How to use the FAR calculator
Floor area ratio is the single number most zoning codes use to cap how much building can go on a piece of land. This calculator works it out in either direction: from a design you already have, or from the limit your zoning sets.
- Choose what to calculate. Pick FAR from floor area to check a scheme, or Max floor area from allowed FAR to find your development envelope before you start drawing.
- Enter the lot area in square metres or square feet. Use the net site area your code specifies; some jurisdictions deduct road widening or easements.
- Enter the gross floor area or the allowed FAR. Gross floor area is the sum of every countable storey, measured to the outside of the external walls unless your code says otherwise.
- Add the footprint (optional) to see lot coverage and the number of storeys the floor area translates to.
The FAR formula explained
FAR = Total gross floor area ÷ Lot area
Max floor area = Allowed FAR × Lot area
Lot coverage = Footprint ÷ Lot area
Storeys ≈ Floor area ÷ Footprint
FAR is a pure ratio with no units, as long as floor area and lot area are measured in the same units. A FAR below 1.0 means the total floor area is smaller than the lot, typical of suburban houses with gardens. A FAR above 1.0 means the building stacks more floor area than the land it occupies, which is normal in towns and cities. Central business districts can allow 10, 15 or more.
The useful insight is that FAR doesn't dictate shape. The same floor area can be a low building that covers most of the lot or a slim tower surrounded by open space. Lot coverage limits, height limits and setbacks then decide which shapes are actually allowed.
Worked examples
Checking an existing design
A house on a 600 m² lot has a 300 m² ground floor and a 240 m² first floor, plus a 300 m² basement your code excludes. Countable floor area is 300 + 240 = 540 m². FAR = 540 ÷ 600 = 0.90. If the zone allows 0.8, you need to lose 60 m², roughly the size of two bedrooms and a bathroom.
Finding the development envelope
A 10,000 ft² urban lot is zoned for FAR 3.0. Maximum floor area is 3.0 × 10,000 = 30,000 ft². With a 6,000 ft² footprint (60% coverage), that's 30,000 ÷ 6,000 = 5 storeys. Shrink the footprint to 4,000 ft² and the same floor area becomes 7.5 storeys, if the height limit allows.
Converting plot ratio percentages
A brief quotes a plot ratio of 250%. Divide by 100 to get a FAR of 2.5 and use the calculator as normal.
Who this calculator helps
- Architects running early feasibility studies, testing massing options and documenting area schedules for planning submissions.
- Architecture and planning students learning how zoning shapes cities and checking studio projects against realistic constraints.
- Developers and real estate investors estimating how much sellable or lettable area a site can deliver before making an offer.
- Builders confirming that an extension or additional storey keeps a property within its permitted floor area.
- Homeowners wondering whether there's room left for a loft conversion, garage apartment or rear extension.
- Interior designers working on fit-outs where change of use or mezzanines could affect counted floor area.
What usually counts, and what usually doesn't
Definitions vary, so treat this as a starting checklist rather than a rule:
- Usually counted: habitable rooms on every above-ground storey, internal corridors and stairs (often counted once per floor), enclosed mezzanines.
- Often excluded: basements fully below grade, parking structures, mechanical plant, open balconies and terraces, attic space below a minimum ceiling height, and shafts.
- Sometimes bonused: affordable housing, public plazas, green roofs or transit proximity can earn extra FAR in some cities.
Once the numbers work, check the vertical side of the scheme with the stair rise and run calculator, and use the architectural scale converter to pull real areas off scaled site plans. To show a client what the massing could actually look like, turn a sketch into a render with Sketch to Render.
