Rentable square footage is the number of square feet a commercial landlord charges you rent on, and it’s always larger than the space your business physically occupies. It equals your usable area inside the suite walls plus a proportional share of the building’s common spaces: lobby, corridors, restrooms, mechanical rooms, and shared amenities. The formula is simple: usable square footage multiplied by a load factor produces rentable square footage. That load factor typically adds 15 to 25 percent on top of the space you actually use, which is why understanding the calculation matters before you sign anything.
Usable Area vs. Rentable Area
Usable square footage (USF) is the area inside your suite where your employees work and your furniture sits. It’s measured from the inside face of shared walls or the centerline of walls between neighboring tenants. Rentable square footage (RSF) takes that private area and layers on your pro-rata share of every common space in the building.1U.S. Government Accountability Office. Federal Real Property: Measuring Actual Office Space Costs Your rent is calculated on the rentable number.
The share is proportional to the size of your suite. If your private area accounts for ten percent of the leasable space on your floor or in the building, you’re assigned ten percent of the common areas. A tenant with a larger footprint carries a bigger piece of the lobby, elevator bank, and shared restrooms. Every occupant chips in for the infrastructure that makes the building function.
The terminology varies. “Load factor,” “loss factor,” and “core factor” all describe the same markup. “Loss factor” simply frames it from the tenant’s side: the share of your rent going to space you can’t put a desk in.
How the Load Factor Works
The load factor is the multiplier that converts usable area into the rentable figure on your lease:
Rentable Square Footage = Usable Square Footage × Load Factor
A business occupying 10,000 usable square feet in a building with a 1.18 load factor pays rent on 11,800 square feet. The extra 1,800 feet is the share of common area you benefit from but don’t exclusively control. Some landlords state the figure as an “add-on” percentage (18 percent in that example) rather than a multiplier. The math arrives at the same place.
Floor Common vs. Building Common
Common areas come in two layers. Floor common areas serve every tenant on a single floor: restrooms, janitor closets, and the corridors linking suites. Building common areas serve the entire property: the main lobby, fitness center, conference facilities, and central mechanical rooms.2BOMA International. BOMA Floor Standards Interpretations Documents: Best Practice Guidance If you share a floor with other tenants, both factors apply to your RSF calculation. If you lease an entire floor, the floor common factor drops away because those spaces are already yours exclusively. Full-floor tenants therefore face a lower overall load factor than a small tenant on a shared floor.
Typical Ranges
In newer Class A and Class B office buildings, load factors generally fall between 15 and 20 percent. Older buildings, especially those built before the 1980s, tend to have wider hallways, multiple elevator banks, and oversized mechanical rooms, which push the figure into the 20 to 25 percent range. In high-density markets like Manhattan, where the REBNY measurement standard is common, the effective loss factor can climb higher still. A quoted load factor below 10 percent or above 30 percent is worth a closer look at exactly how the landlord is measuring.
What Common Areas Include
The shared spaces folded into rentable square footage typically include:
- The building lobby, reception area, and ground-floor corridors.
- Hallways and corridors on each floor connecting suites to elevators, restrooms, and exits.
- Public restrooms on each floor.
- Mechanical rooms housing HVAC equipment, electrical panels, plumbing, and fire control systems.
- Shared amenities such as fitness centers, conference rooms, and tenant breakrooms.
Under the BOMA 2024 standard, ground-level outdoor amenities built exclusively for tenant use, such as courtyards and landscaped terraces, can now also be included.3BOMA International. BOMA Standards
What Gets Excluded
Major vertical penetrations are carved out of the rentable area before the calculation runs. Elevator shafts, public stairwells, flues, pipe shafts, vertical ducts, and their enclosing walls are subtracted from the gross measured area.4Building Owners and Managers Association International. Standard Method for Measuring Floor Area in Office Buildings ANSI/BOMA Z65.1-1996 One exception: if a tenant occupies multiple floors with a private connecting staircase, that penetration isn’t excluded, because it serves only that tenant.
Parking is also outside the RSF calculation. Structured parking garages are not included; parking is assigned and charged by the space, not by square footage.5General Services Administration. National Business Space Assignment Policy A landlord folding parking area into rentable square footage is a signal to ask hard questions.
Structural columns work the opposite way from what many tenants assume. Under BOMA standards, no deduction is made for columns and structural projections necessary to the building.4Building Owners and Managers Association International. Standard Method for Measuring Floor Area in Office Buildings ANSI/BOMA Z65.1-1996 If a concrete column sits in the middle of your suite consuming workspace, you still pay rent on that footprint.
Measurement Standards: BOMA and REBNY
The Building Owners and Managers Association publishes the most widely used guidelines for measuring commercial office space. The current version, ANSI/BOMA Z65.1-2024, is the industry benchmark and is accredited through the American National Standards Institute.6American National Standards Institute. ANSI/BOMA Z65.1-2024 – BOMA 2024 for Office Buildings: Standard Methods of Measurement Its primary purpose is calculating rentable area, and the same framework supports space utilization analysis, property valuation, and expense allocation.3BOMA International. BOMA Standards
BOMA defines precisely where measurements start and stop: whether to measure to the inside of exterior glass, the outside face of a wall, or some other point. Small variations, applied across an entire floor, can shift the rentable area by hundreds of square feet.
In New York City, many landlords use the Real Estate Board of New York (REBNY) guidelines instead. The REBNY method measures usable space to the outside surface of the building’s exterior wall, and corridor wall thickness counts as usable footage.7Real Estate Board of New York. Owners and Managers Rules and Regulations REBNY also applies a “four-inch method” introduced in 2003: elevator shafts, stairwells, and mechanical facilities are subtracted only within a nominal four-inch enclosing wall; anything beyond that is included as usable area. The result is that REBNY measurements produce larger usable and rentable numbers than BOMA for the same physical space. If you’re leasing in New York, ask which standard applies.
Comparing Buildings on Effective Rent
The quoted rent per rentable square foot is almost meaningless without the load factor. A building quoting $30 per RSF at a 15 percent load factor costs less per usable foot than a building quoting $28 per RSF at a 25 percent load factor:
- Building A: $30 RSF × 1.15 = $34.50 per usable square foot.
- Building B: $28 RSF × 1.25 = $35.00 per usable square foot.
Building B looks cheaper and actually costs more for the space you use. Convert quoted rents to a cost-per-usable-square-foot basis before comparing options. A landlord who won’t disclose the load factor is telling you something about the negotiation ahead.
Verifying and Negotiating the Number
Measure the space yourself. A tape measure or laser measurer and the suite’s floor plan are enough to produce your own usable-area figure. Compare it to the landlord’s stated USF; if the numbers don’t match, raise the discrepancy before signing. For larger spaces, hiring an architect or surveyor to measure against the applicable BOMA or REBNY standard produces a professional certification you can bring into negotiations. Landlords should make architectural plans and calculations available on request.7Real Estate Board of New York. Owners and Managers Rules and Regulations
A remeasurement clause gives you the contractual right to have the space independently measured within a set window after taking possession, typically 30 to 60 days, ideally before rent payments begin. Many landlords resist this language, but it’s worth pushing for in spaces that haven’t been recently surveyed. If the remeasurement shows the space is smaller than the lease states, the clause should trigger a rent adjustment reflecting the actual square footage. For buildings still under construction, the protection matters more: negotiate language requiring rent to be recalculated based on actual delivered square footage, with termination rights if the finished space is materially different from what was promised.