Gross floor area (GFA) is the total floor area of a building measured according to the applicable area-measurement or zoning rules. For a simple building, GFA can be calculated by multiplying the floor area of one level by the number of floors.
Use the calculator below to estimate the gross floor area of a rectangular building.
Gross Floor Area Calculator
Calculate floor area and total gross floor area from building dimensions and number of floors.
What Is Gross Floor Area?
Gross floor area (GFA) is a measure of the total floor area contained within a building, subject to the rules used by the relevant authority, zoning code, building standard, or measurement system.
For a simple rectangular floor, the floor area is:
\(Floor\ Area=L\times W\)
where:
- \(L\) = floor length
- \(W\) = floor width
For a building with multiple floors having the same floor area:
\(GFA=Floor\ Area\times N\)
where:
- \(GFA\) = gross floor area
- \(Floor\ Area\) = area of one floor
- \(N\) = number of floors or stories
Example
Suppose a building has a floor measuring 50 ft × 80 ft and contains 4 floors.
First calculate the area of one floor:
\(Floor\ Area=50\times80=4,000\ ft^2\)
Then multiply by the number of floors:
\(GFA=4,000\times4=16,000\ ft^2\)
Therefore, the estimated gross floor area is 16,000 square feet.
Gross Floor Area Formula
For a rectangular building with identical floors:
\(GFA=L\times W\times N\)
For example:
\(GFA=60\times100\times3\)
\(GFA=18,000\ ft^2\)
This formula works well when every floor has approximately the same dimensions.
However, many real buildings have different floor layouts, additions, wings, setbacks, or irregular shapes. In those cases, calculating each section separately can produce a more useful result.
How to Calculate Gross Floor Area
The basic process is:
- Determine the floor area of each section.
- Add the areas together to obtain the floor area for that level.
- Repeat for floors with different layouts.
- Add the applicable floor areas of all levels.
- Apply any exclusions or deductions required by the relevant local rules.
For identical floors, the calculation becomes much simpler:
\(GFA=A\times N\)
where \(A\) is the area of one floor.
Example: Three-Story Building
A building has:
- Length = 40 ft
- Width = 75 ft
- Floors = 3
Floor area:
\(A=40\times75=3,000\ ft^2\)
GFA:
\(GFA=3,000\times3=9,000\ ft^2\)
The estimated GFA is therefore 9,000 ft².
GFA for Buildings With Different Floor Areas
Not every building has identical floors.
For example:
| Floor | Floor Area |
|---|---|
| Ground floor | 5,000 ft² |
| Second floor | 4,500 ft² |
| Third floor | 4,000 ft² |
| Total GFA | 13,500 ft² |
In this situation, multiplying the ground-floor area by three would give an incorrect result.
Instead:
\(GFA=A_1+A_2+A_3+\cdots+A_n\)
So:
\(GFA=5,000+4,500+4,000\)
\(GFA=13,500\ ft^2\)
This approach is particularly useful for buildings with setbacks, terraces, wings, podiums, or changing floor plates.
Gross Floor Area for Irregular Buildings
A building does not have to be a simple rectangle.
An L-shaped or irregular floor plan can be divided into smaller rectangles.
For example, suppose an irregular floor contains three rectangular sections:
- Section A = 40 ft × 50 ft
- Section B = 20 ft × 30 ft
- Section C = 15 ft × 25 ft
Calculate each section separately:
\(A_1=40\times50=2,000\ ft^2\)
\(A_2=20\times30=600\ ft^2\)
\(A_3=15\times25=375\ ft^2\)
Then:
\(Floor\ Area=A_1+A_2+A_3\)
\(Floor\ Area=2,000+600+375=2,975\ ft^2\)
If this same floor plan occurs on four floors:
\(GFA=2,975\times4=11,900\ ft^2\)
Irregular & Multi-Section GFA Calculator
When a building has an L-shaped, T-shaped, stepped, or otherwise irregular floor plan, divide the floor into manageable rectangular sections and calculate each section separately.
Our second calculator makes this process easier by allowing multiple rectangular sections to be combined.
Irregular & Multi-Section GFA Calculator
Calculate gross floor area for L-shaped, T-shaped, stepped, or irregular buildings by combining rectangular sections.
Section Breakdown
| Section | Length | Width | Area |
|---|
GFA vs Floor Area
The terms floor area, gross floor area, net floor area, and usable floor area are sometimes used differently depending on the context.
A simple distinction is:
Floor area:
The area of a particular floor or level.
Gross floor area (GFA):
The total gross floor area across the building, subject to the applicable measurement rules.
Net floor area:
The portion remaining after applicable deductions or exclusions.
Usable or functional area:
The portion of the building that can actually be used for the intended activity.
The important point is that these terms should not automatically be treated as interchangeable.
Gross Floor Area vs Net Floor Area
A building can have a large GFA but considerably less usable or net area.
For example, a building may contain space for:
- Corridors
- Stairs
- Elevators
- Mechanical services
- Electrical services
- Structural elements
- Walls
- Common areas
Depending on the applicable measurement standard, some of these spaces may be included in gross area, excluded, or treated differently.
Therefore, there is no single universal rule that determines exactly which spaces must always be included in GFA.
For zoning or permitting purposes, always use the definition specified by the relevant authority.
Net-to-Gross Calculation
Another common GFA question is:
“I know how much usable or net space I need. How much gross floor area do I need?”
This is a different calculation from simply multiplying floor dimensions by the number of floors.
If the net area represents a percentage of the gross area, use:
\(GFA=\frac{Net\ Area}{Efficiency}\)
If efficiency is expressed as a percentage:
\(GFA=\frac{Net\ Area}{\frac{E}{100}}\)
For example, if the required net area is 80,000 ft² and the assumed net-to-gross efficiency is 80%:
\(GFA=\frac{80,000}{0.80}\)
\(GFA=100,000\ ft^2\)
So approximately 100,000 ft² of GFA would be required under that assumption.
Net-to-Gross GFA Calculator
This calculation is useful during early planning when the user knows the required usable or net area but needs to estimate the gross building area.
Net-to-Gross GFA Calculator
Estimate the gross floor area required when you know the desired net or usable floor area and expected efficiency.
GFA and FAR
Gross floor area is closely related to Floor Area Ratio (FAR).
FAR is calculated by comparing the gross floor area with the lot area:
\(FAR=\frac{GFA}{Lot\ Area}\)
Therefore, if FAR and lot area are known:
\(GFA=FAR\times Lot\ Area\)
Example
Suppose:
- Lot area = 20,000 ft²
- Maximum FAR = 2.5
Then:
\(GFA=2.5\times20,000\)
\(GFA=50,000\ ft^2\)
This means the permitted GFA under that assumed FAR would be 50,000 ft², before considering any local rules, exclusions, bonuses, deductions, or other development controls.
GFA and FAR answer different questions:
- GFA: How much gross floor area does the building contain or require?
- FAR: How does that floor area relate to the size of the lot, and what amount may be permitted?
For a dedicated FAR calculation, see our Floor Area Ratio Calculator.
GFA and Number of Floors
If the total GFA and average floor area are known, the approximate number of floors can be estimated by rearranging the formula:
\(N=\frac{GFA}{Floor\ Area}\)
For example:
- GFA = 30,000 ft²
- Average floor area = 5,000 ft²
Then:
\(N=\frac{30,000}{5,000}=6\)
So the building represents approximately 6 floors at that average floor area.
In real projects, the actual number of stories may differ because floor areas can change between levels.
GFA in Square Feet, Square Meters and Square Yards
GFA is commonly reported in square feet or square meters.
Useful conversions include:
\(1\ m^2=10.7639\ ft^2\)
\(1\ ft^2=0.092903\ m^2\)
And:
\(1\ yd^2=9\ ft^2\)
For example:
\(10,000\ ft^2\times0.092903=929.03\ m^2\)
Therefore:
10,000 ft² ≈ 929.03 m²
Always make sure all dimensions are converted into the same unit before calculating area.
What Areas Count Toward GFA?
This is one of the most important issues when calculating GFA.
The mathematical formula may be straightforward, but the legal or zoning definition of GFA is not universal.
Different jurisdictions and measurement standards can treat areas such as:
- Basements
- Parking areas
- Garages
- Balconies
- Mezzanines
- Mechanical rooms
- Staircases
- Elevator areas
- Covered areas
- Common areas
GFA Calculation Example
Consider a four-story building.
Each floor has a rectangular area of:
60 ft × 80 ft
Floor area:
\(A=60\times80=4,800\ ft^2\)
Number of floors:
\(N=4\)
Therefore:
\(GFA=4,800\times4\)
\(GFA=19,200\ ft^2\)
If the net-to-gross efficiency is assumed to be 80%, the estimated net area would be:
\(Net\ Area=19,200\times0.80\)
\(Net\ Area=15,360\ ft^2\)
This illustrates why gross and net floor area should not be treated as the same measurement.
Common GFA Calculation Mistakes
1. Multiplying the wrong floor area by the number of floors
If upper floors are smaller, do not assume every floor has the same area.
2. Mixing measurement units
Do not multiply feet by meters. Convert all dimensions to the same unit first.
3. Confusing GFA with net area
Gross area generally represents a different measurement from usable or net area.
4. Automatically including or excluding special spaces
Basements, parking, balconies, mechanical areas, and other spaces may be treated differently under different rules.
5. Using FAR rules as a universal GFA definition
FAR and GFA are related, but local regulations determine how floor area is measured for zoning purposes.
6. Ignoring different floor layouts
A building with different floor plates should be calculated floor-by-floor or section-by-section.
How to Use the Three GFA Calculators
Our three calculators address different GFA calculation problems.
Calculator 1 — Basic Gross Floor Area
Use this when you know:
- Floor length
- Floor width
- Number of floors
It is best for simple rectangular buildings with similar floor plates.
Calculator 2 — Irregular / Multi-Section GFA
Use this when your floor plan contains:
- Multiple rectangular sections
- L-shaped layouts
- T-shaped layouts
- Wings or additions
- Irregular sections
Calculate the sections separately and combine them.
Calculator 3 — Net-to-Gross GFA
Use this when you know the required:
- Net area
- Usable area
- Functional area
and want to estimate how much gross floor area is required.
Quick GFA Reference
| Calculation | Formula |
|---|---|
| Rectangular floor area | \(A=L\times W\) |
| Identical multi-story building | \(GFA=A\times N\) |
| Different floor areas | \(GFA=A_1+A_2+\cdots+A_n\) |
| FAR from GFA | \(FAR=\frac{GFA}{Lot\ Area}\) |
| GFA from FAR | \(GFA=FAR\times Lot\ Area\) |
| Floors from GFA | \(N=\frac{GFA}{A}\) |
| GFA from net area | \(GFA=\frac{Net\ Area}{Efficiency/100}\) |
Frequently Asked Questions
What is gross floor area?
Gross floor area is a measure of the total floor area of a building, calculated according to the applicable area-measurement or zoning rules.
How do I calculate GFA?
For identical rectangular floors:
\(GFA=L\times W\times N\)
where length and width determine the floor area and \(N\) is the number of floors.
Does GFA include all floors?
GFA calculations generally consider the applicable floor areas of the building, but exactly which spaces and levels count depends on the relevant local definition.
Is GFA the same as net floor area?
No. Gross and net floor area are different measurements. Net area generally represents a smaller usable portion after applicable deductions or exclusions.
How do I calculate GFA from FAR?
Use:
\(GFA=FAR\times Lot\ Area\)
For example, a 20,000 ft² lot with FAR 2.0 gives:
\(GFA=20,000\times2=40,000\ ft^2\)
subject to the applicable zoning methodology.
How do I calculate GFA for an irregular building?
Divide the floor plan into simple sections, calculate each section’s area, and add them together:
\(Floor\ Area=A_1+A_2+A_3+\cdots\)
Then multiply by the number of identical floors if applicable.
How do I calculate gross area from net area?
If the expected net-to-gross efficiency is known:
\(GFA=\frac{Net\ Area}{Efficiency/100}\)
For example, 80,000 ft² net area at 80% efficiency requires approximately 100,000 ft² GFA.
What is the difference between GFA and FAR?
GFA measures building floor area, while FAR compares GFA with the lot area:
\(FAR=\frac{GFA}{Lot\ Area}\)