Enter what you're mailing and how much it weighs to see the USPS postage and how many Forever stamps it takes.
Quick answer: A standard 1 oz letter needs 1 Forever stamp (82¢). Each extra ounce adds 29¢, so a 2 oz letter needs 2 stamps and a 3 oz letter needs 2. A large envelope (9×12) starts at $1.69 — about 3 Forever stamps. Rates are current as of July 12, 2026.
Forever stamps are always worth the current 1 oz letter rate, so using them slightly overpays on heavier or oversized mail. Exact rates change — confirm at a post office for anything important.
The number of stamps you need depends on the type of mail and its weight, not just its size. USPS charges a base rate for the first ounce and a smaller amount for each additional ounce. Because a Forever stamp is always worth exactly one first-class 1 oz letter, working out "how many stamps" really means working out the total postage and dividing by the Forever stamp value, rounding up.
A letter over 3.5 oz can't go as a standard letter — it becomes a large envelope or a package. A large envelope holds up to 13 oz before it must ship as a package (Priority or Ground Advantage).
Stamps by letter weight
Letter weight
Postage
Forever stamps
1 oz
$0.82
1
2 oz
$1.11
2
3 oz
$1.40
2
3.5 oz
$1.69
3
Worked example
A wedding invitation in a 9×12 envelope weighing 3 oz is a large envelope (flat). Postage = $1.69 + $0.29 × 2 = $2.27. Divided by the $0.82 Forever value and rounded up, that's 3 Forever stamps (a slight overpay of about 19¢).
Frequently asked questions
How many stamps do I need for a letter?
One Forever stamp covers a standard letter up to 1 oz. Add a stamp for roughly each extra ounce — 2 stamps up to about 2 oz, and so on, up to the 3.5 oz letter limit.
How many stamps for a 9×12 envelope?
A large envelope starts at $1.69 for the first ounce, so about 3 Forever stamps for a light one, plus one more for roughly every 2–3 additional ounces.
Can I use two Forever stamps instead of exact postage?
Yes. Forever stamps never expire and you can combine them. Using them on heavier mail slightly overpays, but the mail still goes.
How many stamps for an international letter?
A 1 oz letter to another country uses the Global Forever stamp, which costs more than a domestic Forever stamp. This calculator covers US domestic mail.
Based on USPS domestic rates effective July 12, 2026. Rates change periodically — confirm current pricing at USPS for anything important. Not affiliated with USPS.
Written by the CalcPine team · Reviewed for accuracy · Last updated 24 July 2026 · Method: USPS first-ounce + additional-ounce rates; Forever stamps = postage ÷ $0.82, rounded up.
