Commercial Square Footage: GLA, Rentable, and Usable

Residential square footage has one number and one standard. Commercial space has four numbers, and a building can be honestly described by any of them. Knowing which one a listing, a lease, or a broker is quoting is the difference between comparing two spaces properly and signing for a third more floor than you can use.

The four numbers

Term	What it measures	Who uses it
Gross building area (GBA)	Everything inside the exterior walls, every floor	Appraisers, construction costing, tax assessment
Gross leasable area (GLA)	The floor area available for exclusive occupant use	Retail, shopping centers, investment analysis
Rentable square feet (RSF)	Usable space plus a share of the building's common areas	What your rent is calculated on
Usable square feet (USF)	The space you actually occupy and can put desks in	What you can plan a layout around

The gaps between them are real money. A space marketed at 1,120 rentable square feet might only give you 1,000 square feet to work with.

Gross leasable area

GLA is the retail world's standard, and it is defined by the BOMA Retail Standard. Stevenson Systems' definition of the term describes it as the total enclosed floor area designed for the exclusive use of an occupant, including basements, mezzanines, and upper floors, measured from the outside face of exterior walls and the centerline of the partitions that separate one tenant from the next.

Two details matter in practice. There is no deduction for columns or structural elements, so a space with a column in the middle is measured as though the column isn't there. And the exterior wall thickness counts, which is why a GLA figure sits slightly above what you can measure inside the space.

For a shopping center, total GLA is the sum of every tenant space, excluding the common mall areas, parking, and the areas the landlord keeps for building services. It is the denominator in almost every retail metric: sales per square foot, occupancy cost ratio, and rent comparisons across centers.

Rentable versus usable, and the load factor

Office leasing works differently. You pay on rentable square feet, which adds your proportional share of the lobby, corridors, restrooms, and mechanical rooms to the space you occupy.

The bridge between them is the load factor:

Common area / total building area = load factor

A building with 10,000 square feet of common area in a 100,000 square foot building has a 10 percent load factor, so 1,000 usable square feet is billed as 1,100 rentable. Load factors commonly run 10 to 20 percent, and a building with a dramatic atrium or generous corridors runs higher.

Worked example. Two offices, both quoted at $28 per square foot per year.

	Building A	Building B
Usable square feet	1,000	1,000
Load factor	12%	20%
Rentable square feet	1,120	1,200
Annual rent	$31,360	$33,600
Monthly rent	$2,613	$2,800

Same rate, same working space, $2,240 a year apart. Always ask for both figures, and compare deals on the usable number. The price per square foot calculator runs the rent math and covers what full service, modified gross, and triple net rates include.

Measuring a commercial building yourself

For a straightforward building, square footage is still length times width per floor, added together.

Measure the footprint to the outside of the exterior walls for gross building area.
Measure each floor separately. Mezzanines count in GLA and usually in GBA.
Do not deduct columns, interior partitions, or structural elements from a gross figure.
For a multi-tenant building, measure to the centerline of the walls separating tenants.

Common industrial and commercial footprints run to round numbers: a 30 by 40 building is 1,200 square feet, 40 by 60 is 2,400, and a 50 by 100 bay is 5,000. The square footage chart lists the usual sizes, and the rectangle calculator gives you perimeter as well, which is what you need for fencing and exterior wall estimates.

Warehouses come with a caveat: floor area alone doesn't describe the space. Clear height, column spacing, and dock door count decide what the square footage is actually worth, and two 20,000 square foot warehouses with 16 foot and 32 foot clear heights are not comparable buildings.

What changes by property type

Office. Quoted in rentable square feet under a BOMA office standard. Ask for the load factor and the usable figure.

Retail. Quoted in gross leasable area. Frontage matters alongside area, since a deep narrow unit with 20 feet of frontage trades differently from a shallow wide one.

Industrial and warehouse. Usually quoted close to usable, with little or no load factor, and rates are often lower per square foot but quoted monthly in some markets rather than annually. Confirm which.

Multifamily and mixed use. Residential units within a commercial building are typically measured to ANSI Z765 living area rules for the units themselves, while the building as a whole is described in GBA or GLA.

Common mistakes

Comparing an RSF quote to a USF quote. They're different numbers for the same room. Normalize before you compare.

Assuming the load factor is fixed. It varies by building and sometimes by floor. It's a negotiable assumption in a lease, not a law.

Reading an annual rate as monthly. Commercial rent is almost always quoted per square foot per year in the US. A $28 rate is $2.33 a month per square foot, not $28.

Using residential rules on commercial space. ANSI Z765 is a residential standard. It has nothing to say about GLA or rentable area.

Ignoring what the rate includes. A triple net rate plus operating expenses can land 25 percent or more above the headline number.

Frequently asked questions

What is gross leasable area?

The total enclosed floor area designed for a tenant's exclusive use, including basements, mezzanines, and upper floors. It's measured from the outside face of the exterior walls and the centerline of the walls between tenants, with no deduction for columns.

What is the difference between rentable and usable square feet?

Usable is the space you occupy. Rentable adds your share of the building's common areas, and it's the figure your rent is billed on. The difference between them is the load factor, commonly 10 to 20 percent.

How do I calculate the load factor?

Divide the building's common area by its total area. A 100,000 square foot building with 10,000 square feet of common area has a 10 percent load factor, so multiply usable square feet by 1.10 to get rentable.

How do I calculate commercial square footage?

Measure each floor's footprint to the outside of the exterior walls and add the floors together. For a multi-tenant building, measure to the centerline of the demising walls between tenants. Don't deduct columns from a gross figure.

Is GLA the same as rentable square footage?

No. GLA is a retail measure of the space a tenant exclusively occupies, and it typically excludes common mall area. Rentable square footage is an office measure that deliberately includes a share of the common areas.

How many square feet is a 30x40 building?

1,200 square feet. A 40 by 60 building is 2,400 square feet and a 50 by 100 bay is 5,000.

Why is commercial rent quoted per square foot per year?

It makes buildings of different sizes comparable and matches how landlords underwrite income. You divide by 12 to get the monthly payment, so $28 per square foot on 1,120 rentable square feet is $31,360 a year, or $2,613 a month.