What the 1:5 Ratio Means
USPS requires a minimum of one parcel locker compartment for every five tenant mail compartments on all new centralized mail installations. On a 100-unit property served by Florence 1570 Series CBUs with 100 tenant doors, the minimum parcel locker requirement is 20 compartments. A Florence 1570-16 CBU has 16 tenant compartments and 2 integrated parcel locker compartments. Five of those units provide 80 tenant compartments and 10 parcel locker compartments — which satisfies the 1:5 ratio for an 80-unit property but not for a 100-unit property, where 20 parcel locker compartments are required. The difference matters because USPS calculates this ratio at the system level, not at the individual station level.
The 1:5 ratio represents the minimum, not the optimal. Properties with e-commerce-active demographics — younger renters, urban locations, short-term rental communities — experience package delivery volumes that overwhelm minimum-compliant parcel locker counts daily. When parcel lockers are full, carriers leave packages on the ground, in common areas, or return them to the post office. USPS treats chronic parcel overflow as a failure of the installed mail system and uses it as grounds to initiate a compliance review that can result in a formal suspension notice.
Where the Requirement Applies
The 1:5 ratio applies to all new CBU installations and to substantial alterations of existing mail systems. It applies when USPS designates a property for centralized delivery — which occurs at the time of initial USPS coordination, or retroactively when USPS re-evaluates a property's mail infrastructure. The requirement also applies retroactively when USPS issues a delivery suspension notice triggered by chronic parcel overflow. In that scenario, the property is not given credit for years of operation under a lower ratio — the suspension notice sets a compliance deadline against the current standard.
For new construction, the parcel locker ratio must be satisfied before USPS will issue a postmaster sign-off letter and activate delivery. Developers who submit a mail infrastructure plan that does not meet the 1:5 ratio receive a USPS rejection that requires a revised plan before approval can proceed. In jurisdictions where CO inspectors cross-check USPS sign-off status, this creates a direct CO delay.
Key calculation: Total tenant mail compartments ÷ 5 = minimum parcel locker compartments required (round up to the nearest whole number). A 150-unit HOA with 150 tenant doors requires a minimum of 30 parcel locker compartments. Fifteen Florence 1570-16 CBUs provide 30 tenant compartments and 30 parcel locker compartments — exactly at minimum compliance for a 150-unit property.
How to Calculate Your Requirement
The calculation uses total tenant mail compartments across all CBU stations on the property — not total dwelling units. These numbers are often the same, but not always. Some properties have more tenant compartments than dwelling units (when commercial tenants or HOA amenity spaces are served by the CBU system), and some have fewer (when certain units have alternative mail delivery arrangements). Count physical compartments, not unit counts.
Divide total tenant compartments by 5 and round up to the nearest whole number. That is the minimum number of parcel locker compartments the property must provide in its centralized mail system. Florence 1570 Series CBUs vary in their built-in parcel locker count: the 1570-4T5 has 2, the 1570-8 has 2, the 1570-16 has 2, and the 1570-8T6 has 4. When the built-in parcel locker count across all CBU stations is below the required minimum, standalone Florence 1590 Valiant parcel lockers must be added to reach compliance. The T1 model provides 2 parcel compartments; the T2 provides 4.
A worked example: a 200-unit apartment community has 200 tenant mail compartments across its CBU stations. Required parcel lockers: 200 ÷ 5 = 40 compartments. If the CBU stations provide a total of 26 built-in parcel compartments, the property needs 14 additional parcel locker compartments — which is four Florence 1590-T2 units (4 × 4 = 16 compartments, satisfying the 40-compartment requirement when combined with the 26 built-in).
Certificate of Occupancy Implications
On new construction, USPS parcel locker ratio compliance is a condition for mail service activation. In many Arizona jurisdictions — including the City of Phoenix, Chandler, and Gilbert — CO inspectors flag mail infrastructure non-compliance as a condition item when USPS has not issued a sign-off letter. Without USPS sign-off, the building's mail system is not operational. An inoperational required building system is a CO condition. In the worst case, this means occupancy cannot be granted until the parcel locker count is corrected and USPS re-approves the installation.
A developer who discovers at the CO inspection phase that the mail specification falls short of the 1:5 ratio faces a last-minute scope addition under significant schedule and financial pressure. Adding parcel lockers post-construction — when concrete is set, CBU stations are placed, and the contractor mobilization has been demobilized — is more expensive than specifying correctly pre-construction and more disruptive to the closeout schedule. The risk is entirely preventable with a pre-construction mail infrastructure compliance review.
Compliant Solutions for Under-Ratio Properties
Standalone parcel lockers can be added to existing CBU installations to bring a property into ratio compliance without replacing the CBU hardware. Florence 1590 Valiant T1 (2-compartment) and T2 (4-compartment) standalone parcel lockers are USPS-approved, weatherproof, and designed for outdoor installation on concrete pads adjacent to existing CBU stations. They require the same foundation, anchor bolt, and USPS coordination process as a new CBU installation — they are not bolt-on accessories that can be placed without permits or postmaster approval.
Ironpost Works calculates the required parcel locker count based on the property's existing tenant compartment inventory, specifies the correct 1590 Valiant configuration to reach compliance, and installs with full USPS coordination and postmaster sign-off. For existing properties receiving USPS enforcement pressure, the 48-hour written scope and estimate we provide is the starting point for presenting a remediation plan to USPS that demonstrates a credible compliance pathway.