letter
The envelope must be rectangular and fall inside these limits:
- Minimum: 3½ inches high × 5 inches wide, at least 0.007 inches thick
- Maximum: 6⅛ inches high × 11½ inches wide
Two examples of near-misses, because they are the ones that catch people:
A 3″ × 6″ envelope fails — the short side is under 3½ inches. A 6½″ × 9″ envelope also fails — its short side exceeds 6⅛ inches, so it is a large envelope, not a letter.
The aspect ratio
Length divided by height must fall between 1.3 and 2.5. Length is the dimension parallel to the address.
Below 1.3 the envelope is too close to square for the machines to orient. Above 2.5 it is too elongated. Either way it becomes non-machinable, which is why square envelopes always attract the surcharge — a square has a ratio of exactly 1.0.
What else triggers the surcharge
Beyond the ratio, mail is non-machinable if it is:
- Rigid — it will not bend around the machine's rollers
- Uneven in thickness — buttons, ribbon, anything lumpy inside
- Sealed with wax — inside or outside the envelope
- Too thin — under 0.009 inches for mail over 4¼ inches high or 6 inches long
- Addressed along the short edge — vertical addressing counts
The surcharge is $0.17, and it applies to international letters at the same rate.
Worked examples
Standard letter, under 1 oz: 60 cents — one Forever stamp.
Square envelope under 1 oz, or a rectangular one between 1 and 2 oz: 84 cents, or a single two-ounce Forever stamp. This covers most wedding invitations.
A 1.05 oz letter with a wax seal: $1.01 — 84 cents by weight plus the 17-cent surcharge. Two separate charges, which is why people underestimate it.
A flat at 1.05 oz: $1.44. Large envelope rates start at $1.20 and rise in 24-cent steps by weight.
Large envelopes and packages
Over 6⅛ inches tall, 11½ inches wide, or ¼ inch thick, and it is a large envelope — a flat — with its own rate table.
The letter-shape rules do not carry over to flats. What matters there instead: if a flat is rigid, or contains something that varies its thickness by more than ¼ inch, it may be reclassified as a package, which is a considerably larger jump in price.
Paying for extra weight properly
First-Class letters can weigh up to 3.5 ounces. Above one ounce, each additional ounce costs 24 cents — not another full-rate stamp.
That is the saving people miss. An additional-ounce stamp at 24 cents does the job of a second 60-cent Forever stamp, for less than half the price.
Small make-up denominations of 5 and 10 cents exist, but an additional-ounce stamp is simpler and harder to get wrong.
You can put three Forever stamps on a three-ounce flat and it will be accepted — you will simply have overpaid. Postage above the required amount is never refused; it is just wasted.
Best deals on Forever stamps
Keep three denominations on hand and most of this becomes automatic: standard Forever, additional-ounce, and two-ounce. Between them they cover nearly everything a household or small office sends.
Where to buy
The USPS official store for the specific denominations and postcard rates, or a verified surplus channel such as Forever Stamp Center for postage stamps in volume.
And the advice that never stops applying: take one assembled sample to a counter and have it weighed and measured before buying for the whole mailing. The online calculator gives a good estimate; a clerk with the actual envelope gives an answer.
The short version
Rectangular, between 3½ × 5 and 6⅛ × 11½ inches, aspect ratio between 1.3 and 2.5, nothing rigid or lumpy. Miss any of those and add 17 cents. Above one ounce, use a 24-cent additional-ounce stamp rather than a second Forever stamp.
More on buying Forever stamps
- 👉 Square, Lumpy, Rigid or Dark: Four Things That Add a Surcharge
- 👉 Two-Ounce Stamps for Heavy Letters and Invitations
- 👉 How Many Stamps Do I Need for a Small Package?
USPS specialist based in Los Angeles with extensive experience in postal operations and customer service. Certified in mail management and trained at the USPS Business Mail Academy, she offers reliable guidance on Forever Stamps and practical mailing solutions.
