Floor to Area Ratio Calculator

Work out how much you can build on a lot. This floor to area ratio calculator computes FAR from floor and lot area, the maximum buildable floor area from an allowed FAR, or the minimum lot size for a target building, with square feet, square meters and acres converted for you. FAR is a zoning control, so the page also shows what the number means in real buildings and where the legal limit for your parcel actually comes from.

Quick Answer

How to use the floor to area ratio calculator

Pick a mode, enter the two numbers that mode asks for, and click Calculate. Results stay hidden until you calculate, and every result shows the substituted formula so you can check it by hand.

1. FAR from floor area + lot area: enter the building’s total floor area across all stories and the lot area, each in sq ft, sq m or acres. The result is the ratio to three decimals plus a density reading from very low to ultra high. The pre-filled example, 8,000 sq ft on a 10,000 sq ft lot, gives 0.800.
2. Max floor area from lot + allowed FAR: enter the lot area and the FAR your district allows. The result is the largest total floor area that ratio permits, the number developers screen parcels with.
3. Min lot size from floor area + target FAR: enter the floor area the project needs and the target FAR. The result is the smallest lot that holds it, in sq ft and acres.
4. Mixed units are fine: each area converts to square feet before any division, using 10.7639 sq ft per sq m and 43,560 sq ft per acre. If your parcel is described in acres and you want it in square feet first, the acreage calculator does that conversion on its own.

Two guardrails are built in. Areas and FAR values must be greater than zero, with a specific message for each missing or invalid field, and a FAR entry over 30 in the two FAR-input modes stops with “FAR over 30 looks like a typo; please check.” Note that the cap applies to the FAR you type in, not to a computed ratio: FAR mode will happily report a ratio above 30 if the areas you enter produce one.

One formula, three questions

Every FAR problem is the same equation rearranged: FAR = total floor area ÷ lot area. Know any two of the three numbers and the third is one operation away.

The ratio itself is unitless, which is why the units cancel as long as both areas are measured the same way. Municipal codes write the measurement rules out in full: the Tucson Unified Development Code, Section 6.4.6, for example, spells out that the maximum permitted FAR is based on the most favourable situation and may not be achievable once height, setback and coverage rules are applied to the same lot. That sentence is the whole relationship between FAR and design in one line: the ratio gives a budget, the other controls decide whether you can spend all of it.

One display detail to know: areas inside results are rounded to whole square feet. A minimum lot of 10,000 ÷ 3 shows as 3,333 sq ft, and the formula line in sq m conversions shows the converted square feet rounded the same way (100 sq m of floor on a 200 sq m lot shows as 1,076 ÷ 2,153 = 0.500). The ratio itself is always computed from the unrounded values, so the rounding never moves the answer.

Worked examples, checked against this exact tool

Each row below is a real run of the calculator above, in the mode named. Type the same numbers in and you get the same results, including the density reading and the formula line.

Read the max-floor row the way a developer reads it: 20,000 sq ft is the envelope the land price has to justify. Divide the asking price by the buildable area and you have land cost per buildable square foot, the figure that decides whether a parcel pencils out; the cost per acre calculator covers the land side of that same screening, and once a buildable area is set, the build cost per square foot calculator prices what goes on it.

What FAR means in buildings, not decimals

A FAR of 1.0 means total floor area equal to the lot: one story covering the whole lot, two stories covering half of it, or four stories covering a quarter, before setbacks and coverage rules bite. The tool’s density reading follows six bands, and the boundaries are exact, so 0.500 is already Low, not Very low, and 2.000 is already High.

Why regulate bulk this way at all? The Rhode Island Chapter of the American Planning Association, in testimony on FAR regulation, puts the purpose plainly: tying building size to lot size leaves room for parking, stormwater facilities and yard space, and keeps a structure in balance with the lot it sits on. FAR is the dimensional standard that does that job in most American zoning codes, which is why the same ratio turns up in residential, commercial and industrial districts alike.

Typical FAR limits, taken from real zoning codes

There is no national FAR table: each city’s ordinance sets its own numbers by district. The two tables below are real values from two published codes, included so you can see the format your own city’s table will use. Read your parcel’s district, not these examples, for any actual decision.

The full tables, including how each city measures floor area and what bonuses add on top of the base figure, are in the Chicago Zoning Ordinance bulk and density standards and San Francisco Planning Code Section 124, both linked in the References. Chicago’s downtown chapter is where its bonus system lives: the downtown district standards allow floor area bonuses above the base FAR in D districts, and the business and commercial district standards add a transit-served FAR increase near rail stations. New York runs the same structure at larger scale, with district FAR tables and bonus provisions throughout its Zoning Resolution bulk regulations.

What counts as floor area, and what does not

It depends on the jurisdiction, and the exclusions are where real projects gain or lose area. Structured parking, mechanical rooms, cellars below grade and open balconies are excluded in many codes; enclosed living and working space nearly always counts.

New York’s Zoning Resolution is the most cited example: its floor area definition excludes cellar space and much mechanical and parking area, which is why two buildings with identical physical bulk can report different zoning floor areas. The practical rule is to never carry a floor area figure from a brochure or a tax record into a FAR calculation without checking it against your code’s definition of gross or zoning floor area. A broker’s square footage and a zoning floor area are different numbers, measured for different purposes.

FAR also gets confused with its neighbour control, lot coverage. FAR totals every story; coverage measures only the footprint, and confusing the two understates a multi-story building badly. The distinction is worked through with diagrams on the coverage area calculator page. Height limits complete the trio: a lot can hold FAR 2.0 worth of floor area and still be unbuildable at that bulk if the height cap forces a footprint the coverage rule forbids. All three controls have to pass at once, which is why this calculator reports the FAR answer as an envelope rather than an approval.

Finding your allowed FAR, and using the unused part

Your allowed FAR comes from your parcel’s zoning district, in five steps: find the district, read its bulk table, run the max-floor mode here, check the companion controls, then confirm the floor area definition with planning staff.

1. Find the district: look the parcel up on your city’s zoning map, or ask the planning department. District first, numbers second, because the same street can cross two districts.
2. Read the bulk table: the district’s table in the zoning ordinance lists the maximum FAR, often with a higher figure when affordable housing or another public benefit is included.
3. Run max-floor mode: lot area times allowed FAR gives the buildable envelope. Compare an existing building against it and the difference is unused FAR: a building at 0.800 on a lot zoned for 2.0 has (2.0 − 0.8) × 10,000 = 12,000 sq ft of unused floor area on paper.
4. Check the companion controls: height, setbacks, lot coverage, yards and any historic or special-district overlay can each cut into the envelope. Seattle’s Neighborhood Residential zoning summary shows the pattern for houses: maximum floor area set by a FAR that varies with the lot’s density, with exceptions listed beside it.
5. Confirm before you rely on it: unused FAR is potential, not a right to build. Landmark status, a sky exposure plane or a rear-yard rule can block an addition the arithmetic allows, and in some cities unused development rights can be transferred to another lot instead. Investors screening rentals run this same sequence before valuing a parcel’s expansion potential, alongside tools such as the buying rental property calculator.

What competing calculators miss

FAR tools tend to fail in one of two directions: a bare ratio with no zoning context, or a deep explainer with no usable tool at all.

• Omni Calculator’s floor area ratio page: built around carpet area, built-up area and super built-up area, returning a loading factor and open space ratio alongside FAR. Useful inside that housing workflow, but the FAR inputs are buried in it, and there is no lot-plus-allowed-FAR mode for screening a parcel or a min-lot mode for sizing land.
• The calculator.city FAR pages: several near-identical pages with shifting input models, some asking for floors times average floor area, some auto-calculating as you type. The fragmentation makes it hard to know which formula you just used, and none of them classify the result or show a code table.
• Everydaybudd’s FAR/FSI calculator: buildable GFA and what-if runs in five unit systems, the closest rival concept. It stops at the GFA figure, with no density reading and no discussion of exclusions or bonuses.
• Guide pages with no tool (NestApple, HN Republic, LegalClarity): strong plain-English and legal explanations of New York FAR, unused FAR and zoning analysis, but the reader still has to do the arithmetic elsewhere, and the reverse questions, how much can I build and how much land do I need, get prose instead of a calculation.
• The gap this page fills: all three modes in one tool with acre and square meter conversion, the substituted formula printed under every result, a six-band density reading whose boundaries match the article’s table, real FAR tables quoted from the Chicago and San Francisco codes, and the exclusions warning stated where it belongs, next to the number.

Frequently asked questions

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