Gross Floor Area Calculator

Calculate gross floor area (GFA) by multiplying floor area by the number of floors, or from dimensions or sections, plus FAR, net area and cost.

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Calculator Functions

Function	What it does	What it requires
GFA from typical floor area	Finds gross floor area from the area of one typical floor	Requires the floor area and the number of equal-area floors
GFA from rectangular floor	Finds gross floor area for a building with a rectangular floor plate	Requires the floor length, the floor width and the number of equal-area floors
GFA from rectangular sections	Finds gross floor area for an irregular floor split into rectangles	Requires up to 100 non-overlapping rectangles as length and width, one per line, and the number of equal-area floors
Floor area ratio (FAR)	Finds the ratio of building floor area to lot area	Requires the floor area and the lot area
Net usable area estimate	Finds the usable part of a gross floor area	Requires the floor area and a measured or assumed efficiency in percent
Construction cost estimate	Finds a construction cost from a floor area and a cost rate	Requires the floor area and a cost in USD per square foot or per square meter

Areas are entered in ft² or m² and lengths in ft or m. The three GFA functions give the result in either unit with the other alongside, and each has an optional area to deduct per floor, where a blank means zero.

Gross Floor Area Formulas

GFA from typical floor area

The floor area, less any deduction, is multiplied by the number of floors. A building with 40 floors of 1,000 ft² each has a GFA of 40,000 ft², or 3,716.12 m². Deducting 50 ft² per floor leaves 38,000 ft².

GFA from rectangular floor

L and W are the length and width of the floor. A floor of 50 ft by 20 ft is 1,000 ft², and three such floors give 3,000 ft².

GFA from rectangular sections

An L-shaped or stepped floor is split into rectangles that do not overlap, and their areas are added. Sections of 40 ft by 25 ft and 20 ft by 10 ft make a 1,200 ft² floor, so two floors give 2,400 ft².

Floor area ratio (FAR)

Both areas use the same unit. A building with 20,000 ft² of floor area on a 10,000 ft² lot has a FAR of 2. A zoning FAR uses the floor area as the zoning code defines it, and FAR is not the share of the lot the building covers.

Net usable area estimate

E is the efficiency in percent, the usable share of the gross area. The calculator assumes no percentage for any building type. At 85%, 40,000 ft² of gross area leaves 34,000 ft² usable.

Construction cost estimate

The area is converted to the unit of the cost rate first. At $250 per ft², 40,000 ft² costs $10,000,000.

How to Calculate GFA

Measure each floor to the outside face of the exterior walls, add the floors together, and leave out any space that the applicable rule excludes. When every floor is the same size, that is the area of one floor multiplied by the number of floors.

What counts depends on who is asking. For federal buildings, the U.S. General Services Administration measures gross area as the total constructed area to the outside of the exterior enclosing walls. It includes basements, permanent mezzanines, mechanical floors, penthouses and structured parking, and it leaves out voids, crawl spaces and unexcavated space [1]. A zoning code can count differently: Portland, Oregon excludes qualifying basements and areas with a ceiling lower than 6 feet 8 inches from the floor area used for FAR [3].

Limits of This Estimate

• The result is planning arithmetic. It is not a zoning or building code determination, and the measurement rules and exclusions of the local authority decide the official figure.
• The GFA functions assume every floor has the same area. A building with different floor plates needs each floor measured and added separately.
• The efficiency and the cost rate are whatever is entered. Taxes, site work, professional fees and contingencies are in the cost only if the rate includes them.

Sources