If your project's been flagged for STD-4C compliance, you need a clear answer to one question: what does USPS STD-4C actually mean for what you order and how it gets installed? A 4C mailbox is a centralized, wall-mounted, USPS-approved unit that holds tenant compartments and integrated parcel lockers in a single bank. This guide covers the USPS 4C mailbox requirements end to end, including dimensional specs, mounting heights, the 1:5 parcel locker ratio, ADA overlap, and which project types trigger which standard. It's written for builders, developers, architects, and property managers who need the regulatory language translated into buildable guidance. For a broader view across all USPS mailbox formats, see our USPS mailbox requirements builder's guide.
What Is USPS STD-4C and Why It Exists
USPS STD-4C is the 4C mailbox standard that governs the design and construction of wall-mounted centralized mail delivery receptacles. It became the active standard on October 5, 2006, and applies to all new construction and major renovations that use centralized mail. The rule was published in the Federal Register, Vol. 69, No. 171, under 39 CFR Part 111.
Why was it introduced? The older 4B and 4B+ formats were built for an era of letter-heavy mail and small package volume. As e-commerce changed delivery patterns, USPS needed a standard with larger compartments, integrated parcel lockers, improved security, and a more accessible mounting profile. STD-4C replaced the 4B+ horizontal format, the now-obsolete 4B+ vertical format (5"W × 6"D × 15"H), and legacy 4B installations as the active mandate. Today, 4C horizontal mailboxes are the only USPS-approved option for new multi-unit residential and commercial centralized delivery.
Which Projects Require 4C Mailboxes
Three project types determine whether STD-4C applies. Here's the quick framework:
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New construction: STD-4C required
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Major renovation: STD-4C required
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Like-for-like replacement: 4B+ still permitted, with conditions
The full USPS mailbox requirements for builders cover every format, but for 4C specifically, the rules below tell you which standard applies. Confirm the call with your local postmaster before ordering.
New Construction
STD-4C is mandatory for any new multi-unit project that uses centralized wall-mounted delivery. That covers apartments, condos, offices, and mixed-use developments. If your project doesn't have wall depth for a recessed install, cluster mailboxes (CBUs) are a USPS-pre-approved outdoor alternative.
Major Renovation
The new construction mailbox requirements also apply when a building goes through a major renovation. A major renovation is any modification that changes the wall cavity or rough opening so existing mailboxes can't be reinstalled one-for-one. Typical examples include enlarging the rough opening, replacing a wall section, or relocating the mailbox bank. Any of these triggers the USPS 4C mailbox requirements automatically, even if the building was originally fitted with 4B+ units.
Replacement / Retrofit (Like-for-Like)
If the existing rough opening is preserved and the original mailboxes can be reinstalled on a one-for-one basis, 4B+ remains acceptable. This is the only scenario where 4B+ is still permitted today. If you're considering a 4B+ replacement to stay within budget, browse our 4B+ horizontal mailboxes and confirm with your local postmaster first. Even a like-for-like replacement can be ruled a major renovation if the rough opening shifts during the work.
STD-4C Dimensional and Design Specifications
The 4C mailbox specifications cover compartment size, door types, materials, locks, and labeling. STD-4C specifications exist to enforce consistency across USPS-approved manufacturers, so any compliant unit meets the same dimensional and material baseline regardless of brand. Use the table below as your dimensional reference when matching a 4C unit to your project.
| Specification | Requirement | Notes |
|---|---|---|
| Minimum compartment size | 12"W × 15"D × 3"H | Replaces the eliminated 5"W × 6"D × 15"H vertical form factor |
| Tenant door types | MB1, MB2, MB3, MB4 | MB1 is the standard single-height tenant door; MB2 is double-height; MB3 and MB4 are larger tenant doors used for higher-volume mail |
| Parcel locker sizes | PL3, PL4, PL4.5, PL5, PL6 | PL3 (10-1/4"H), PL4 (13-3/4"H), PL4.5 (15-1/2"H), PL5 (17-1/4"H), PL6 (20-3/4"H) |
| Construction materials | Heavy-gauge aluminum body, 300-series stainless steel components | Powder-coat finish for weather and corrosion resistance |
| Locks | USPS Arrow lock (master access), patron lock with dust/rain shield | 3 keys per compartment supplied by manufacturer |
| Labeling | 1-1/2"H × 1-3/4"W decals (up to 5 characters) or USPS Block font engraving (up to 12 characters) | Sequential numbering recommended for ADA flexibility |
| Finish | Polyester powder-coat finish, USPS approved colors | USPS-STD-4C compliant finishes for exterior use |
A few clarifications on what these specs mean for ordering. The Arrow lock is supplied and installed by USPS after the unit is mounted, not by the manufacturer. Patron locks come pre-installed. The minimum compartment size ensures every tenant box can hold standard envelope and small-package mail; if you need to handle larger items routinely, scale up to MB2 doors or add parcel locker capacity. These 4C horizontal mailbox requirements are codified in the USPS STD-4C engineering specification and the supporting USPS Postal Bulletin 22206, which together set the authoritative requirements for every dimensional and material spec above.
USPS 4C Installation Height Requirements (AFF)
All USPS 4C mailbox installation requirements for mounting heights are measured Above Finished Floor (AFF). Get these heights wrong and your install will fail inspection, regardless of which manufacturer you ordered from. The five required heights cover the top of the tenant lock row, the ADA reach ceiling, the Arrow lock zone, the bottom tenant shelf, and the lowest parcel locker shelf.
The five required heights for any USPS 4C mailbox install:
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67" maximum: top row of patron locks (the highest tenant compartment lock)
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48" minimum: at least one tenant compartment must sit at or below this height for ADA forward-reach accessibility
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36" to 48": the USPS Arrow lock must fall within this range
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28" minimum: the lowest tenant compartment shelf
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15" minimum: the lowest parcel locker shelf, which is the only exception that allows hardware below 28" AFF
The 48" ceiling and 15" floor map directly to the 2010 ADA Standards for Accessible Design, Section 308.2.1, which sets forward-reach ranges for unobstructed accessible elements. Hit the STD-4C heights correctly and you'll satisfy ADA reach requirements at the same time. Plan the layout against these five heights before placing your order, because retrofitting the bank height after installation usually means rework.
Parcel Locker Ratio Requirement (1:5)
The 4C mailbox requirements set a fixed parcel locker ratio: at least one parcel locker for every five tenant compartments in any USPS 4C mailbox installation of five or more units. For a 10-unit building, that's a minimum of two parcel lockers; for 30 units, six; for 100 units, twenty. Installations of fewer than five mailboxes don't require a parcel locker, though one is still recommended.
The 1:5 parcel locker ratio replaced the older 1:10 ratio in a 2020 USPS update, codified in current USPS postal operations guidance. Some online sources still cite the obsolete 1:10 number, so double-check anything you read against current USPS material before placing an order.
Quick reference for common project sizes:
| Tenant Compartments | Minimum Parcel Lockers |
|---|---|
| 5 | 1 |
| 10 | 2 |
| 15 | 3 |
| 20 | 4 |
| 25 | 5 |
| 30 | 6 |
| 50 | 10 |
| 75 | 15 |
| 100 | 20 |
A few practical notes on getting the parcel locker ratio right. Smaller PL3 and PL4 lockers count toward the ratio identically to taller PL5 and PL6 lockers, so you can use locker height to control footprint without affecting compliance. Each parcel locker counts as one unit regardless of door height. If your building skews high-volume, exceed the 1:5 minimum by adding extra PL3 or PL4 lockers rather than under-spec'ing the count, since underordering means a costly reorder after install. Always confirm the current ratio with your local postmaster before placing the order.
Mounting Types and USPS Approval
4C horizontal mailbox requirements allow three mounting configurations. Each is STD-4C compliant by design, but USPS direct delivery isn't automatic for all three. The mounting type you specify determines whether your unit qualifies as a USPS approved 4C mailbox for direct carrier delivery, or whether it serves as a compliant private-access installation.
Recessed Mount (USPS-Approved for Direct Delivery)
The recessed unit installs flush into a framed wall opening, with the body of the mailbox sitting inside the wall cavity. This is the configuration USPS pre-approves for direct carrier delivery when the unit meets STD-4C. Plan for adequate wall depth (typically 15" or more) and a precisely sized rough opening to manufacturer spec.
Surface Mount (Collar Enclosure)
A heavy-gauge aluminum collar wraps around the back of the unit so it can mount on a wall surface where there isn't enough depth to recess. The unit itself remains STD-4C compliant, but USPS classifies most surface-mount installs as private-access only, with no Arrow lock fitted. If you need USPS direct delivery on a surface mount 4C mailbox, local postmaster approval is required on a case-by-case basis. Build that approval into your project timeline before ordering, since it isn't guaranteed.
Free-Standing (Depot Cabinet)
The 4C unit sits inside a free-standing cabinet or kiosk, used where wall mounting isn't possible. These are deployed in private-delivery and controlled-access environments such as gated communities, university housing, and office campuses. For projects where USPS direct delivery is required, a recessed wall mount is the safer specification. Free-standing depot cabinets are best treated as a private-delivery solution, not a USPS-approved primary mailbox.
ADA Compliance Overlay for 4C Mailboxes
STD-4C was written with ADA in mind, but the two standards aren't identical and neither overrides the other. Both apply, and 4C mailbox compliance means meeting both at once.
The 2010 ADA Standards for Accessible Design, Section 308.2.1, set forward-reach ranges of 15" minimum and 48" maximum AFF for accessible elements. Those numbers match the lower and upper bounds of the STD-4C mounting requirements, which is why a correctly installed 4C unit usually satisfies both standards in one pass.
Where projects fail: parcel lockers mounted above 48" AFF. The unit is still USPS-compliant, but those parcel lockers aren't ADA-accessible, and the install fails the disability accommodation requirement even though USPS would otherwise approve it.
Sequential compartment numbering (typically 1 through 300 across a full 4C suite) gives you flexibility to assign lower-mounted compartments to tenants who need ADA accommodation, rather than locking yourself into a fixed numbering scheme that might place an accessible tenant at the top row. In practice, an ADA compliant 4C mailbox install built around 6-door-high or 10-door-high configurations is the easiest path to full compliance, because those layouts keep more compartments inside the accessible reach range.
For a deeper look at where the two standards diverge, see our guide on USPS vs. ADA compliance, and browse our ADA-compliant 4C mailboxes for configurations that ship pre-specified for accessibility. Always confirm local building code requirements, since some municipalities require additional accommodations beyond the federal baseline.
Pre-Purchase Compliance Checklist
Run through this 4C mailbox compliance checklist before placing an order. Every item maps back to a USPS or ADA requirement covered above, and the goal is a USPS approved 4C mailbox install that passes inspection on the first visit.
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Project type: Confirm whether your project is new construction, major renovation, or like-for-like replacement
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Approved manufacturer: Verify the 4C unit is from a USPS-approved manufacturer (Florence Corporation or Salsbury Industries, both available in our catalog)
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Mounting matches delivery: Match the mounting type to your delivery type, with recessed for USPS direct delivery and surface or free-standing for private access
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Parcel locker count: Calculate parcel locker count using the 1:5 ratio against tenant compartment count
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AFF mounting heights: Plan all five AFF heights before ordering: 67" max patron lock, 48" max for at least one accessible compartment, 36" to 48" Arrow lock range, 28" min tenant compartment, 15" min parcel locker
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ADA-accessible compartments: Confirm at least one compartment is specified for tenants who need ADA accommodation
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Postmaster confirmation: Contact your local postmaster before installation to confirm location and the approval pathway
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Documentation on site: Keep manufacturer cut sheets and full spec documentation on site for USPS inspection
Why Choose Our USPS-Approved 4C Mailboxes
Budget Mailboxes carries the full range of USPS approved 4C mailbox units from Florence Corporation and Salsbury Industries. Both are on the USPS-approved STD-4C manufacturer list, and every 4C product on our site meets the dimensional, security, and locking requirements covered in this guide.
Our 4C horizontal mailbox catalog covers every mounting configuration: recessed, surface mount with collar, and free-standing depot cabinets. ADA-aware door arrangements ship pre-configured for accessibility. For builders, we provide spec sheets, manufacturer cut sheets, bulk pricing for multi-unit projects, and submittal package support. Browse our full 4C horizontal mailboxes collection or contact our builder support team if you'd like help spec'ing your project.
Conclusion
STD-4C has been the active USPS standard for centralized mail delivery since October 2006, and it's required for all new multi-unit construction and major renovations. The specs to keep front of mind: minimum compartment 12"W × 15"D × 3"H, AFF heights at 67" / 48" / 36" to 48" / 28" / 15", and a 1:5 parcel locker to tenant compartment ratio. The next step before placing your 4C order is a short conversation with your local postmaster to confirm delivery type, parcel locker count, and any district-specific approval steps. Browse our USPS-approved 4C collection or reach out to our builder support team if you'd like help walking through the specs before the call.
Frequently Asked Questions
A 4C mailbox is a wall-mounted, centralized mail receptacle designed to USPS STD-4C specifications. It's been the required format for new multi-unit construction and major renovations since October 2006, and it covers tenant compartments, integrated parcel lockers, and ADA-aware mounting heights in a single unit.
4C replaced 4B and 4B+ as the active USPS standard. 4B+ remains acceptable only for like-for-like replacement in existing rough openings. 4C requires larger compartments (minimum 12"W × 15"D × 3"H), integrated parcel lockers at the 1:5 ratio, USPS Arrow lock master access, improved security materials, and ADA-aware mounting heights.
One parcel locker for every five tenant compartments (1:5 ratio), applied to any installation of five or more mailboxes. So 30 units requires six parcel lockers, 50 units requires ten, and 100 units requires twenty.
The Arrow lock is a master access lock installed by USPS after delivery of the mailbox unit. It gives the carrier single-key access to the entire 4C bank for mail loading. It's required for USPS direct-delivery installations and isn't fitted on private-access units.
Surface-mount 4C units are STD-4C compliant by construction, but they're typically installed for private delivery rather than USPS direct delivery. USPS delivery to a surface-mount unit requires case-by-case local postmaster approval, so confirm before ordering if direct delivery matters.
