Gross Floor Area

Gross floor area is the total floor space inside a building, every storey added together, measured to the outside face of the exterior walls. It's the number zoning codes care about, because it's what floor area ratio is calculated from, and floor area ratio is what decides how much building a piece of land is allowed to hold.

Add each floor's footprint below to get the total.

How gross floor area is measured

Three rules cover most of it:

Measure to the outside of the exterior walls. Wall thickness is inside the number, unlike a usable or carpet area figure.
Count every storey. Ground floor, upper floors, mezzanines, and any occupiable level.
Don't deduct interior walls, columns, stairwells, or shafts. Gross means gross. Stairs and elevator shafts are counted on each floor they pass through.

That last point surprises people. A ten-storey building with a lift shaft counts that shaft ten times, once per floor, because the code is measuring building bulk rather than usable space.

What counts and what doesn't

Inclusions and exclusions are set by each city's zoning code, not by a national standard. Portland's zoning guide is a representative example, and it includes stairwells and elevator shafts, covered balconies enclosed on most of their perimeter, spaces with at least 6 feet 8 inches of ceiling height, above-grade structured parking, and garages attached to a residence.

Usually counted	Usually excluded
All enclosed floors above grade	Basements with 50 percent or more of the wall below grade
Stairwells and elevator shafts, once per floor	Unenclosed roof areas and open decks
Mezzanines	Rooftop mechanical equipment
Attached garages and above-grade parking	Areas 4 or more feet below the adjacent street level
Enclosed balconies and covered porches	Unenclosed porches and open balconies

The basement exclusion is the one with the most money attached, and it echoes the residential rule: a level mostly below grade is treated differently from one above it, which is the same logic behind above grade vs below grade in home measurement.

Check your own city's code before relying on any of this. Definitions of floor area vary enough between jurisdictions that a building can gain or lose thousands of square feet by moving across a municipal line.

Gross floor area versus every other area measure

This is where the terms collide. Five different figures describe the same building, and each has its own audience.

Measure	What it covers	Who uses it
Gross floor area (GFA)	All floors, to the exterior walls, subject to zoning exclusions	Planners, zoning, developers
Gross building area (GBA)	Everything inside the exterior walls, every floor	Appraisers, construction costing
Gross leasable area (GLA)	Tenant space for exclusive use, excluding common mall areas	Retail leasing, investment analysis
Rentable square feet (RSF)	Usable space plus a share of building common areas	Office leasing, what rent is billed on
Gross living area (GLA, residential)	Finished, heated, above-grade living space	Appraisers on homes

Note the collision on the acronym: GLA means gross leasable area in retail and gross living area in residential appraisal. They are unrelated. Commercial square footage covers the leasing side, and ANSI Z765 governs the residential one.

Floor area ratio

Floor area ratio, or FAR, is gross floor area divided by the lot area.

FAR = gross floor area / site area

It's the zoning lever that controls building bulk. A city sets the maximum FAR for each zone, and that ceiling tells you the most floor area you can build on a given parcel, as the formula relating building area to site area sets out.

Worked examples.

A 20,000 square foot site with a 3.0 FAR allows 60,000 square feet of building.
A 5,000 square foot lot with a 0.5 FAR allows 2,500 square feet.
A two-storey building of 10,000 square feet per floor on a 20,000 square foot lot has an FAR of exactly 1.0.

To find what a site allows: multiply the lot area by the permitted FAR. To find a building's FAR: divide its gross floor area by the lot area. If your lot is described in acres, the acreage calculator converts it first, since an acre is 43,560 square feet.

The same FAR, different buildings

FAR controls volume, not shape. A 1.0 FAR can be built as:

One storey covering 100 percent of the lot
Two storeys covering 50 percent
Four storeys covering 25 percent

That flexibility is the point. Zoning pairs FAR with separate rules on height, setbacks and lot coverage to shape what actually gets built, which is why two sites with identical FAR limits can produce very different buildings.

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

Calculating your own

Measure each floor's footprint to the outside of the exterior walls. Length times width for a simple rectangle, or split an irregular outline into pieces.
Add the floors together. Don't deduct interior walls, stairs or shafts.
Remove whatever your local code excludes, most commonly the below-grade level and open decks.
Divide by the lot area for FAR.

For a house, the same exterior measurement is how gross living area starts, though living area then excludes the garage, the basement and anything unheated, so a home's GFA is always the larger number. The house square footage calculator works through that residential version.

Common mistakes

Using interior dimensions. Gross floor area is measured to the outside of the walls. Interior measurements understate it by the wall thickness on every floor.

Deducting stairwells. They count, once per floor.

Assuming one definition applies everywhere. Each municipality writes its own. The exclusions above are typical, not universal.

Confusing the two GLAs. Gross leasable area and gross living area share an acronym and nothing else.

Mixing lot units. An acre is 43,560 square feet. Feeding acres into an FAR calculation makes the number look 43,560 times too generous.

Frequently asked questions

What is gross floor area?

The total floor space of every storey in a building, measured to the outside face of the exterior walls, without deducting interior walls, columns or stairwells. It's the figure zoning codes use to control building bulk.

What is included in gross floor area?

All enclosed floors above grade, stairwells and elevator shafts counted once per floor, mezzanines, attached garages, above-grade structured parking, and enclosed balconies. Basements that are mostly below grade, open decks and rooftop mechanical equipment are typically excluded.

What is floor area ratio?

Gross floor area divided by the site area. A 20,000 square foot building on a 20,000 square foot lot has an FAR of 1.0. Zoning sets a maximum FAR per zone, which caps how much building the lot can hold.

How do I calculate FAR?

Divide the building's gross floor area by the lot area. To work the other way, multiply the lot area by the permitted FAR: a 5,000 square foot lot at 0.5 FAR allows 2,500 square feet of building.

What is the difference between gross floor area and gross living area?

Gross floor area is a zoning measure covering every storey to the exterior walls, including the garage and often the basement. Gross living area is a residential appraisal measure covering only finished, heated, above-grade space. A house's GFA is always larger.

Does the basement count in gross floor area?

Usually not, where 50 percent or more of its wall height is below grade. That mirrors the residential rule, though the exact threshold is set by local code.

Is gross floor area the same as gross building area?

They're close and often used interchangeably, but GFA is the zoning term with code-specific exclusions, while GBA is the appraisal and construction term for everything within the exterior walls. The two figures can differ on the same building.