How many stamps do I need?

How the calculator works out your postage

What a letter costs depends on two things: what kind of mail piece it is and how much it weighs. The calculator starts with your envelope, because that settles the first part: a #10 business envelope or a 5×7 invitation envelope travels as a letter, while a 9×12 is always a large envelope with higher prices. Then you count the sheets and add a card, or type in the weight if you've already weighed the letter, and the calculator shows the format, the price and which stamps to put on.

USPS charges in one-ounce steps, so a letter that is a fraction over one ounce pays the full two-ounce price. The steps stop before four ounces, because USPS treats a piece as a letter only within these limits: Up to 11½ × 6⅛ in, ¼ in thick, 3.5 oz. A heavier piece pays large-envelope prices, so a letter of 3¾ or four ounces costs $2.56. A kitchen scale is fine for checking at home: weigh the sealed envelope with everything inside, and remember that one ounce is about 28 grams. Counting sheets only gives an estimate from typical paper and envelope weights, so if your letter is close to a step, weigh it or pay for the next step up.

When size, shape or padding changes the price

An envelope that is longer, taller or thicker than the letter limits allow is priced as a large envelope, which USPS also calls a flat, so a 9×12 envelope with a single sheet in it costs $1.69 instead of $0.82. A large envelope has limits of its own: Bigger than a letter, up to 15 × 12 in, ¾ in thick, 13 oz; must be flexible and evenly thick. An envelope that is stiff, bulging or past those limits doesn't count as a flat either, so it is priced as a parcel.

Shape matters for ordinary letters too, because the sorting machines need letters that are rectangular and bend easily. A square invitation small enough to count as a letter, an envelope closed with a clasp or buttons and a stiff photo mailer that is still within the letter limits all pay a non-machinable surcharge of $0.49 on top of the weight price. A square letter of up to one ounce therefore costs $1.31, which two Forever stamps cover. A rigid mailer bigger than the letter limits is priced as a parcel, because a large envelope has to be flexible.

A wax seal, ribbon or twine left exposed on the outside adds the same surcharge, because a ribbon counts as string and a seal makes the envelope lumpy. There are two ways to send such an envelope: for specially decorated envelopes, USPS lets you pay the non-machinable surcharge and ask the Post Office counter to hand-cancel them, and for wax seals, strings, ribbons and other decorations on the outside it advises not to send them exposed but to put the envelope inside another envelope. With an outer envelope, the surcharge depends on whether the finished piece is still flexible and evenly thick, which a thick seal can spoil.

Bigger square envelopes, such as the 6½ and 7 in sizes many wedding invitations come in, are taller than the letter limits allow, so they are priced as large envelopes from $1.69 rather than as a letter plus the surcharge. Padded and bubble mailers usually end up as parcels, because once something is inside they are too rigid or lumpy for a letter or a large envelope. Parcel prices depend on weight and distance, so for those use USPS's own calculator.

Using Forever stamps for letters over one ounce

A Forever stamp always covers a one-ounce letter at today's price, which is currently $0.82. A two-ounce letter costs $1.11, and you can pay it in two ways. One Forever stamp plus $0.29 in extra postage matches the price exactly. Two Forever stamps cost a little more, but they save you a trip to the post office if a book of Forever stamps is all you have at home. The US stamp prices page lists every step.

Paying too much is allowed, and USPS simply keeps the difference. Paying too little is the real risk. A letter without enough postage is either returned to you or delivered "postage due," which means the person receiving it pays what's missing. On a wedding invitation or a letter to a client, a few extra cents is worth it to avoid that.

Five worked examples

A one-page letter in a #10 envelope weighs well under an ounce, so it needs one Forever stamp. Five sheets of ordinary copy paper and the envelope together weigh about an ounce, which puts the letter right on the line. The safe choice is the two-ounce price of $1.11: one Forever stamp plus the extra ounce, or two Forever stamps.

A 5×7 wedding invitation on heavy card, with an RSVP card and its own small envelope, usually stays under an ounce in an A7 envelope. One Forever stamp covers it as long as it bends easily and has nothing lumpy on or in it, though a details card, an envelope liner or a vellum wrap can push it over an ounce. If the invitation is stiff, has an exposed wax seal or ribbon, or comes in a 5½ or 6 in square envelope, the surcharge explained above applies, so it needs $1.31, which two Forever stamps cover. Put a Forever stamp on the RSVP envelope too, and before you buy stamps for the whole guest list, take one finished invitation to the post office to have it weighed and checked for the surcharge.

Ten unfolded sheets in a 9×12 envelope weigh about two and a quarter ounces: roughly half an ounce for the envelope and the rest for the paper. That puts them at the three-ounce large-envelope price of $2.27. Three Forever stamps cover it, while two wouldn't even cover the one-ounce price for a large envelope. If you add a cardboard stiffener, the envelope is no longer flexible and is priced as a parcel.

A two-page letter to Canada stays under an ounce and costs $1.75, which is exactly one Global Forever stamp; a regular Forever stamp alone isn't enough. Prices go up faster for mail abroad, so at two ounces the same letter costs $2.35. Our guide on how to address an envelope shows how to lay out an address in another country.