First class postage is the standard mail service from USPS for sending personal and business correspondence, including letters, postcards, and large envelopes. A single one-ounce letter costs $0.78 using a Forever stamp. This is the service most people use when they drop a bill payment, greeting card, or personal letter in the mailbox.
What a Forever Stamp Costs
A Forever stamp currently costs $0.78 at the Post Office. That price covers a standard letter weighing up to one ounce. “Forever” means the stamp will always cover a one-ounce first class letter, even if rates go up after you buy it. So a book of stamps purchased today at $0.78 each will still work next year without needing extra postage.
Each additional ounce on a letter costs extra. If your letter weighs two ounces because you stuffed in a few extra pages or a photo, you’ll need to add postage beyond the single Forever stamp. These additional-ounce stamps are sold separately at the Post Office or online.
What Counts as a Letter
USPS has specific size rules that determine whether your mail piece qualifies as a standard letter or gets bumped into a more expensive category. A letter must be between 5 and 11.5 inches long, between 3.5 and 6.125 inches high, and no thicker than 1/4 inch. It also can’t weigh more than 3.5 ounces. A standard #10 business envelope and most greeting card envelopes fit these dimensions easily.
If your piece exceeds any of those limits, even by a fraction, USPS reclassifies it as a “flat” (large envelope) and charges a higher rate. This catches people off guard when they mail something slightly bulky, like a padded greeting card or a letter with a key taped inside that pushes the envelope past 1/4 inch thick.
Large Envelopes and Flats
Large envelopes, which USPS calls “flats,” still travel as first class mail but cost more than a standard letter. A flat can be up to 15 inches long, 12 inches high, and 3/4 inch thick. The maximum weight for any first class mail piece, whether letter or flat, is 13 ounces.
You’d use this category for things like manila envelopes holding legal documents, 8.5-by-11 papers you don’t want to fold, or magazines you’re forwarding to someone. The postage goes up with weight, so a heavier flat costs more than a lighter one.
What Happens Above 13 Ounces
Once your mail piece weighs more than 13 ounces, it no longer qualifies for first class mail. For packages, USPS now uses a service called Ground Advantage, which handles items from under a pound all the way up to 70 pounds. For lightweight packages up to 15.999 ounces, Ground Advantage pricing is based on weight (rounded up to 4, 8, 12, or 15.999 ounce tiers) and the distance it’s traveling. Heavier packages are priced by the pound, rounded up.
This matters if you’re mailing a small parcel and wondering why the clerk doesn’t just slap first class postage on it. The 13-ounce ceiling is a hard cutoff for letters and flats. Packages have their own pricing structure through Ground Advantage.
How First Class Differs From Other Services
First class mail gets priority handling over bulk mail categories like Marketing Mail (the ads and catalogs that fill your mailbox). A first class letter typically arrives in two to five business days for domestic delivery. It also includes forwarding, so if the recipient has moved and filed a change of address, the letter follows them to their new address. Bulk mail doesn’t get this treatment.
First class is not the same as Priority Mail, which costs more and promises faster delivery (usually one to three business days). If you need guaranteed speed or tracking on a letter, Priority Mail or Priority Mail Express is a separate, pricier service. Standard first class letters don’t come with tracking, though first class packages do.
Ways to Buy Postage
You can buy Forever stamps at any Post Office, through usps.com, at most grocery stores, and at many pharmacies and big-box retailers. Stamps come in books of 20, coils of 100, or individual stamps at the counter. The price per stamp is the same regardless of where you buy them.
If you’re mailing something that needs more than one stamp, like a heavy letter or a large envelope, bring it to the Post Office counter or use the self-service kiosk so the clerk or machine can weigh it and print the exact postage. Guessing and sticking on extra stamps works but often means you’re overpaying. Underpaying means your mail comes back to you or the recipient owes the difference.