Gross Floor Area Calculator
Gross floor area is the total floor space inside a building across every floor, and it drives zoning checks, permit fees and early construction budgets. This calculator adds up your floors three ways (a floor-by-floor list, footprint times floors, or a mixed-use list), subtracts the areas your standard excludes, converts the result between square feet, square meters and acres, and works out the floor area ratio when you add the lot area.
Gross floor area
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Quick Answer
A three-story building with 2,000 sq m per floor has a gross floor area of 64,583 sq ft (6,000.0 sq m, 1.483 acres). Excluding a 150 sq m open balcony leaves a net GFA of 62,969 sq ft, and on its 8,094 sq m lot the floor area ratio is 0.74. The calculator above is pre-filled with this exact building.
Key takeaways
- GFA counts everything inside the exterior walls on every floor. What gets excluded depends on the standard you follow (BOMA, RICS IPMS, or your local code), not on the calculator.
- The floor area ratio here is gross GFA divided by lot area. Exclusions lower the net GFA line, not the FAR.
- Rent is never charged on GFA. Tenants pay on rentable area, which is smaller, so never quote GFA as leasable space.
- Quick Answer
- How to use the gross floor area calculator
- The GFA and FAR formulas
- Worked example: a three-story office building
- What counts toward GFA, and what usually does not
- GFA vs. net leasable area vs. rentable area
- What competing calculators miss
- Frequently asked questions
- Add This Calculator to Your Website
- References
How to use the gross floor area calculator
Pick the entry mode that matches the numbers you have, set the input unit, then click Calculate GFA. The three modes total the same building in different ways.
- Floor-by-floor: type one floor area per line. Best when floors differ, such as a podium with a tower above it.
- Footprint × floors: enter length, width and floor count. Best for a simple block where every floor is identical.
- Mixed-use list: enter each use as Name: area, for example Retail: 1200. Best when a permit or appraisal needs the split by use, and the breakdown table keeps the names.
Add any excluded areas as Name: area lines and the tool reports a net GFA beside the gross figure. Add the lot area and it also reports the floor area ratio, the number zoning codes cap. If you are checking a parcel before you design, the floor to area ratio calculator works the same FAR question from the zoning side.
The GFA and FAR formulas
GFA is a sum, not a model: add the area of every floor, measured to the outside face of the exterior walls. FAR then divides that total by the lot area.
Two details of this tool are worth knowing before you rely on the output. First, the FAR uses the gross figure, before exclusions, which matches how most zoning codes define the ratio; the net GFA is reported separately for cost and efficiency work. Second, the list parser reads the first number on each line, so a line like "2000 sq ft" works, but a minus sign is ignored: -500 is read as 500. Enter positive areas only. The conversion factors are the standard ones published in NIST Special Publication 811, rounded to 10.7639 sq ft per sq m. One more tool behaviour to know: Footprint mode is built for square feet and square meters. With Acres selected, the tool multiplies length by width and treats the product itself as acres, which produces outsized totals, so run footprint work in square feet or square meters and reserve Acres for floor lists and lot areas, where it behaves normally.
Worked example: a three-story office building
The pre-filled example is a three-story office building in square meters: three floors of 2,000 sq m, a 150 sq m open balcony to exclude, and an 8,094 sq m lot (2 acres). Here is exactly what the tool returns:
| Line | Value |
|---|---|
| Gross floor area (3 × 2,000 sq m) | 64,583 sq ft |
| Same total in the other units | 6,000.0 sq m = 1.483 acres |
| Average floor | 21,528 sq ft |
| Excluded: open balcony (150 sq m) | 1,615 sq ft |
| Net GFA after exclusions | 62,969 sq ft |
| Floor area ratio (64,583 ÷ 87,122 sq ft of lot) | 0.74 |
Against a zoning cap of FAR 1.0, this building uses about three quarters of its allowed bulk, so a fourth floor of the same size (which would push FAR to about 0.99) is the obvious next test: change one line in the floor list and rerun. For early budgeting, multiply the GFA by a rate from the build cost per square foot calculator to get a first construction estimate.
What counts toward GFA, and what usually does not
The arithmetic is universal; the inclusion list is not. Which spaces count is set by the measurement standard or zoning code you are working to, and the differences are big enough to change a permit outcome.
| Space | Usually in GFA? | Notes |
|---|---|---|
| Offices, retail, residential floors | Yes | The core of every definition. |
| Corridors, lobbies, stairwells, elevator shafts | Yes | Inside the envelope, so they count toward gross even though nobody rents them. |
| Enclosed basement with adequate ceiling height | Usually yes | Parking or storage basements are excluded by many codes. |
| Open balconies, roof terraces, open decks | Usually no | Enclosed balconies generally count; open ones are the classic exclusion line in this tool. |
| Parking structures | Varies | BOMA gross area can include structured parking; many zoning codes exempt it from FAR. |
| Attics and spaces with very low ceilings | Usually no | Codes set a minimum ceiling height, often around 6 to 7 feet. |
Measurement frameworks such as the BOMA standard methods of measurement and RICS IPMS each publish their own boundary and exclusion rules. Put the spaces your standard excludes into the Exclusions box and the spaces it counts into the floor list, and the tool's net figure follows your standard rather than a generic one.
GFA vs. net leasable area vs. rentable area
GFA is the biggest of the three area measures, and mixing them up overstates a building. Net leasable area is GFA minus the common areas (lobbies, corridors, restrooms, mechanical rooms): the space tenants actually occupy. Rentable area is net leasable area plus the tenant's share of those common areas under the BOMA load factor: the area rent is charged on.
Use GFA for zoning, FAR checks, permits and cost-per-square-foot estimates. Use leasable or rentable area for leasing and valuation. The gap between GFA and leasable area is the building's efficiency: a tower at 85% efficiency earns rent on far more of its bulk than one at 70%. If your lot figure itself is uncertain, settle it first with the lot size calculator or the acreage calculator, because every FAR result inherits the lot area's error.
What competing calculators miss
Most rival GFA tools multiply one floor area by a floor count and stop. That fails on real buildings in four ways this page handles directly:
- Uneven floors. Podiums, setbacks and mezzanines break the single-footprint assumption. The floor-by-floor list totals any stack of floors as drawn.
- Mixed uses. Rivals give one anonymous total; the mixed-use mode keeps Retail, Office and Residential as named lines in the breakdown, which is how permits and appraisals ask for it.
- Exclusions. A gross figure with no way to subtract the parking level or the open balcony forces hand edits in a spreadsheet. Here exclusions are itemized in red in the breakdown and produce a separate net GFA.
- FAR. Almost no rival computes the floor area ratio alongside GFA, even though FAR is the number the zoning code actually caps. Add the lot area and it is part of the same result.
Frequently asked questions
Usually yes, if the basement is enclosed and has adequate ceiling height. Parking levels and unheated storage basements are excluded by many local codes, so check the definition your permit authority uses and list the basement in Exclusions if it is out.
Enclosed balconies generally count toward GFA; open balconies and roof terraces are typically excluded. The worked example in this article excludes a 150 sq m open balcony, which is why the net GFA is lower than the gross figure.
There is no universal good FAR; zoning sets the maximum for each parcel, often between 0.5 and 3.0 in commercial areas and higher downtown. A design only has to stay at or under its cap. Above the cap, the project cannot be approved as drawn.
Gross. The FAR line divides the gross floor area by the lot area, before exclusions, which matches how most zoning codes define the ratio. The net GFA after exclusions is shown separately for cost estimates and efficiency checks.
Multiply by 10.7639. So 100 sq m is 1,076.39 sq ft, and the 6,000 sq m worked example is 64,583 sq ft. This calculator shows square feet, square meters and acres in every result, so you rarely need the manual conversion.
The terms are often used interchangeably. Strictly, some standards define gross building area as GFA plus areas such as parking structures, so check which definition your jurisdiction applies before comparing figures from two different reports.
The list reader takes the first number on each line and ignores everything else, including a minus sign: a line entered as -500 is read as 500. Enter positive areas only, one per line, and use the Exclusions box for anything you want subtracted.
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References
- Building Owners and Managers Association (BOMA) International. Standard Methods of Measurement. Accessed October 21, 2026.
- Autodesk Help. About Calculating Spaces Based on the BOMA Standard. Accessed October 21, 2026.
- UBC Sauder School of Business, Real Estate Division. BUSI 401 Review Guide: BOMA Measurement Questions and Answers. Accessed October 21, 2026.
- National Institute of Standards and Technology. Special Publication 811: Guide for the Use of the International System of Units. Accessed October 21, 2026.
- RICS. International Property Measurement Standards (IPMS). Accessed October 21, 2026.
- New York City Department of City Planning. NYC Zoning Handbook 2025: Floor Area and Floor Area Ratio. Accessed October 21, 2026.