Floor Area Ratio (FAR) Calculator
Calculate a building’s Floor Area Ratio, and the maximum floor area a zoning FAR limit allows.
Reviewed by the ToolNestr Editorial Team — July 2026
How the floor area ratio calculator works
Floor Area Ratio (FAR) is a zoning measurement that compares a building's total floor area — summed across every story, not just its ground footprint — to the size of the lot it sits on. FAR = total floor area ÷ lot area. A FAR of 1.0 means the building's combined floor space equals the lot's area, which could be a single-story building covering the whole lot, or a two-story building covering half of it with the same total square footage.
Cities and counties set a maximum allowed FAR for each zoning district as a way to control overall development density and bulk, independent of separate rules on building height or lot coverage. Given a lot size and a zoning FAR limit, the maximum buildable floor area is simply the lot area times that limit — a 5,000 square foot lot with a 0.5 FAR limit allows up to 2,500 total square feet of floor area, distributed however the design calls for across one or more stories, subject to other zoning constraints like height and setbacks.
What counts toward "floor area" for FAR purposes is defined by local code and varies significantly between jurisdictions — many exclude unfinished basements, low-ceiling attics, or sometimes garages, while others count everything under roof. Always confirm your specific municipal zoning code's definition before using a FAR calculation to plan an actual project, since local exclusions can meaningfully change how much buildable area you actually have.
FAR definitions and exclusions vary by municipal zoning code; this guidance reflects common practice seen in U.S. zoning ordinances (e.g. NYC Zoning Resolution Section 12-10 definition of "floor area").
Pick a mode
Calculate an existing FAR, or find max buildable area.
Enter lot & floor area
Or the lot area and your zoning's FAR limit.
Confirm with your code
Check what counts as floor area locally.
The formula explained
FAR
= total floor area ÷ lot area. 2,500 sq ft on a 5,000 sq ft lot: 2,500 ÷ 5,000 = 0.5 FAR.
Max buildable area
= lot area × FAR limit. Same lot at a 0.5 limit: 5,000 × 0.5 = 2,500 sq ft.
Distributing floors
2,500 sq ft could be one 2,500 sq ft story, or two 1,250 sq ft stories — FAR doesn't dictate the layout.
Worked example
An 8,000 sq ft lot zoned with a 0.35 FAR limit.
Where FAR is used
Downtown and mixed-use zoning
Dense commercial districts often allow FAR limits of 2.0 or higher, letting developers stack many stories of floor area onto a single lot, sometimes with bonuses for public plazas or affordable housing.
Single-family residential zoning
Suburban residential districts typically cap FAR much lower, often 0.3 to 0.6, to limit how large a house can be relative to its lot and preserve neighborhood scale and yard space.
Pre-purchase due diligence
Buyers and developers calculate a property's existing FAR before purchase to see how much additional floor area, if any, the zoning still allows to be added.
Variance and rezoning requests
When a proposed project exceeds the as-of-right FAR, architects and attorneys use FAR math to quantify exactly how much of a variance or upzoning they need to request from the planning board.
Real-world context
A downtown mixed-use lot of 10,000 square feet zoned at a FAR of 2.0 allows up to 20,000 square feet of total floor area — enough for a five-story building covering the entire lot footprint, or a taller tower on a smaller footprint with setbacks. This is why dense urban cores with high FAR limits produce tall buildings even on modest-sized lots.
Contrast that with a 6,000 square foot suburban residential lot zoned at a FAR of 0.4: the maximum buildable floor area is only 2,400 square feet, which typically translates to a modest single-story home or a smaller two-story home, once setback and height rules are also factored in. Low FAR limits like this are a common tool for keeping new construction proportionate to existing neighborhood homes.
For an existing building, FAR also works as a diagnostic: a 12,000 square foot building sitting on an 8,000 square foot lot has an existing FAR of 12,000 ÷ 8,000 = 1.5. If the zoning district allows up to FAR 2.0, the property owner still has 0.5 FAR of additional buildable area — worth 4,000 more square feet on that lot — which is exactly the kind of calculation developers run before buying a property to expand.
Common misconceptions
"FAR and lot coverage are the same thing." They measure different things. Lot coverage is the percentage of the lot covered by the building's footprint (a 2D measure), while FAR sums floor area across every story (closer to a 3D measure). A tall, narrow building can have a low lot coverage but a high FAR.
"A FAR of 1.0 always means a one-story building." Not necessarily. A FAR of 1.0 just means total floor area equals lot area — that could be a single story covering the whole lot, or two stories each covering half the lot, or many other layouts. FAR says nothing about height or footprint on its own; those are governed by separate zoning rules.
Related calculators
Frequently asked questions
What is Floor Area Ratio (FAR)?
FAR is the ratio of a building's total floor area (summed across all stories) to the size of the lot it sits on. FAR = total floor area ÷ lot area. A FAR of 1.0 means the building's combined floor area equals the lot size — for example, a two-story building covering half the lot.
How do I calculate maximum buildable floor area from a FAR limit?
Multiply the lot area by the zoning's maximum allowed FAR. A 5,000 sq ft lot with a zoning FAR limit of 0.5 allows a maximum of 5,000 × 0.5 = 2,500 total square feet of floor area, which can be arranged across one or more stories.
Does FAR include garages, basements, or attics?
It depends on the local zoning code — many jurisdictions exclude unfinished basements, attics with low ceiling height, and sometimes garages from the FAR calculation, while others count them. Always check your specific municipal code's definition of "floor area" for FAR purposes.
Why do cities regulate FAR?
FAR controls the overall bulk and density of development relative to lot size, independent of building height or lot coverage rules. It's a zoning tool used to manage neighborhood density, sunlight and privacy between buildings, and infrastructure load, and it varies significantly by zoning district within the same city.
Sources & references
This tool uses standard formulas and reference values from:
- • American Concrete Institute — ACI 318, Building Code Requirements for Structural Concrete. concrete.org
- • ICC — International Residential Code (IRC), span, footing and framing tables. codes.iccsafe.org
- • APA – The Engineered Wood Association, allowable span and load guidance.
Estimates for planning only. Span, load and code values vary by jurisdiction — verify against your local adopted code and a licensed engineer before building.