When you tour a commercial office and the broker quotes "5,000 square feet," that number rarely matches the space you can put desks in. Commercial leases are priced on two different measurements — usable square feet and rentable square feet — and the gap between them is money you pay every month. Understanding how a landlord gets from one figure to the other is the single most useful skill for reading a lease honestly and comparing quotes fairly.
This guide explains what usable and rentable square feet actually mean, how the load factor inflates one into the other, why New York brokers talk about "loss factor" instead, and how the BOMA standards define the whole exercise. Once you can convert a rentable quote back to a usable one, you can line up competing offices on the same basis and see which deal is genuinely cheaper. If you are sizing a space from scratch, our building square footage tools can help you pin down the raw numbers first.
Usable square feet: the space you actually occupy
Usable square feet (USF) is the area a tenant physically occupies and controls — the floor inside your suite where your desks, offices, conference rooms, and break area go. It is measured from the inside faces of the walls that bound your premises, and it excludes shared building elements like public corridors, elevator shafts, stairwells, and mechanical rooms that serve the whole property.
Usable square footage is the honest answer to "how much room do I get?" It is what you would measure if you walked your suite with a tape and calculated the floor area, the same way you would measure any office space or workroom. Because it reflects real capacity, usable square feet is the figure you should use for space planning: how many workstations fit, whether a 12-person boardroom is realistic, and how much circulation space is left over.
Rentable square feet: usable plus your share of the commons
Rentable square feet (RSF) is usable square feet plus the tenant's pro-rata share of the building's common areas — the lobby, hallways, shared restrooms, elevator lobbies, and other spaces that every tenant benefits from but nobody occupies exclusively. Rather than leave those areas uncharged, landlords spread their cost across all tenants in proportion to how much space each one leases.
Crucially, rent is charged on rentable square feet, not usable. So if you occupy 10,000 usable square feet but your rentable figure is 11,500, you pay rent on all 11,500. The extra 1,500 square feet represents your slice of the shared spaces you walk through every day. This is entirely standard and not a trick — but it does mean the quoted "size" of a space and the space you can use are two different things, and you should always know which one a broker is citing.
The load factor: how usable becomes rentable
The number that bridges usable and rentable square feet is the load factor, also called the add-on factor or core factor. It is simply the ratio of rentable to usable area, expressed as a percentage of extra space added on top of what you occupy. The formula is straightforward:
Rentable SF = Usable SF × (1 + load factor). Take a suite of 10,000 usable square feet in a building with a 15% load factor: 10,000 × 1.15 = 11,500 rentable square feet. That 15% is your contribution to common areas. A building with generous lobbies, wide corridors, and lavish shared amenities carries a higher load factor; a plain, efficient building carries a lower one. Load factors commonly range from about 10% to 20%, though grand trophy towers can push higher.
- Load factor = Rentable SF ÷ Usable SF (a 1.15 result means a 15% load).
- Rentable SF = Usable SF × (1 + load factor).
- Usable SF = Rentable SF ÷ (1 + load factor) — use this to recover the real occupancy from a rentable quote.
- A 10,000 USF suite at a 15% load = 11,500 RSF; at a 20% load it would be 12,000 RSF.
Loss factor: the same idea, worded backwards
In some markets — New York City above all — brokers quote a "loss factor" instead of a load factor, and the two are easy to confuse because they measure the gap from opposite directions. Loss factor is the share of the rentable area that is not usable: (Rentable − Usable) ÷ Rentable. It answers "what fraction of the space I pay for can I not actually occupy?"
Because the denominator is rentable rather than usable, a loss factor and a load factor for the very same space produce different percentages. A 15% load factor (11,500 RSF over 10,000 USF) equals a loss factor of about 13% (1,500 ÷ 11,500). New York loss factors are famously high — often 25% to 35% — which is one reason a Manhattan office can feel small relative to its rentable quote. Always confirm which term a broker is using before you compare buildings, or you will be comparing figures that are not measured the same way.
Why two identical suites can have different rentable figures
Here is the part that surprises most first-time commercial tenants: two suites with exactly the same usable square footage can carry completely different rentable numbers. Because rentable area depends on the building's common areas and how the landlord allocates them, the same 10,000 usable square feet becomes 11,000 rentable in an efficient building with a 10% load and 12,000 rentable in an amenity-heavy building with a 20% load.
That difference flows straight into your rent. At $40 per rentable square foot, the 10% building costs $440,000 a year while the 20% building costs $480,000 — for the identical amount of space you can actually work in. Neither landlord is doing anything improper; they simply have different amounts of shared space to spread around. This is exactly why savvy tenants never compare deals on rentable square feet alone. They convert everything back to a common basis first.
BOMA standards and comparing quotes on a $/usable basis
The Building Owners and Managers Association (BOMA) publishes the measurement standards that define how usable and rentable areas are calculated, so that landlords, tenants, and appraisers measure the same building the same way. The widely used BOMA Office Standard sets out exactly which spaces count toward usable area, which count as common area, and how the load factor is derived. When a lease says it follows a BOMA standard, ask which edition — methods have evolved over the years and can shift the numbers.
To compare offices fairly, translate every quote into rent per usable square foot. Multiply the rentable rate by the rentable area to get annual rent, then divide by usable square feet. In the example above, both buildings might advertise "$40/SF," but the effective cost is $44 per usable foot in the efficient building versus $48 in the amenity-heavy one. Running the numbers through a cost per square foot calculation exposes the real price of each space — and the same discipline applies whether you are leasing an office or a bulk warehouse, where load factors are typically much lower.