Gross square footage (GSF) is the total floor area of a building measured from the outside faces of its exterior walls, on every floor. It includes everything within those outer walls: the rooms you can use, the walls themselves, hallways, stairwells, elevator shafts, mechanical spaces, and any other area that takes up space inside the building’s footprint. It is the largest square footage number you’ll encounter for any building.
What Gets Included in the Measurement
The key word in gross square footage is “gross,” meaning nothing inside the building’s exterior walls is subtracted. You’re measuring the entire building envelope, floor by floor, then adding those floors together. The measurement is taken from the outside face of the exterior walls, so even the thickness of those walls counts toward the total.
Beyond the obvious rooms and hallways, GSF also includes:
- Basements, including excavated below-grade areas
- Stairways and elevator shafts, counted on every floor they pass through
- Mechanical spaces, such as equipment floors and vertical duct shafts
- Mezzanines, penthouses, and attics
- Garages
- Covered porches and balconies, whether walled or not, as long as they’re within the building’s footprint or covered by a roof above
- Corridors and walkways, walled or open, if they fall within the building’s exterior lines or under a roof
Decorative elements that project beyond the wall face, like cornices or pilasters, are ignored. The measurement stops at the flat plane of the exterior wall itself. One notable exception: the top, unroofed floor of a parking structure is included in GSF as long as parking is available there.
How to Calculate It
The basic process is straightforward. Measure or scale the exterior dimensions of each floor from the outside face of the walls. Multiply length by width to get that floor’s area. Then add up every floor in the building, including basements, mezzanines, and any other levels. The formula boils down to: GSF equals net usable area plus structural space.
For a simple two-story rectangular building that measures 50 feet by 100 feet on the outside, each floor is 5,000 square feet. The gross square footage is 10,000. If there’s also a full basement at the same dimensions, the total jumps to 15,000 GSF. Irregularly shaped buildings require breaking the footprint into smaller rectangles, calculating each one, and summing them up.
GSF vs. Net Square Footage
Gross square footage will always be larger than net square footage because net square footage strips out structural elements. Net square feet represents only the interior area that’s accessible by foot, measured from the inside face of walls rather than the outside. It excludes the space taken up by the walls, columns, and other structural components.
Net square footage is further divided into two categories: assignable area (spaces you can actually use, like offices, classrooms, or apartments) and non-assignable area (hallways, restrooms, mechanical rooms, and lobbies). The gap between gross and net gives you a sense of how much of a building is “lost” to structure and shared spaces. In a typical commercial building, the usable space might be 15% to 30% smaller than the gross figure, depending on how thick the walls are, how many elevator shafts exist, and how much corridor space the layout requires.
GSF vs. Rentable Square Footage
If you’re leasing commercial space, you’ll encounter a third measurement: rentable square footage (RSF). This sits between gross and usable. Rentable square feet starts with the space you’ll actually occupy (your usable square feet) and adds your proportional share of common areas like lobbies, shared hallways, and restrooms.
The difference between rentable and usable square footage is called the load factor (sometimes called the common area factor). Most commercial buildings have load factors between 10% and 25%, though some run as low as 3%. A building with 200,000 rentable square feet and 190,000 usable square feet has a load factor of about 5.2%. This matters because your rent is calculated on the rentable number, not the usable number. You’ll pay for space you share with other tenants.
Gross square footage is rarely used directly in lease calculations, but it determines the building’s total capacity and is the number used for property tax assessments, insurance, and construction cost estimates.
Residential Measurements Work Differently
When you’re buying or selling a home, the relevant number is typically gross living area (GLA), not gross square footage in the commercial sense. Residential standards, guided by ANSI Z765 and used by Fannie Mae, have specific rules that differ from commercial measurement.
Measurements are taken to the nearest inch or tenth of a foot, with the final number rounded to the nearest whole square foot. Staircases count toward the floor from which they descend. Two-story foyers and other openings to the floor below are not included in the upper floor’s area. Basements, defined as any space partially or completely below grade, are excluded from GLA entirely, even if they’re finished.
Ceiling height matters too. Finished areas need at least a 7-foot ceiling. In rooms with sloping ceilings, at least 50% of the room’s floor area must have 7-foot ceilings, and any portion under 5 feet cannot be counted at all. This is why some Cape Cod-style homes with sloped upstairs ceilings report lower square footage than you might expect from the footprint.
Why GSF Matters for Zoning
Local zoning codes use gross square footage to calculate something called floor area ratio (FAR), which controls how large a building can be relative to its lot. FAR is calculated by dividing a building’s gross floor area by the lot’s area. If a 10,000-square-foot lot has a FAR limit of 1.5, the maximum gross floor area allowed is 15,000 square feet. That could be a single-story building covering most of the lot or a three-story building with a smaller footprint.
This is one reason gross square footage is the standard measurement for construction planning, building permits, and municipal records. It captures the full physical size of a structure in a way that net or usable measurements don’t.
Measurement Standards by Building Type
Different industries follow different formal standards. The Building Owners and Managers Association (BOMA) publishes the most widely used commercial standards in the United States. Their standard for gross area measurement is ANSI/BOMA Z65.3-2009. Separate BOMA standards cover office buildings (Z65.1-2010) and industrial buildings (Z65.2-2012), each with rules tailored to those building types.
Internationally, the International Property Measurement Standards Coalition (IPMSC) defines IPMS 1 as the floor area measured to the external extent of the exterior walls, including any notional boundaries for external floor areas or sheltered areas. This aligns closely with the U.S. concept of gross square footage and is intended to create consistency for global real estate transactions.
Knowing which standard applies matters when you’re comparing buildings, reviewing appraisals, or negotiating a lease. A gross square footage number calculated under one standard may not match a figure calculated under another, even for the same building, because of small differences in what gets included or excluded.