Short Answer: One $0.55 First-Class Mail stamp covers one ounce. However, you do not need one stamp per ounce. First-Class Mail prices anything above one ounce at an additional $0.15 per ounce ($0.70 for two ounces, $0.85 for three ounces, etc.), and you can purchase Additional Ounce stamps for $0.15 each. For more details about how many stamps to use, see below.
How Many Stamps per Ounce/Ounces for One Stamp?
One First-Class Mail stamp (including Forever stamps) costs $0.55 and covers one ounce. However, this does not mean you need one $0.55 stamp per ounce when sending letters or packages.
You can use United States Postal Service (USPS) First-Class Mail stamps for letters up to 3.5 ounces, and the price will increase $0.15 with each ounce ($0.70 for two ounces, $0.85 for three ounces, and $1 for 3.5 ounces).
Note that even though the price per ounce is not equivalent to using one stamp per ounce, you should not cut any of your stamps in halves or thirds and add a partial stamp to your package. This will only invalidate the stamp. Instead, use one of the following options:
- Use one Forever stamp, then pay the remainder of the postage at the post office counter when you mail the letter.
- Use one Forever stamp and purchase First-Class Mail Additional Ounce stamps ($0.15 each) to cover the remaining postage.
Our related research lists where to buy stamps.
Postage for Large Letters and Postcards
Letters larger than 3.5 ounces are considered large envelopes and anything over 13 ounces is considered a parcel. Letters and packages of these types are priced differently than stamped letters based on their weight, shipping method, and destination. If you know how much your envelope weighs, you can use the U.S. Postal Service’s postage rate calculator to determine how much you’ll have to pay.
Additional Costs for International Mail
It costs $1.20 to send a First-Class Mail letter or postcard that weighs one ounce or less internationally. This includes Canada and Mexico. If your letter weighs more than one ounce, the cost of the postage will vary by country — ranging from $1.20 to $4.32 for letters up to 3.5 ounces.
Where to Place Your Stamp(s)
You should place your stamp on the upper right of the envelope you wish to mail, on the same side where you write the address. If you need more than one stamp, place them all in the upper right corner, but be sure that they do not overlap.
There is no maximum number of stamps you can place on an envelope, but the USPS does require that the stamps not cover the addresses. Keep in mind, too, that your First-Class Mail letter should not exceed 3.5 ounces.
Want to make it easy? See our list of places that sell pre-stamped envelopes.
Costs for Other Classifications of Mail Service
The costs quoted above are for First-Class Mail as defined by the USPS. There are other services available that will get your letter to its destination faster, but these will be more expensive. These services are also suitable if what you are mailing exceeds the 13-ounce First-Class Mail weight limit. See pricing, timing, and more details in the table below.