Gross square feet is the total floor area of a building, measured from the outside faces of its exterior walls and added together across every level. The number captures everything inside the building envelope: finished rooms, hallways, mechanical spaces, stairwells, and the walls themselves. Because it is tied to the outer shell rather than any particular layout, it stays the same whether the interior is wide open or chopped into a hundred offices.
Appraisers and tax assessors rely on this figure as a baseline because it reflects the physical mass of the structure. Developers use it to estimate construction costs. Lenders use it in underwriting. If you have seen a building described two different ways by two different sources, odds are one of the numbers was gross and the other was something smaller.
What the Number Includes
The calculation captures every enclosed area within the exterior walls on every floor. Finished offices, lobbies, restrooms, and break rooms all count. So do spaces most people never think about: mechanical rooms, electrical closets, elevator machine rooms, custodial closets, and utility chases. Stairwells and elevator shafts are counted on each floor they pass through, not just once at the bottom.
Below-grade space counts too. Excavated basements, finished or unfinished, add to the gross figure. Mezzanines, penthouses, enclosed porches, attached garages, and attic levels within the exterior wall lines are all included. Structural elements like columns, load-bearing walls, and interior partitions are part of the total as well, because the measurement makes no distinction between space you can occupy and space the building consumes for itself.
One consequence of measuring to the outside face of the wall: a building clad in thick brick veneer will have higher gross square footage than an identical floor plan wrapped in thin siding, even though the usable rooms are the same size.
What the Number Leaves Out
The line between in and out hinges on whether the space is enclosed within the exterior walls. Uncovered patios, open courtyards, and rooftop decks without a roof overhead are excluded. Unenclosed breezeways, open-air walkways, and detached structures that are not physically connected to the main building also fall outside the total. Surface parking is never part of gross square footage, though enclosed parking garages inside the building shell typically are.
Open-to-below voids are a judgment call. When a floor has been cut away to create a double-height atrium, there is no actual floor surface at the upper level to measure. Some measurement standards exclude the void for that reason; others count it. If a building has large atriums or multi-story lobbies, confirm which standard was applied before trusting the number.
How the Measurement Is Taken
Surveyors start at the exterior face of the outside walls, not the interior finish. They record the length and width of the footprint at ground level, then repeat the process on each additional floor. Where floors have different footprints, such as an upper-story setback or a cantilevered section, each level is measured independently. Cornices, pilasters, and decorative projections beyond the wall face are disregarded; the measurement follows the plane of the wall itself.
Each floor’s area is calculated by multiplying its measured dimensions, and the results are summed. For irregular shapes, surveyors break the floor plate into rectangles and triangles, calculate each section, and add them. Laser distance meters and building information modeling software have largely replaced tape measures in professional practice, but the underlying geometry is straightforward. Two different surveyors measuring the same building from the same exterior walls should arrive at the same number, give or take minor rounding.
How It Differs From Net, Usable, and Rentable
Gross is always the largest of the common square footage figures. The others get smaller as they strip away different layers of the building, and the differences matter most in commercial leasing, where rent is quoted on one of these numbers and the gap directly affects cost.
- Net square feet is the interior area measured from the inside finishes of the walls. It excludes exterior walls, interior partitions, and utility chases. Gross equals net plus structural area.
- Usable square feet is the portion of a floor a specific tenant controls exclusively, measured from the tenant side of corridor walls and the inside surface of exterior walls. Common areas like lobbies, restrooms, and mechanical rooms are excluded.
- Rentable square feet is usable area plus the tenant’s proportionate share of the building’s common spaces. This is the figure landlords use to calculate rent.
The gap between usable and rentable is called the loss factor, and in dense urban office markets it can run 15 to 30 percent. A tenant paying for 5,000 rentable square feet might have closer to 3,500 to 4,250 square feet they can actually furnish. Gross will be larger still, because it adds back the structural mass of walls, columns, and shafts. When you are comparing buildings or reading a lease, knowing which of these four numbers is on the page is more important than the number itself.
Which Standard Produced the Number
Two measurement standards govern most of the figures you will encounter, and they are not interchangeable.
Residential: ANSI Z765-2021
The American National Standards Institute publishes ANSI Z765-2021, the current standard for calculating square footage in residential properties. Fannie Mae now requires appraisers to measure and report square footage in compliance with this standard, with no option to opt out.1Fannie Mae. Standardizing Property Measuring Guidelines Because most lenders sell loans to Fannie Mae, the requirement effectively reaches the whole mortgage industry.
One of the standard’s most important rules is the treatment of below-grade space. Finished basement area must be reported separately from above-grade living area and can never be combined into a single total. A home with 2,000 square feet of above-grade living space and an 800 square-foot finished basement is reported exactly that way, not as a 2,800 square-foot house. Unfinished basement and garage space is excluded from finished square footage entirely, though it can be noted separately. Below-grade space is generally valued at a lower rate per square foot than above-grade living area, so the distinction affects appraised value directly.
Commercial: BOMA Standards
Commercial buildings follow standards published by the Building Owners and Managers Association. The most widely used is ANSI/BOMA Z65.1-2024, designed for office buildings.2BOMA International. BOMA Standards Its primary purpose is calculating rentable area for lease agreements, not gross area.3American National Standards Institute. ANSI/BOMA Z65.1-2024 – BOMA 2024 for Office Buildings: Standard Methods of Measurement BOMA publishes a separate standard, Z65.3, for gross area measurement specifically.
This trips people up. A landlord quoting “BOMA square footage” is almost certainly quoting rentable area under Z65.1, not gross area. If you are comparing a building’s gross figure to its rentable figure, confirm which standard produced each number. BOMA also publishes separate standards for industrial, retail, multi-family, and mixed-use buildings, each with its own rules for allocating shared and common spaces.
Why Accuracy Matters
Getting gross square footage wrong cascades through other numbers. In residential sales, an overstated figure inflates apparent value, and buyers who discover the discrepancy after closing may have grounds for a misrepresentation claim against the listing agent or seller. On the tax side, square footage errors can distort depreciation deductions for investment property; the IRS allows an amended return within three years of the original filing date or two years from when you paid the tax, whichever is later.4Internal Revenue Service. Publication 946, How To Depreciate Property
In commercial leasing, a small measurement error multiplied across thousands of square feet and a long lease term adds up quickly. A 3 percent overstatement on a 20,000 square-foot lease at $40 per square foot costs the tenant an extra $24,000 a year. Over a ten-year term, that is $240,000. Institutional tenants routinely hire independent space planners to verify the landlord’s measurements before signing, and anyone evaluating a building should treat the measurement standard behind the number as part of the number itself.