Short answer

The correct format

Write the recipient name first. On the next line, write the street address followed by the apartment designator and number, such as 123 MAIN ST APT 4B. Finish with the city, two-letter state abbreviation, and ZIP Code. If the apartment information will not fit, place it on the line directly above the street address.

Copy-ready example
JORDAN LEE
123 MAIN ST APT 4B
CHICAGO IL 60610

The standard apartment address format

An apartment number is part of the delivery address, not an optional note. It tells the carrier which unit should receive the mail after the letter reaches the building.

For most US addresses, keep the street address and apartment number together on the delivery address line. Use a recognized unit designator such as APT, UNIT, STE, or BLDG, followed by the unit identifier.

  • Recipient full name
  • Street number and street name + APT and unit number
  • City + state abbreviation + ZIP Code

Where the apartment number goes

The preferred position is at the end of the street-address line: 123 MAIN ST APT 4B. This keeps the building and unit information in one readable block.

When that line would be too long, move the apartment information to the line immediately above the street address. Do not place it below the city, state, and ZIP Code line, where it can be separated from the delivery information.

APT, Unit, #, or a second address line?

APT is the clearest choice for an apartment. UNIT is appropriate when the property uses that wording. STE is normally used for a suite, often in a commercial building. A bare number sign may be understood, but a written designator gives the unit number more context.

Online forms often provide “Address line 2.” That field is a form convention, not a separate postal rule. Put the apartment information there when the form asks for it; the printed label may still combine both fields onto one delivery line.

Checks that prevent apartment mail problems

Use the unit identifier exactly as the resident or building provides it. Do not guess whether 4B should be written as B4, #4-B, or another variation. Confirm the ZIP Code and make sure the recipient name matches the name used for mail at that address.

  • Include the apartment number before mailing
  • Use dark, legible writing on a light envelope
  • Keep the address away from the lower barcode area
  • Check unfamiliar addresses with the USPS ZIP Code lookup

Official references

Reviewed against the postal guidance listed below. EnvelopeFormat is independent and is not affiliated with USPS or Royal Mail.

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