Gross Floor Area (GFA): How to Calculate It
Gross floor area (GFA) is the total floor area of every story of a building, measured from the outside face of the exterior walls. Zoning codes divide it by the lot area to get the floor area ratio (FAR), which caps how much building a lot can hold. What's left out, such as basements, depends on the code.
How to calculate gross floor area
Measure each story to the outside of its exterior walls and add the stories together.
Gross floor area = story 1 + story 2 + story 3 … each measured to the exterior walls
- Measure each story's outside dimensions. Split an L-shaped or irregular outline into rectangles.
- Add the stories. Don't deduct interior walls, columns, stairwells or elevator shafts.
- Count tall spaces once. An atrium or two-story lobby counts on its base level only.
- Take out what your code excludes, most often basements and uncovered roof decks.
L-shaped floor: split it into two rectangles and enter each in the calculator above. A 40 by 30 foot block plus a 20 by 15 foot wing is 1,200 + 300 = 1,500 sq ft per story.
What gross floor area includes
It depends on whose definition you use, and the Metropolitan Council notes each community can measure differently. A zoning code and EPA's ENERGY STAR treat basements and parking in opposite ways.
Counts stairwells on every floor, house garages and above-ground parking; leaves out basements at least half below grade. It's the same split as above grade vs below grade in homes.
Counts basements, stairwells, elevator shafts and mechanical rooms; leaves out parking, balconies, patios and crawl spaces (Portfolio Manager glossary).
Portland's full floor area rules
| Counts as floor area | Doesn't count |
|---|---|
| Stairwells and elevator shafts, on every floor (one level in single-dwelling zones) | Basements with at least 50% of their wall area below grade |
| Space with a ceiling of 6 ft 8 in or more | Floors 4 ft or more below an adjacent right-of-way |
| Above-ground structured parking | Roof area and uncovered roof decks |
| Garages accessory to houses and other dwellings | Rooftop mechanical equipment |
| Roofed porches and balconies walled over 42 in high on 75% or more of the perimeter | |
| Atriums, lofts and tall lobbies, on the main floor only |
Floor area ratio
Floor area ratio is gross floor area divided by the lot area, so a lot's maximum floor area is its area times the allowed FAR.
FAR = gross floor area ÷ lot area
Maximum floor area by lot size and FAR
| Lot area | FAR | Maximum floor area |
|---|---|---|
| 5,000 sq ft | 0.5 | 2,500 sq ft |
| 5,000 sq ft | 1.0 | 5,000 sq ft |
| 10,000 sq ft | 2.0 | 20,000 sq ft |
| 20,000 sq ft | 3.0 | 60,000 sq ft |
| 43,560 sq ft (1 acre) | 1.0 | 43,560 sq ft |
| 43,560 sq ft (1 acre) | 4.0 | 174,240 sq ft |
The 0.5 and 3.0 rows match the worked examples in Portland's FAR guide.
Lot in acres? Convert it to square feet first with the acreage calculator. An acre is 43,560 sq ft, so plugging in acres makes the FAR look 43,560 times too high.
Gross floor area vs other area measures
GFA measures the whole building; the other terms measure part of it. The closest match is gross building area, which INREV defines as all floor area to the outside of the walls, below-grade space included.
Finished, heated, above-grade space under ANSI Z765, without the garage or basement. See gross living area and the house square footage calculator.
The area rented to tenants, including basements and mezzanines (INREV). Same acronym as gross living area, unrelated meaning.
Usable is the space a tenant occupies; rentable adds a share of the common areas. See commercial square footage.
Takes out walls and space you can't use. Gross vs net square footage does the math.
For homes on their own, see whether garages are included in square footage. Porches and decks follow their own rule, covered in whether porches and decks count as square footage.
Frequently asked questions
What is the gross floor area of a building?
It's the floor area of every story added together, measured from the outside face of the exterior walls. Interior walls, columns and stairwells aren't deducted.
Is gross floor area the same as gross building area?
Close. Gross building area is the appraisal and investment term for all floor area measured from the outside of the walls, including below-grade space but not unenclosed areas, per INREV's definition. GFA is the zoning term, and local codes often exclude basements.
How do you calculate floor area ratio?
Divide the building's gross floor area by the lot area. A 20,000 sq ft building on a 20,000 sq ft lot has an FAR of 1.0; to find what a lot allows, multiply its area by the permitted FAR.
Are stairwells included in gross floor area?
Yes. Portland counts stairwells and elevator shafts on every floor (one level in single-dwelling zones), and ENERGY STAR includes them too.
Does the basement count in gross floor area?
For zoning, usually not: Portland excludes a basement with at least half its wall area below grade. ENERGY STAR benchmarking counts basements.
What is the difference between gross and net floor area?
Gross floor area includes the walls and every part of each story. Net area takes out walls and unusable space, so it's always smaller; gross vs net square footage shows by how much.