A floor area ratio calculator does one piece of arithmetic that sits at the centre of almost every development question: it divides the floor area of a building by the area of the lot it stands on. That single ratio is how most zoning codes in the United States express density, and it is the number that decides whether a scheme is even legal before anyone argues about its design. This page computes the ratio a building achieves, and runs the same relationship backwards to give the maximum floor area a stated FAR limit permits on a stated lot.
Arb Digital publishes free calculators for people who need a defensible number quickly. The important thing to understand about this one is that it takes your FAR limit and your exclusions as inputs. There is no national FAR table and this page does not pretend to hold one; the limit is set district by district in local zoning text, and so is the definition of what floor area even counts.
What This Floor Area Ratio Calculator Does
Enter the lot area, the gross floor area of the building, any floor area your code excludes from the calculation, the FAR limit for the district and the number of storeys. The calculator returns the FAR the building actually achieves on countable floor area, the maximum floor area the limit allows, the floor area still available under that limit, the average footprint per storey, and the lot coverage that footprint implies. Three bars show how much of the limit is used, how much headroom is left, and how much of the lot is covered.
The storey count is there because FAR and lot coverage are commonly confused, and seeing them side by side is the fastest way to separate them. The same 15,000 square feet of permitted floor area can be a single-storey building covering most of the site or a five-storey building covering a fifth of it. FAR is indifferent between those two; almost every other rule in the code is not.
How to Use It
- Enter the zoning lot area, not the parcel area. Codes define a zoning lot in their own terms, and adjacent parcels in common ownership can sometimes be treated as one. Our land area calculator converts acres, hectares and other units into square feet first.
- Enter gross floor area as your code measures it. Most codes measure to the outside face of exterior walls and count every storey. Our square footage calculator handles the geometry if you are working from dimensions.
- Enter the exclusions your ordinance actually grants. Some codes exclude cellar space, some exclude structured parking, some exclude mechanical floors, and many exclude none of it. This field is blank of assumptions on purpose.
- Enter the FAR limit from the zoning text for the district. Not from a neighbouring district, and not from a memory of what a similar city allows.
- Read the headroom figure as a maximum, not a plan. Available floor area under FAR is the ceiling that other rules then cut into.
The Formula / How It's Calculated
The definition is FAR = countable floor area ÷ lot area, where countable floor area is gross floor area minus whatever the code excludes. Running it backwards, maximum floor area = FAR limit × lot area, and the headroom remaining is that maximum minus the countable area already built. Average footprint is countable floor area divided by storeys, and lot coverage is that footprint divided by lot area.
Worked through the values the page loads with: a lot of 7,500 sq ft carrying a building of 11,250 sq ft gross, with 1,000 sq ft excluded by the code, has 10,250 sq ft of countable floor area and therefore an FAR of 10,250 ÷ 7,500 = 1.37. Against a limit of 2.0 the lot could carry 2.0 × 7,500 = 15,000 sq ft, so 4,750 sq ft of floor area remains available and 68.3% of the limit is used. Spread over three storeys the countable area implies an average footprint of 10,250 ÷ 3 = 3,416.67 sq ft, which covers 45.6% of the lot.
New York City's Zoning Resolution is a useful public example of how a code handles this in practice. Its floor area regulations for residence districts set maximum floor area ratios district by district and add a rule for mixed-use lots: the maximum FAR applies per use, and the total of all such ratios may not exceed the greatest ratio permitted for any single one of those uses on the zoning lot. The city's zoning glossary carries the underlying definitions of floor area, zoning lot and lot coverage.
FAR Is Not Lot Coverage and Not Height
This is the confusion that causes the most wasted design work. FAR governs quantity of floor area. Lot coverage governs how much of the ground the building sits on. Height and setback rules govern the envelope it can occupy. A scheme can satisfy FAR comfortably and still be illegal because it breaches a rear yard, exceeds a height limit, or covers too much of the site, and none of those failures will show up in the ratio.
The practical consequence is that the maximum floor area this page returns is an upper bound that the rest of the code then erodes. On a narrow or awkward lot, the envelope left after setbacks and height limits frequently cannot physically hold the floor area FAR would permit, and the site is said to be envelope-constrained rather than FAR-constrained. Working out which of the two binds first is one of the first things a zoning analysis establishes, and it changes the whole approach to a scheme.
What Counts as Floor Area Is a Local Question
Two buildings identical in every physical respect can produce different FARs in different cities because the codes count different things. Cellar and basement space, attached and structured parking, mechanical penthouses, balconies, elevator shafts, atria and the thickness of exterior walls are all treated differently from one ordinance to the next. Some codes exclude a fixed area, some exclude a percentage, some exclude nothing at all.
That is why the exclusion field on this page is empty of assumptions and why the tool never fills it in for you. The honest workflow is to open the definitions section of your own zoning ordinance, read what it says floor area includes and excludes, add up the excluded portions of your scheme, and enter that number. Any calculator that quietly applied a rule from somewhere else would be producing a legally meaningless answer with two decimal places of false precision.
Bonuses, Transfers and Overlays
The base FAR in a district is often not the end of the story. Many codes attach bonuses to public benefits — the NYC provisions cited above, for instance, allow maximum ratios to be exceeded through mass transit improvement and public plaza bonuses. Other jurisdictions permit development rights to be transferred from one lot to another, so a receiving site can legally build above its base ratio because a sending site has agreed to build below its own.
Overlays run in the other direction as often as not. Historic districts, coastal zones, airport approach surfaces, view corridors and inclusionary housing requirements can all modify what a base FAR appears to permit. The way to use this calculator alongside those is to run it once on the base limit for a floor, then again on the bonused limit for a ceiling, and treat the gap between them as the value at stake in whatever approval process the bonus requires.
Where This Sits Against Our Other Property Tools
The nearest tool on this site is the cost per square foot calculator, and the boundary between them is clean: that one prices floor area, this one tests whether the floor area is permitted in the first place. They are usually run in sequence — establish the buildable area here, then price it there. Once a scheme exists, the home renovation cost calculator covers the works side and the home value estimate calculator the finished-value side.
FAR also does quiet work in valuation. Two adjacent lots of identical size in different districts are not worth the same, because one can carry twice the floor area of the other. Where a site is well under its permitted ratio, that unused headroom is a real component of value, which is part of why the residual land value approach exists at all. Our property appreciation calculator covers the time dimension of the same asset, and the percentage calculator handles the general arithmetic.
Arb Digital designs and maintains free tool libraries that earn organic search traffic, with every definition sourced to a named authority before it goes live.
Browse All Free Tools Talk To Arb DigitalCommon Mistakes to Avoid
- Using the parcel area instead of the zoning lot area. Codes define a zoning lot in their own terms, and the difference changes every figure on this page proportionally.
- Assuming a basement or garage is excluded. Exclusions are entirely local, and assuming one you do not have overstates your headroom by exactly the area you assumed away.
- Treating the FAR maximum as a buildable scheme. Setbacks, height limits and coverage rules routinely make the permitted floor area physically impossible on the site.
- Applying a base ratio where a bonus, overlay or transfer is in play. The operative limit for a specific lot can sit well above or below the district's headline number.
- Mixing measurement conventions. Gross area measured to the inside face of walls and a code that measures to the outside face will disagree by a few percent, which matters at the margin of a limit.
Related Free Tools From Arb Digital
Convert land units before you start with the land area calculator, and work out areas from dimensions with the square footage calculator. Once the buildable area is settled, the cost per square foot calculator prices it, the home renovation cost calculator covers works, and the home value estimate calculator and property appreciation calculator cover value. Browse the full free online tools hub for the rest.
Frequently Asked Questions
It is the countable floor area of a building divided by the area of the zoning lot it stands on. A building with 10,250 sq ft of countable floor area on a 7,500 sq ft lot has a floor area ratio of about 1.37.
Multiply the lot area by the FAR limit for the district. A 7,500 sq ft lot in a district with a limit of 2.0 permits 15,000 sq ft of countable floor area, before any other rule reduces what can physically be built.
No. Floor area ratio measures how much total floor area is permitted; lot coverage measures how much of the ground the building occupies. The same permitted floor area can be a low building covering most of the site or a tall one covering a fraction of it.
There is no typical figure and this page does not supply one. Limits are set district by district in local zoning text and range from well under 1.0 in low-density residential areas to double figures in dense commercial cores. Look up the limit for your specific district.
It depends entirely on the ordinance. Some codes exclude cellar space, structured parking or mechanical floors; others exclude none of them. That is why exclusions are an input on this page rather than a built-in assumption.
In some jurisdictions, yes, through mechanisms the code itself provides. New York City's resolution, for example, allows maximum ratios to be exceeded via mass transit improvement and public plaza bonuses, and many places permit development rights to be transferred between lots.
Because FAR is only one of several controls. Height limits, setbacks, yard requirements and coverage rules shape the envelope, and on a constrained lot that envelope frequently cannot hold the floor area the ratio would allow.
This calculator performs zoning arithmetic on figures you supply and is not a zoning determination. Floor area definitions, exclusions and limits are set locally and change; confirm any figure with your municipal planning or zoning department, and take professional advice before relying on it.
