Calculate gross floor area (GFA) from floor dimensions or sections and story count, then convert it to floor area ratio, net usable area, or an estimated construction cost.

Planning arithmetic only. Use the measurement rules and exclusions required by your authority; this is not a zoning or code-compliance determination.

Per-floor deduction — blank means zero

Deduct only exclusions required by your measurement rules.

Gross Floor Area Formula

The following formula is used to calculate a gross floor area.

GFA = A × F
  • Where GFA is the gross floor area (ft^2)
  • A is the typical or average counted area per floor, after any applicable per-floor deduction (ft² or m²)
  • F is the total number of floors in the building

Multiply the average counted floor area by the number of floors. The dimensional modes assume equal-area floors; subtract only applicable exclusions. FAR divides the applicable building floor area by lot area. Usable area requires your project-specific efficiency, and construction cost requires your own USD rate per area; no universal building-type efficiency is assumed.

The area of each individual floor can further be calculated using the following formula:

A = L * W

L and W are the length and width of a rectangular floor. For an irregular floor, sum non-overlapping rectangular sections.

Gross Floor Area Definition

Gross floor area generally sums measured floor areas, but its legal measurement basis and exclusions depend on the jurisdiction and purpose. There is no universal 7.5-foot ceiling-height rule.

Confirm whether garages, mezzanines, basements, voids and other spaces count under your applicable standard. GSA gross-area guidance illustrates an exterior-wall measurement basis; zoning floor area may have different exclusions.

Example Problem

How to calculate gross floor area?

First, determine the average area of each floor. In this example, the average area of the floors is found to be 1,000 ft^2.

Next, determine the total number of floors in the building. In this case, there are 40 floors in the entire building.

Finally, calculate the gross floor area using the formula above:

GFA = A * F

GFA = 1000 * 40

GFA = 40,000 ft^2