Rentable vs. Usable Square Footage, Explained by the People Who Write the Standard

Usable square footage (USF) is the space a tenant exclusively occupies. Rentable square footage (RSF) is that space plus the tenant’s share of the building’s common areas: lobbies, corridors, restrooms, mechanical rooms. The two are connected by the load factor:

RSF = USF × load factor

Rent, CAM charges, and building valuation all run on RSF, not USF. That single fact explains most of the confusion, and most of the money, in commercial leasing.

Stevenson Systems has measured more than 12,500 buildings over 40 years, and our team co-authors the BOMA standards those measurements follow. Here’s how the two numbers actually work.

What Counts as Usable Square Footage?

Usable square footage is the area inside a tenant’s demising walls: offices, conference rooms, private restrooms, storage, the space where the tenant’s operations happen.

It does not include the elevator lobby the tenant walks through, the shared restrooms down the hall, or the corridor connecting suites. Those areas serve everyone on the floor, so no single tenant “uses” them on paper.

But somebody pays for them. That’s where rentable square footage comes in.

What Is Rentable Square Footage?

Rentable square footage adds each tenant’s proportionate share of the building’s common services areas to their usable area. A tenant with 5% of a building’s usable space carries roughly 5% of its lobbies, corridors, and shared facilities.

The logic is fair on the surface: every tenant benefits from the building lobby, so every tenant shares in its use. But this distinction is where a building gains or loses square footage and income, because the allocation depends entirely on how the building’s measurements were produced: which measurement standard was applied, and who has maintained the square footage over time.

“Square footage is the language of a building. Every lease, every CAM charge, every valuation speaks it.”

How Does the Load Factor Work?

The load factor (also called the R/U ratio) is the building’s total rentable area divided by its total usable area.

Example: say a tenant occupies 10,000 USF in a building with a 1.15 load factor:

• 10,000 USF × 1.15 = 11,500 RSF
• At $40 per RSF, annual rent is $460,000
• The same 10,000 USF in a building with a 1.20 load factor rents as 12,000 RSF: $480,000 per year

Same physical space, $20,000 a year apart. Over a 10-year lease, the load factor alone moved $200,000.

Why Do Two Buildings Measure the Same Space Differently?

The load factor is only as good as the building calculations, the measurements produced, and how they’ve been applied to occupants over time.

BOMA (the Building Owners and Managers Association) publishes the BOMA Measurement Standards referenced in many US office leases. BOMA has revised those standards repeatedly since 1915, most recently in the 2024 edition. With each revision, changes have been made to what is included as rentable. Ground-level outdoor amenity areas, for example, can now be included as rentable area under BOMA 2024 if they’re built for tenant use.

As occupancies change and tenants reconfigure, corridors can move and common areas can get reallocated. A load factor calculated under measurement standards from fifteen years ago no longer describes the building standing today. That gap is rarely anyone’s fault. It’s what time does to a fixed number in a building with changing occupancies. It’s also why owners who remeasure routinely find rentable area they weren’t capturing.

Who Decides What the Right Number Is?

The BOMA standard does, and this is where it pays to know who you’re working with.

Peter Stevenson, Stevenson Systems’ founder, co-authored the BOMA standards your leases reference, domestic and international, and is one of only two official BOMA Standard interpreters internationally. When a measurement question has two plausible readings, an interpreter’s reading is the definitive one.

That matters far more in practice than on paper. Whether a mechanical penthouse, a shared loading dock, or a covered terrace counts as rentable can swing a building’s RSF by thousands of feet. Owners who get those questions answered by someone with the authority to answer them lease with confidence. The numbers hold up in negotiations, in financing packages, and if it comes to it, in court.

What Should Owners Do With This?

Three things, in order.

Know which standard your stated RSF was measured under. If the answer is BOMA 2010 or older, the building almost certainly measures differently today. If nobody knows the answer, that’s the answer.

Know whether the building was field-verified. Numbers inherited from a prior owner, scaled from old drawings, or stitched together suite by suite weren’t measured; they were assumed. Stevenson Systems is often the only firm to field-verify what’s actually built: an accurate snapshot in time establishing square footages, tenant improvements, and demising walls.

**Treat measurement as an investment, not a cost.** A remeasurement using the current standard typically pays for itself many times over when found area flows into leases. Every found foot of rentable area is recurring revenue that capitalizes directly into asset value.

Frequently Asked Questions

What is the difference between rentable and usable square footage?

Usable square footage is the space a tenant exclusively occupies. Rentable square footage adds the tenant’s proportionate share of building common areas. Rent is charged on rentable square footage.

How is the load factor calculated?

Total building rentable area divided by total usable area. Typical office load factors fall between 1.10 and 1.25.

Can rentable square footage change without construction?

Yes. When a building is remeasured under the current BOMA standard, areas that weren’t previously counted (or were misallocated) often become rentable. SSI clients average 3 to 4% additional rentable area in Class A properties, with no physical changes to the building.

Which BOMA standard is current?

ANSI/BOMA Z65.1-2024, published in 2024. Buildings measured under earlier standards can usually be brought current, and frequently gain rentable area in the process.