Home mailbox
Put the letter inside and raise the flag if the box has one, so the carrier on the regular delivery route knows to collect on the way past.
One envelope, five steps
How to mail a letter in an envelope
Five things have to happen in order: fold the contents flat, seal the envelope, address it, add postage, and get it into the mail stream. usps.com lays out that sequence on its own send-a-letter page, read 2026-09-01, and the first two steps are the ones a plain addressing or postage guide tends to skip past.
USPS steps, citedRead 2026-09-01Five steps, in order
Fold, seal, address, stamp, send
In that order. Skipping the fold-flat limit in step one is what quietly turns a letter into a nonmachinable surcharge later.
1
Fold the contents and slide them in
usps.com puts a real limit on this step: whatever goes inside has to lie flat at no more than a quarter inch thick once it is folded, read 2026-09-01. A single folded sheet, a check, a greeting card or a handful of photos all sit well under that. A stack of pages, a keepsake taped in, or anything with genuine thickness pushes the piece past the letter rate before it ever reaches the flap.
Fold along creases the paper wants to make and skip the paperclip or staple — a raised bump is exactly what USPS calls nonmachinable, adding $0.49 to anything rigid, lumpy, or carrying a clasp, string or button, per the same page. The envelope itself tops out at 11-1/2 by 6-1/8 inches, well above what a standard No. 10 envelope (9-1/2 by 4-1/8 inches) already holds, so size is rarely the binding constraint.
2
Seal the flap
Once the contents are inside, usps.com's own instruction is direct: “put what you're sending inside the envelope, then close and seal it (using the envelope's glue or tape),” read 2026-09-01.
Most envelopes sold today are self-seal — peel a strip, press the flap down. Older gummed stock needs moisture instead. Either way, the seal has to hold through a sorting machine and a truck ride, so a flap that keeps popping back open is worth reinforcing with a strip of tape rather than resealing it twice. Tape on the flap does not trigger the nonmachinable surcharge; a rigid object or a metal clasp underneath it does.
3
Address it
An envelope carries the same two names as a package — sender and recipient — but the layout exists for a different reason. There is no barcode label to leave room for, so usps.com puts three things directly on the paper: the return address in the top left corner, the delivery address in the bottom center of the same face, and the stamp in the upper right corner, read 2026-09-01. A package label usually carries one printed block plus a routing barcode across most of the face, with the return address squeezed into a smaller corner; a plain stamped envelope has no barcode to plan around, just two written blocks and a stamp.
Each address keeps to the same three-line structure USPS Publication 28 uses everywhere: name on the first line, street address or PO Box on the second, then city, state and ZIP together on the last line, printed in capital letters rather than script since sorting equipment reads block letters more reliably than a signature. Our envelope addressing guide works through that block corner by corner with a labeled diagram, and our package addressing guide covers the barcode-label version of the same layout.
4
Add the postage
One Forever stamp, 0.82 USD, covers a standard envelope up to 1 oz, going in the upper right corner next to the address block. Every ounce past that adds 0.29 USD to a plain letter; once the piece stops being letter-shaped — heavier, stiffer or larger than the size from step one — it prices as a large envelope starting at 1.69 USD, and a postcard uses its own 0.65 USD stamp instead. All four figures are read from usps.com's send-a-letter page on 2026-09-01.
The full weight ladder past 1 oz, and how many stamps to reach for at each step, live on our Forever stamp price guide and stamp-count guide. This page only needs the one number that gets a normal letter out the door.
5
Get it into the mail stream
Sealed, addressed and stamped, a letter has four ways to leave your hands, all named on usps.com's send-a-letter page and read 2026-09-01.
Cluster mailbox slot
Apartment and condo mail rooms usually run a separate slot for outgoing mail rather than sharing the locked delivery compartment; that slot, not the box with your name on it, is where a stamped letter goes.
Blue collection box
Fine for anything stamped 10 oz or under and no thicker than half an inch. Heavier or thicker stamped mail past that line has to go to a person at a counter instead.
Post Office lobby drop
Open whenever the building is, even outside window hours, and the option a collection box would turn away always falls back to this one.
Our drop-off breakdown goes deeper on the flag, the collection box and the lobby option, including how the 10 oz and half-inch limits get checked. This page adds the cluster-mailbox slot, which most apartment buildings use and which that guide does not name; our mailbox locator guide covers finding a collection box or branch nearby.
When plain postage is not the right call
This process covers a standard First-Class letter moving inside the United States. A few situations branch off it early. A letter that needs proof of mailing or proof of delivery follows a different set of steps, covered in our certified mail guide. A letter leaving the country needs a Global Forever stamp and a different customs framing, in our international letter guide. And once the contents run past the quarter-inch limit from step one, the piece stops being a letter at all — our package addressing guide and the rate calculator pick up from there.
Where these rules come from
- USPS, How to Send a Letter or Postcard — envelope size and thickness, folding, sealing, address and stamp placement, postage prices and the four drop-off options. Read 2026-09-01.
usps.com revises this page on its own schedule, without an announcement. The steps and figures above reflect what it stated on 2026-09-01 — confirm anything time-sensitive against the live page before you seal the envelope.
Common questions
What is the maximum thickness for a letter envelope?
A quarter inch once the folded contents are inside, per usps.com's send-a-letter page, read 2026-09-01. Past that, or with a clasp, string or rigid object inside, the piece is nonmachinable and carries a $0.49 surcharge on top of the letter rate.
Do I need to seal the envelope before mailing it?
Yes. USPS's own instruction is to put the contents inside the envelope, then close and seal it using the flap's glue or a strip of tape, read 2026-09-01. An unsealed flap can come open going through a sorting machine.
How do you put a return address on an envelope?
In the top left corner of the same side that carries the delivery address, three lines: your name, then street address or PO Box, then city, state and ZIP together. That corner placement is specific to a plain stamped envelope; a shipping label carries a smaller return-address block because the rest of the face is built around a tracking barcode instead, which an envelope does not have.
Where does the stamp go on a letter?
The upper right corner, opposite the return address, per usps.com, read 2026-09-01. A First-Class Forever stamp there covers a standard envelope up to 1 oz; heavier letters need additional postage in the same corner.
Can I drop a stamped letter in an apartment mailroom slot?
Yes, when the building uses a cluster mailbox. USPS names the outgoing mail slot on a cluster box as a valid drop-off point alongside a home mailbox, a blue collection box and a Post Office lobby, read 2026-09-01.
What happens if a letter goes out without enough postage?
It becomes postage due, and USPS collects the shortfall from whoever ends up holding it, usually the recipient. Our postage-due guide covers how that charge gets collected and how to avoid triggering it.