Usable vs Rentable Square Footage: How the Load Factor Works

An office listing advertises 1,650 sq ft. You walk the space, pace out the rooms, and the floor you can actually put desks on feels closer to 1,250 sq ft. Nobody has lied to you — at least not in the narrow sense. The 1,650 sq ft figure and the 1,250 sq ft figure measure different things: the first is the rentable area, the second is the usable area. The gap between them is the load factor, and understanding it is the single most useful piece of arithmetic a tenant or buyer can learn before signing a lease.

This guide defines usable and rentable square footage precisely, gives the formulas that connect them, works a complete example with real numbers, and ends with the checks to run before you commit — because landlords almost always quote rent per rentable square foot, while you work in usable space.

The two areas, defined

Usable square footage is the actual space you occupy and use exclusively — the floor inside your suite, measured from the inside faces of the walls. It is the area where your desks, conference rooms, kitchen and storage actually sit.

Rentable square footage is the usable area plus your proportionate share of the building’s common areas: lobbies, corridors, restrooms, elevator lobbies, stairwells, and sometimes the building’s fitness center or shared conference facilities. This share is not space you control; it is your suite’s allocated portion of areas shared by all tenants. The Building Owners and Managers Association (BOMA) publishes the measurement standards most US landlords follow.

The relationship is:

Usable area < Rentable area

And the connecting formula is:

Rentable area = Usable area + proportionate share of common areas

The load factor

The gap between usable and rentable area is expressed as the load factor (sometimes called the loss factor or common-area factor):

Load factor = (Rentable area − Usable area) ÷ Usable area × 100

Equivalently, if you know the load factor, you can recover the usable area from a quoted rentable figure:

Usable area = Rentable area ÷ (1 + Load factor ÷ 100)

The percentage calculator handles the same division-and-multiply-by-100 arithmetic if you want to check a brochure’s load figure against its own area numbers.

A load factor of 15% to 30% is typical for modern office buildings; buildings with grand lobbies, large atriums and extensive shared amenities can reach 35% or more. A lower load factor means more of what you pay for is space you actually use — which is why two suites with the same rentable area and the same rent per square foot can be very different value.

Worked example: a 1,650 sq ft listing

Take a suite advertised at a rentable area of 1,650 sq ft, with a load factor of 32% stated in the listing details.

Step 1 — Recover the usable area.

Usable area = 1,650 ÷ (1 + 0.32) = 1,650 ÷ 1.32 = 1,250 sq ft.

So of the 1,650 sq ft you are paying for, 1,250 sq ft is actual working space. The remaining 400 sq ft is your share of common areas.

Step 2 — Find the rent per usable square foot.

Suppose the landlord quotes $32 per sq ft per year on the rentable area. The total annual rent is:

1,650 × $32 = $52,800 per year.

But the rent per square foot of space you actually use is:

$52,800 ÷ 1,250 = $42.24 per usable sq ft per year.

That $42.24 figure, not $32, is the number to compare with other suites. A competing suite quoted at $34 per rentable sq ft with a 20% load factor costs $34 × 1.20 = $40.80 per usable sq ft — cheaper in real terms than the first suite, despite the higher headline rate.

Step 3 — Check the load factor against the floor plan.

The listing’s 32% should also reconcile with the suite’s own measurements. Measuring the suite room by room — the square footage calculator does the length-times-width arithmetic for each room, including irregular shapes — gives:

That total matches the 1,250 sq ft recovered from the load factor — which is exactly the check to run. If your room-by-room sum comes out materially lower than the listing’s implied usable area, either the room dimensions are larger than the ones you measured, the plan includes spaces you have not counted, or the listing’s load figure is wrong. The correct response is to ask the landlord for the BOMA measurement exhibit — that figure is the binding one — and to measure the suite yourself before signing.

The full build-up is then:

• Usable area: 1,250 sq ft
• Share of common areas (lobby, elevators, corridors, restrooms): 400 sq ft
• Rentable area: 1,250 + 400 = 1,650 sq ft ✓

And the load factor checks out: (1,650 − 1,250) ÷ 1,250 × 100 = 32%.

Why listings quote rentable area

Listings quote rentable area because it is the larger number, and because the rent per square foot therefore looks lower. It is also the industry convention: BOMA standards define rentable area as the basis for quoting, and virtually every US office and retail listing leads with it. The usable figure may appear in a BOMA exhibit or a space plan, but the headline number — and the number the rent is calculated on — is rentable. The only reliable figure anywhere is the one written into the lease, measured the way the lease defines it.

Usable vs rentable at a glance

Mistakes tenants make

Comparing headline rates across buildings. Two suites at the same rent per rentable square foot are not the same price if their load factors differ. Always convert to rent per usable square foot before comparing.

Ignoring the load factor on renewal. When you renew or expand, the landlord will quote the same way. A suite with a 35% load factor costs visibly more per usable foot than an equivalent suite at 20%.

Confusing usable area with the floor plan’s “total”. Marketing floor plans sometimes shade common corridors as part of the suite. The BOMA exhibit, not the brochure graphic, is the authority.

Mixing measurement standards. BOMA has different standards for office, retail, industrial and mixed-use buildings, and they allocate common areas differently. A load factor computed under the office standard is not comparable with one computed under the retail standard.

Paying for common areas twice. The rentable area already includes your share of common areas. Separate charges for building amenities or operating expenses are a different matter, but the area itself should not be charged again as if it were additional space.

What to check before you sign

1. Ask for the BOMA measurement exhibit showing the usable area and the load-factor calculation, in writing, not just in the listing.
2. Ask for the load factor and the usable area, so you can verify the arithmetic: rentable ÷ (1 + load factor) should equal the usable area.
3. Measure the suite room by room and add the areas up. A gap of more than a few percent against the stated usable area deserves an explanation.
4. Compare on rent per usable square foot, not rent per rentable square foot.
5. Check what the common-area share includes. A building with a large atrium, fitness center and conference facilities will have a higher load factor — that may be worth it to you, but it should be a conscious choice, not a surprise.

Frequently asked questions

Is usable area the same as the space I can furnish?

Broadly, yes. Usable area is the floor inside your suite that you control. Minor differences arise at the edges: columns and recesses inside the suite are included, while shared corridors and restrooms are excluded. If a landlord uses a non-BOMA definition, the figure is not comparable with a BOMA usable area.

How do I calculate usable area from rentable area?

Divide the rentable area by one plus the load factor expressed as a decimal. With a rentable area of 1,650 sq ft and a 32% load factor: 1,650 ÷ 1.32 = 1,250 sq ft. Without the load factor, you cannot do this calculation reliably — you need the landlord’s figure, or you must measure the suite and work from the rooms upward.

What is a good load factor?

For a modern office building, 15% to 25% is good and 25% to 30% is acceptable. Above 35%, a large share of the rent buys corridors and lobbies rather than working space, and the suite will compare poorly with alternatives. Low load is not automatically better, though: a building with minimal lobbies and narrow corridors may achieve a low load factor by cutting shared amenities you would actually want.

Does rentable area include the parking space?

No. Parking is normally leased or allotted separately and is not part of the rentable-area calculation. Check the lease: parking should appear as a separate line item, not inside the area figures.

Is there a rule of thumb for converting rentable to usable?

Some brokers use “rentable × 0.85” as a rough guess, but the actual ratio depends on the building’s common areas — in the worked example above, usable was 1,250 ÷ 1,650 = 75.8% of rentable, not 85%. Use the actual load factor, not the rule of thumb, for any figure you will pay on.

Which figure should I use when comparing two suites?

Rent per usable square foot, for value, and usable area itself, for whether the space fits your team. Rentable area is useful only as the starting point of the calculation — it is the number you convert from, not the number you compare on.

The bottom line

Usable area is what you work in. Rentable area is that plus your share of the building’s common spaces. The load factor connects them, and converting every quoted rent to a rent per usable square foot is the one habit that stops a headline rate from misleading you. Before signing, get the usable area in writing in the lease, check it against a room-by-room measurement, and compare suites on the space you will actually use. Once the rent is settled, the mortgage calculator turns a purchase price into the monthly payment you will actually live with.