Rentable square footage and usable square footage describe two different things on the same lease: usable is the space inside your suite that you actually occupy, and rentable is that same space plus a proportional share of the building’s hallways, lobbies, restrooms, and other common areas. Rentable is always the larger number, and it’s the one your rent is calculated against. The gap typically runs 15–25%, which means you’re paying for considerably more area than you can fill with desks and people.
What Usable Square Footage Covers
Usable square footage is the space inside your suite that belongs to you alone. It runs from the interior face of your exterior windows to the walls separating your office from the hallway. Everything inside those boundaries counts: private offices, conference rooms, reception, storage closets, break rooms, and any area dedicated exclusively to your business.
This is the number that tells you how many employees you can seat and whether a layout actually works. Architects use it to draw floor plans. When you walk a space and think it feels big enough, you’re reacting to usable area.
One detail catches tenants off guard. Structural columns inside your suite are not subtracted from the usable figure. Under BOMA measurement standards, the measurement line passes through columns as though they aren’t there.1BOMA International. Answers to 26 Key Questions About the ANSI/BOMA Standard Method of Measuring Floor Area in Office Buildings A column eating into your open floor still counts as usable space on paper, even though no desk fits there. Factor that in when evaluating whether a suite really fits your headcount.
What Rentable Square Footage Covers
Rentable square footage is the number on your lease, and it drives your rent. It takes your usable area and adds a proportional share of every shared space in the building: lobbies, corridors, restrooms, mechanical rooms, and similar areas. When a lease quotes 5,000 square feet at $30 per foot for a $150,000 annual base rent, that 5,000 is rentable area, not the space you physically occupy.
The shared spaces fall into two layers. Floor common areas are shared only on your floor: the restrooms, elevator lobby, janitorial closets, and corridors connecting suites. Building common areas are shared by every tenant in the building: the main lobby, loading docks, management offices, fitness centers, and shared conference facilities. Floor common areas typically add around 8% to your usable area, and building common areas generally add another 6–8%.2Society of Industrial and Office Realtors (SIOR). Understanding the Common Factor
If you lease a partial floor, both layers apply. Your usable area is multiplied by the floor common factor, then by the building common factor. If you lease an entire floor, the floor common areas effectively become part of your usable space because no one else shares them, and you only pick up the building factor on top. Full-floor tenants generally carry a lower overall load.
Rentable square footage also drives your share of operating expenses. Most commercial leases calculate your proportionate share of property taxes, insurance, and maintenance by dividing your rentable area by the building’s total rentable area. If your suite is 5,000 of a 100,000-square-foot building, you pay 5% of those shared costs. Some leases use “gross leased and occupied area” as the denominator instead, which excludes vacant suites from the total and pushes each tenant’s share higher. Catch that distinction before you sign.
Tenant improvement allowances are also typically quoted per rentable square foot. A landlord offering $50 per square foot in build-out money is calculating against the rentable figure, so a higher load factor can actually work in your favor on the construction budget even as it raises your rent.
The Load Factor That Connects Them
The load factor, sometimes called the add-on factor, is the percentage that bridges usable and rentable square footage. A 15% load factor means your rentable area is 15% larger than your usable area. A suite with 2,000 usable square feet and a 15% load factor produces 2,300 rentable square feet.
For most office buildings, the total load factor falls between 12% and 20%, though buildings with especially generous lobbies or extensive amenities can run higher.2Society of Industrial and Office Realtors (SIOR). Understanding the Common Factor Newer Class A buildings with fitness centers, rooftop terraces, and conference suites typically carry higher load factors than older, no-frills buildings. Whether that’s worse value depends on whether you and your employees actually use those amenities.
BOMA’s current standard allows two calculation approaches. The legacy approach (Method A) produces different factors for multi-tenant floors and single-tenant floors. The newer approach (Method B) applies a single factor across the entire building, which simplifies comparisons but can shift costs between tenants on different floor types.3BOMA International. BOMA 2017 For Office Buildings Standard Methods of Measurement When a landlord quotes a load factor, ask which method the building uses. The same physical building can produce different rentable areas depending on the calculation.
How to Compare Spaces
Most tenants get tripped up by the load factor during comparison shopping. Two suites can offer identical usable square footage but carry very different rental costs because their load factors differ. A 3,000-usable-square-foot suite with a 12% load factor costs you rent on 3,360 rentable square feet. The same usable area in a building with a 20% load factor means paying rent on 3,600 square feet. That’s 240 additional square feet of pure common-area allocation.
The cleanest way to compare competing spaces is to calculate the effective cost per usable square foot rather than relying on the quoted price per rentable foot. Divide total annual rent by usable square footage. A space quoted at $28 per rentable foot in an efficient building can beat a space quoted at $25 per rentable foot in a less efficient one. The math also forces landlords to compete on what actually matters: the workspace you can fill with people.
Building efficiency, the ratio of usable space to total gross area, generally runs between 75% and 90% for office buildings. Low-rise buildings with simple floor plates tend toward the higher end. High-rise towers with large cores lose more space to elevator banks and mechanical systems. Open-plan layouts are inherently more efficient than buildings carved into many private offices. None of this appears on a listing, so you have to ask for the usable figure and run the numbers yourself.
Operating Expenses and Gross-Up Clauses
The rentable figure doesn’t just set your rent. It also sets your share of operating expenses, and that share interacts with another lease provision worth knowing about.
If 25% of the building sits empty, the landlord’s actual operating costs don’t drop by 25%. The lights still run, the elevator still needs maintenance, and the lobby still gets cleaned. A gross-up clause lets the landlord calculate operating expenses as though the building were fully occupied, or at some specified threshold like 95%. The expenses are inflated for the purpose of dividing them among tenants, but the total collected shouldn’t exceed actual costs.
The load factor matters here because your pro-rata share is based on rentable square footage. A higher load factor means a bigger slice of grossed-up expenses. Review the occupancy threshold in the gross-up provision carefully. Some leases gross up to 100%, others to 95%, and the difference compounds over a long term.
Verifying the Numbers Before You Sign
Landlords have the measurement advantage. They hired the architect, they control the floor plans, and they chose which BOMA method to apply. Tenants rarely question the numbers, which is exactly why you should.
Before signing, request the building’s measurement methodology, including which BOMA edition and method were used. The current office building standard is ANSI/BOMA Z65.1-2024, which builds on the widely adopted 2017 edition.4BOMA International. BOMA Standards Buildings remeasured under newer BOMA editions can legitimately show higher rentable areas than the same space measured a decade ago, so the edition matters. If the building predates current standards, the measurement approach may not conform to any recognized methodology at all. BOMA adoption isn’t legally required, but it has become the market expectation in most major commercial markets, and lenders and appraisers generally require it. If a landlord can’t tell you which edition their building was measured under, treat that as reason for closer scrutiny.
Hiring an independent space planner or architect to verify the usable square footage of your suite is the most direct protection. The cost is modest against the lease obligation. Even a 3% measurement error on 10,000 square feet at $35 per foot adds up to over $10,000 a year. Over a seven-year lease, that’s real money. Negotiate a measurement verification clause that lets you audit the square footage within a set period after move-in and requires the landlord to adjust rent if the measurements are off beyond a stated tolerance, typically 1–2%.