Apartment mailboxes that receive USPS delivery must meet USPS-STD-4C if the building is new or substantially remodeled. In practice that means horizontal, wall-mounted commercial mailboxes with compartments of at least 3 x 12 x 15 inches, tenant locks no higher than 67 inches, enough parcel lockers, and a location your local postmaster approves before you buy.
This guide walks through each requirement in plain terms: what USPS-STD-4C says, when it applies, what the Postal Operations Manual adds, how ADA rules fit in, and what to do if your building still has older 4B or 4B+ boxes. Where a number matters, we link the USPS source so you can show it to your architect or board.
What is USPS-STD-4C?
USPS-STD-4C is the Postal Service standard for wall-mounted centralized mail receptacles: the horizontal mailbox banks you see in apartment lobbies, dorms and office buildings. It covers how the units are designed, tested and approved, from compartment sizes to lock placement. You can read the full standard on the USPS website.
4C replaced the older USPS-STD-4B standard. The final rule was published in the Federal Register in September 2004, and the committee that wrote it set mandatory compliance for new construction at two years after publication, which is why you will see "since 2006" in most guides. The old vertical box format (about 5 inches wide and 15 inches tall) was eliminated.
Two things changed most for owners: compartments got bigger so they hold magazines and flats without folding, and parcel lockers became part of the standard.
When are 4C mailboxes required?
The USPS delivery planning handbook (PO-632) is direct: "All new or remodeled apartment houses must install USPS-approved 4C centralized mailbox equipment." The Postal Operations Manual (section 632.62) says the same and adds that obsolete receptacles should be replaced with currently approved ones when a building is substantially renovated or when box locations change.
4C is required for:
- New apartment buildings, condos, dorms and mixed-use buildings that get USPS delivery to a central mail area.
- Buildings undergoing a substantial renovation or remodel, especially work that changes the structure or the mail area.
- Any project where the mailbox location is being moved.
4C is generally not required when you are simply repairing or replacing worn boxes in an existing building and nothing else changes. USPS guidance from 2008 notes that 4C is "primarily intended for new construction" and "not as a means of retrofitting existing complexes." That is where 4B+ comes in (see below). Your postmaster decides which case your project falls into, so ask before you order.
USPS-STD-4C requirements at a glance
These are the numbers that matter when you lay out a mail area. Heights are measured from the finished floor.
| Requirement | USPS-STD-4C rule |
|---|---|
| Minimum customer compartment | 3 in. high x 12 in. wide x 15 in. deep (taller sizes allowed) |
| Standard parcel locker | 15 in. high x 12 in. wide x 15 in. deep |
| Large parcel locker | 18 in. high x 12 in. wide x 15 in. deep |
| Highest tenant lock | No more than 67 in. above the floor |
| Lowest customer compartment | Interior bottom shelf at least 28 in. above the floor |
| Lowest parcel locker | Interior bottom shelf at least 15 in. above the floor |
| Accessible compartment | At least one customer compartment below 48 in. |
| USPS Arrow lock (carrier access) | Between 36 and 48 in. above the floor |
| Outgoing mail | One reinforced collection compartment per unit |
| Mounting | Designed for fully recessed wall mounting, indoors or outdoors |
The 67-inch and 28-inch limits are why a single 4C column tops out at a certain number of doors. If you need more compartments, you add columns rather than stacking higher.
How many parcel lockers does an apartment building need?
This is the most misquoted rule, because two USPS documents give two ratios:
- The USPS-STD-4C specification (section 3.7.2.1) sets a minimum of one standard parcel locker for every ten customer compartments, with no mandatory locker below ten compartments.
- The Postal Operations Manual (632.622) and the current PO-632 handbook require at least one parcel locker for every five mailbox compartments, and say there is no maximum.
Plan for the stricter one: one parcel locker per five doors. It is the rule in the current USPS manuals for new or remodeled buildings, and with today's package volume most properties want more lockers than the minimum anyway. A 40-unit building needs at least 8 parcel lockers.
If your mail area cannot fit enough lockers, outdoor parcel lockers can make up the difference. See our guide to what parcel lockers are and how they work for the options.
Where apartment mailboxes can go
POM 632.622 and PO-632 set the location rules:
- Put the receptacles and parcel lockers reasonably close to the entrance, in a vestibule, hall or lobby.
- Keep the carrier's working area clear of swinging or open doors, and make sure it is well lit.
- Exterior installations on a building wall can be approved, but not directly on a street or public sidewalk. USPS asks for at least 15 feet of separation from the street or sidewalk where possible, and requires a canopy for weather protection and adequate night lighting.
- If the boxes sit behind a locked entry door, agree with the postmaster how the carrier will get in before you finalize the layout.
Get the postmaster's sign-off on the location in writing before you frame the wall. Moving a recessed opening later is expensive.
Front-loading vs rear-loading 4C mailboxes
Every 4C unit is either front-loading or rear-loading, and the choice drives the floor plan.
| Front-loading | Rear-loading | |
|---|---|---|
| How the carrier loads | Opens a master door on the front of the unit | Works from a room behind the wall |
| Space needed | Wall space only | A secure mail room behind the boxes |
| Extra USPS rules | Fixed solid back on the unit | Mail room door with an ANSI 156.13 F15 lock, carrier key in a USPS-B-3180 key keeper, cover on the rear of the panel (POM 632.623) |
| Typical use | Most apartments and small buildings | Large buildings where staff also sort packages |
Front-loading is simpler and cheaper for most properties. Rear-loading suits buildings with a staffed package room where the carrier and staff need to work out of sight of residents.
Recessed, surface-mount or free-standing?
The 4C standard is written around fully recessed wall mounting, which is the cleanest and most secure installation. Manufacturers also sell surface-mount kits and free-standing enclosures that house 4C units, which help when you cannot open up a wall or the mail area is in an open lobby.
- Recessed: best for new construction; needs a framed rough opening sized to the unit.
- Surface-mount: bolts to a finished wall with a trim collar; good for renovations.
- Free-standing: a pedestal-style enclosure, no wall work. For example, the Salsbury 10 Door High Free-Standing 4C Horizontal Mailbox is front-loading, holds 9 tenant doors and 2 parcel lockers, and comes with its enclosure. The version we list is private access, which suits sites where staff, not USPS, distribute mail; for USPS delivery ask us for the USPS-access version.
Whatever you choose, confirm the mounting method with your postmaster before ordering. Browse our 4C mailboxes or ask us for configurations we do not list online.
ADA requirements for apartment mailboxes
ADA rules sit on top of the USPS standard. Under the 2010 ADA Standards, section 228.2:
- Where mailboxes are in an interior location, at least 5 percent, and no fewer than one, of each type must comply with section 309 (operable parts).
- In residential facilities with a mailbox for each unit, every unit required to have mobility features must also get a compliant mailbox.
Section 309 points to the reach ranges in section 308: operable parts between 15 and 48 inches above the floor for an unobstructed forward or side reach, with clear floor space in front and locks that work without tight grasping or twisting. That is why 4C requires at least one compartment below 48 inches. Assign those lower compartments to accessible units first.
Many state and local codes go further than the federal minimum, so check with your architect or building department.
Keys, labels and directories
The Postal Operations Manual also sets rules that owners often miss (POM 632.625 to 632.627):
- Keep a record matching key numbers to box numbers, and never stamp key numbers on the outside of the locks.
- Masterkeying tenant locks is prohibited, and combination locks are not approved.
- Buildings with 15 or more receptacles must keep a current, alphabetical directory of everyone receiving mail.
- Owners must keep boxes in good repair. Failing to do so can justify withholding delivery, normally after about 30 days' written notice.
Replacement tenant locks and key blanks for Salsbury 4C units are in our 4C locks and key blanks collection.
What about older 4B and 4B+ mailboxes?
If your building has pre-2006 horizontal or vertical boxes, you are not required to tear them out. The 4C rule "imposes no general retrofit requirement." For replacement, USPS approved security-enhanced USPS-STD-4B+ boxes that fit the openings of old 4B units. According to USPS guidance:
- 4B+ boxes are approved for existing indoor installations or existing protected outdoor locations.
- USPS does not approve 4B+ for new installations and will not install Arrow locks in them there.
So 4B+ is a like-for-like replacement for an existing wall. If you are renovating the mail area anyway, or the postmaster asks for 4C, plan on 4C. Our guide to 4C vs 4B+ vs cluster mailboxes covers this choice in more detail.
Can an apartment complex use cluster mailboxes instead?
Yes, for outdoor delivery. Garden-style complexes and townhome communities often use pedestal-mounted cluster box units (CBUs) rather than a lobby wall. CBUs are built to the USPS-B-1118 CBU specification rather than 4C, include parcel lockers, and the postmaster still approves the location. Read our cluster box unit buying guide if your site has no suitable indoor space.
Checklist before you order apartment mailboxes
- Call your local postmaster or USPS Growth Management and confirm the delivery mode and mail area location.
- Count units, then add parcel lockers at one per five doors at minimum.
- Choose front- or rear-loading based on the floor plan.
- Choose recessed, surface or free-standing mounting.
- Check heights: tenant locks at 67 inches or below, accessible compartments at 48 inches or below.
- Assign ADA-compliant compartments to accessible units.
- Get written approval, then order.
Planning a building or replacing a full mail wall? Send us your unit count and we will quote the full project, including parcel lockers, locks and the right mounting for your site.
Frequently asked questions
Are 4C mailboxes required for apartments?
Yes for new or substantially remodeled apartment buildings that get USPS delivery to a central mail area. Existing buildings with older boxes can usually replace them like-for-like with 4B+ boxes if the postmaster agrees.
What is the minimum size of a 4C mailbox compartment?
USPS-STD-4C sets a minimum customer compartment of 3 inches high, 12 inches wide and 15 inches deep. Larger compartment heights are allowed as long as the unit still meets the height limits.
How high can apartment mailboxes be?
No tenant lock may be more than 67 inches above the floor, and no customer compartment's bottom shelf may be lower than 28 inches. At least one compartment must be below 48 inches for accessibility.
How many parcel lockers does an apartment building need?
Plan for at least one parcel locker per five mailbox compartments, the ratio in the Postal Operations Manual and USPS handbook PO-632. The 4C spec itself sets a lower minimum of one per ten, but the manuals' 1:5 rule is the one to plan for.
Who pays for apartment mailboxes?
The building owner or developer buys, installs, maintains and replaces the receptacles. USPS supplies and installs only its own carrier access lock.
Can I still buy 4B+ mailboxes?
Yes, but only to replace existing 4B boxes in an existing indoor or protected outdoor location. USPS does not approve 4B+ for new installations.
******It is always recommended to contact your local postmaster prior to installation. While most of our mailboxes are USPS-approved your local postmaster may have specific preferences in terms of mailbox types and installation location******