Should I use APT or a pound sign for unit numbers?

USPS Publication 28 says to use a designator such as APT or UNIT rather than a pound sign when the correct one is known.

What is the best way to number units?

Floor plus position, such as 101 and 201, is common for multi-floor buildings. Pick one rule and use it on every floor.

Can I renumber units after tenants move in?

You can, but it disrupts mail and deliveries. Update leases, labels and the post office together, and give tenants notice.

How to Number Apartment Units in a Building

Give every unit one short, unique number that follows the floor, such as 101 to 104 on the first floor and 201 to 204 on the second, and use the same number on the door, the mailbox, the lease and the rent memo. USPS says a unit designator such as APT or UNIT is required in the address and should be used instead of a pound sign.

Why consistent numbers matter

A unit number is used by carriers, delivery drivers, ambulance crews, your tenants, your leases, your bank memos and your books. If the lease says Unit B, the door says 2 and the mailbox says 2B, packages go to the wrong door and a payment memo is hard to match. One number everywhere fixes most of it.

Common formats

Format	Example	Works best for
Floor plus position	101, 102, 201, 202	Multi-floor buildings: the first digit is the floor
Letters	A, B, C, D	Small buildings on one level, or a single-floor walk-up
Floor plus letter	1A, 1B, 2A, 2B	Smaller buildings where letters read cleanly
Building plus unit	Bldg 2, Unit 12	Several buildings on one lot

Number left to right or clockwise from the entrance, and apply the same rule on each floor. Skip numbers that cause confusion, such as 13 if your tenants would object, and avoid mixing letters and numbers inside one building.

What USPS requires in the address

USPS Publication 28 says secondary unit designators are required on mail for addresses that have them, preferably at the end of the delivery address line, and lists APT and UNIT among the common designators. It adds that the pound sign should not be used if the correct designator is known, and if one is used there must be a space between it and the number (USPS Publication 28, read 2026-10-31). Use a format like 410 Cedar Row Ln APT 2B in your leases and listings.

Label everything the same

Door: large, reflective numbers visible from the hallway and, where possible, in low light.
Mailbox: the same number, with the name as an optional label that you change at move-out.
Lease and unit record: the identical number, plus the address format above.
Floor and stair signs: a simple "Units 201 to 204" sign at the landing.
Entry and intercom: unit numbers on the directory or buzzer.

When you renumber, update every place at once, and tell tenants, the post office and delivery services before the change. Avoid renumbering an occupied building unless you must.

Common numbering mistakes

The most common error is two numbering styles in one building, such as 2B on the lease and 202 on the door. Another is a unit with no number on the mailbox, so the carrier returns mail. A third is leaving out the unit number on listings, which sends applicants to the wrong door.

Fix them in one pass. Walk the building with the lease list, compare each door and mailbox to the record, and correct whichever one is off. Then check your listing, your rent memo format and your vendor job links so every one carries the same number.

If you buy a building with messy numbers

After a purchase you may inherit numbers that do not match the plans, the leases or the doors. Resist renaming everything on day one. First list what exists, find which numbers the tenants and the post office already use, and pick the version most people already know. Then change only what is needed and give notice before you do.

How DoorKeeping handles it

DoorKeeping uses the unit number as the key for each lease. Each tenant gets a rent payment memo built from it, such as 12B SMITH, and an exact amount with its own cents so deposits are easy to tell apart. A clean unit list makes matching reliable. It costs $2 per unit per month with every feature included.

Sources

USPS Publication 28: secondary address unit designators, read Oct 31, 2026

Questions

Should I use APT or a pound sign for unit numbers?: USPS Publication 28 says to use a designator such as APT or UNIT rather than a pound sign when the correct one is known.
What is the best way to number units?: Floor plus position, such as 101 and 201, is common for multi-floor buildings. Pick one rule and use it on every floor.
Can I renumber units after tenants move in?: You can, but it disrupts mail and deliveries. Update leases, labels and the post office together, and give tenants notice.