How Many Stamps Do I Need? USPS Stamp Calculator by Weight and Envelope Type

Most of us know that a basic letter needs one stamp. The confusing part starts when the envelope is heavier than one ounce. I used to assume that meant adding another Forever Stamp, but USPS sells cheaper additional-ounce stamps for exactly this situation.

This guide will help you estimate the number of stamps your finished envelope needs. If you are mailing a postcard, large envelope, or international letter, check the full USPS postage-rate guide because those pieces use different prices.

Skip the Line With a USPS Self-Service Kiosk

A lot of larger Post Offices have quick and easy self-service kiosks. Typing an address on the screen can be a little cumbersome, but they are a great option when you want to skip the counter line and make sure you are paying the correct postage.

The kiosk walks you through entering the mailing information, weighing your letter or package, paying for postage, and printing what you need in one stop. I use these when I want the Post Office's scale and postage calculation without waiting for the retail counter.

How Many Stamps Do You Need by Letter Weight (2026)

A Forever Stamp covers a standard letter weighing up to one ounce. USPS rounds any fraction of an ounce up to the next ounce, which is why a 1.5-ounce letter costs the same to mail as a 2-ounce letter. After the first ounce, use one $0.29 additional-ounce stamp for each new ounce or fraction of an ounce.

Sources: USPS Notice 123 effective July 12, 2026 for the current rates and Domestic Mail Manual 133 for ounce rounding and the 3.5-ounce letter limit.

What Counts as Additional Ounces When Calculating Stamps?

It is surprisingly hard to guess how much a finished envelope weighs. People are not very good human scales, and small differences in paper stock, envelope thickness, photos, cards, or a return envelope can push a letter into the next ounce.

If you are mailing a standard envelope with a few sheets of ordinary paper, a page-count estimate will usually get you into the right range. The best answer is still to weigh the sealed envelope. If the estimate is close to the next ounce, use a kitchen or postal scale, a USPS self-service kiosk, or the counter at a Post Office instead of guessing. Keys, USB drives, padding, and other rigid or lumpy contents are a different issue because they can change how USPS classifies the mailpiece.

When Regular Letter Postage Does Not Apply

The stamp table above is only for standard, flexible rectangular letters. Size, shape, rigidity, and destination can move a mailpiece into a different USPS price category.

Large Envelopes

An envelope moves into the large-envelope, or flat, category when it exceeds any standard letter maximum: 11½ inches long, 6⅛ inches high, or ¼ inch thick. A flexible, rectangular, uniformly thick flat starts at $1.69 for one ounce, then increases with weight. If it is rigid or uneven, USPS may charge parcel prices instead.

Square, Rigid, or Uneven Letters

A letter that is square, too rigid to bend, closed with clasps or string, or uneven because of its contents may be nonmachinable. The current surcharge is $0.49, making a one-ounce nonmachinable letter $1.31. A very thick, rigid, or lumpy piece may be classified as a flat or parcel instead of a letter, so take it to the counter when you are unsure.

International Letters

One $1.75 Global Forever Stamp covers a standard one-ounce First-Class Mail International letter to an eligible country. Heavier international letters do not use the simple domestic additional-ounce calculation because prices vary by destination group and weight.

Sources: USPS Notice 123 effective July 12, 2026 and Domestic Mail Manual 101.

Examples: How Many Stamps for Common Letters

I have mailed a lot of letters, and this is the rough cheat sheet I use when I do not have a postage scale nearby. I strongly recommend weighing your finished envelope before sending it, or taking it to the Post Office when you are unsure.

If you are in a hurry and need to estimate an ordinary paper letter, these examples provide a reasonable starting point. Paper, envelopes, photos, and cards vary, so treat them as estimates rather than exact weights.

Rate sources: USPS Notice 123 effective July 12, 2026. Classification source: Domestic Mail Manual 101.

Postcard Stamps and Greeting Card Postage

A lot of people outside the mailing industry do not realize that USPS sells stamps specifically for postcards. A Postcard Stamp currently costs $0.65, which saves you $0.17 compared with using a regular Forever Stamp.

You can use a regular Forever Stamp on a domestic postcard because it provides more than enough postage, but USPS will not refund the difference. The lower postcard price only applies when the card meets the USPS size, shape, and thickness rules.

A domestic card qualifies for the postcard price when it is rectangular, at least 5 inches long by 3½ inches high, no more than 6 inches long by 4¼ inches high, and between 0.007 and 0.016 inch thick. A larger card may pay the letter rate if it still meets letter standards. Pieces outside the letter standards may be priced as flats or parcels.

Sources: USPS Notice 123 effective July 12, 2026 for postcard prices and Domestic Mail Manual 101 for postcard and letter dimensions.

Where to Buy Stamps

Believe it or not, stamps are everywhere. People still mail things every day, so you usually do not need to make a special trip to a Post Office just to find a booklet of Forever Stamps.

The Post Office and USPS self-service kiosks are the most dependable places to find the exact stamp type you need, including postcard and additional-ounce stamps. Regular Forever Stamp booklets are also commonly available at grocery stores, pharmacies, big-box stores, office-supply stores, and some bank branches. Availability varies, so call ahead if you need something other than ordinary Forever Stamps.

Buying stamps in person

Start with a Post Office or self-service kiosk when you need a specific denomination. For a basic booklet of Forever Stamps, check the customer-service desk or checkout area at nearby grocery stores, pharmacies, office-supply stores, and large retailers.

Buying stamps online

The safest online option is the USPS Postal Store. Other retailers sell stamps online, but compare the total price and seller carefully. A third-party listing may include a markup, shipping charge, or stamps from an unverified seller.

USPS resource: USPS Location Finder for nearby Post Offices, approved postal providers, and self-service kiosks.

FAQs

What happens if I don't use enough stamps?

USPS may mark an ordinary shortpaid letter "Postage Due" and ask the recipient to pay the missing amount. If the recipient refuses it, or USPS cannot deliver it, First-Class Mail with a return address is returned for additional postage. A shortpaid nonmachinable First-Class Mail letter is returned to the sender instead of being offered postage due.

Can I use multiple Forever Stamps instead of additional ounce stamps?

Yes, but it is more expensive. Two Forever Stamps provide $1.64 in postage, while a two-ounce letter needs $1.11.

Do I need special stamps for important documents?

No. The postage depends on the size, shape, and weight of the mailpiece, not how important the contents are. Certified Mail is an optional extra service that provides a mailing receipt and electronic verification that the item was delivered or that delivery was attempted. It does not make the letter travel faster, guarantee successful delivery, or insure the contents. A Return Receipt is an additional service when you need a signature record.

How many stamps do I need for a 9x12 envelope?

A flexible, rectangular, uniformly thick 9x12 envelope is generally treated as a large envelope, or flat. Postage starts at $1.69 for the first ounce and increases with weight. One Forever Stamp plus three additional-ounce stamps provides exactly $1.69. A rigid or uneven 9x12 mailpiece may pay parcel prices instead.

How many stamps do I need for a 5-page letter?

There is no exact answer based only on page count. USPS gives four ordinary sheets plus a business envelope as a rough one-ounce example, so a five-page letter may be close to or over one ounce depending on the paper and envelope. Weigh the sealed letter for an exact answer.

How many stamps for a 1.5 oz letter?

A 1.5-ounce letter needs one Forever Stamp plus one additional-ounce stamp, for $1.11 in total postage. USPS counts any fraction of an ounce as the next whole ounce for First-Class Mail pricing.

FAQ sources: Domestic Mail Manual 604 for insufficient postage, USPS domestic extra services for Certified Mail, and USPS letter-mailing guidance for the paper-sheet rule of thumb.