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Floor Area Ratio (FAR) Calculator: Maximum Floor Area, Storeys and Height

Calculate floor area ratio (FAR, also called FSI or plot ratio) from lot size and gross floor area, or the maximum buildable floor area under a zoning FAR limit with bonuses. See the storeys and building height that area needs at a given lot coverage, whether a height limit binds before FAR, excluded areas, net rentable area and the number of units it could hold.

Lot area

Zoning maximum FAR

FAR bonus (affordable housing, TDR, plaza …)

Proposed gross floor area (0 to size to the max)

Building footprint as % of lot (max lot coverage)

Floor area excluded from FAR (structured parking, mechanical, cellar)

Net rentable / sellable efficiency

Floor-to-floor height

Height limit (0 = none)

Average gross floor area per dwelling unit (0 = skip)

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How it works

FAR = gross floor area ÷ lot area · maximum floor area = lot × (FAR + bonus) · footprint = lot × coverage · storeys = floor area ÷ footprint · height ≈ ⌈storeys⌉ × floor-to-floor height · with a height limit, buildable area = min(lot × FAR, footprint × ⌊height limit ÷ floor-to-floor⌋)

Floor area ratio is the zoning code’s main dial for development intensity. It caps the total floor area a lot may carry as a multiple of the lot’s own area, and leaves the designer to trade footprint against height. A FAR of 2.5 can be a squat five-storey block covering half the site or a slender tower on a quarter of it, which is why FAR is always paired with lot coverage, setback and height limits. The same ratio is called floor space index (FSI) or plot ratio in many countries, sometimes written as a percentage (FSI 250% = FAR 2.5). Enter the base FAR and any bonus (for affordable housing, transferred development rights or a public plaza) to get the maximum counted floor area, or enter a proposed floor area to check it against the limit. The calculator turns the floor area into storeys and an approximate building height at your lot coverage and floor-to-floor height. With a height limit it shows which rule binds first: on a tight height limit and a small footprint, the full FAR may not fit. Codes differ in what counts: basements, structured parking, mechanical floors and balconies are commonly excluded, so the “true” gross building can be larger than the FAR area. Add an average unit size to see how many dwellings the floor area could hold.

Worked example

A 20,000 ft² lot (0.46 acres) zoned FAR 2.5 allows 20,000 × 2.5 = 50,000 ft² of counted floor area. At 60% lot coverage the footprint is 12,000 ft², so the building needs 50,000 ÷ 12,000 = 4.2 storeys: five floors with a top floor 17% of the plate, about 60 ft tall at 12 ft floor-to-floor. At 85% efficiency it yields 42,500 ft² of rentable space. With a 48 ft height limit only four floors fit (48,000 ft²), so height, not FAR, would bind.

Sources: Floor area ratio — Wikipedia · Omni floor area ratio (FSI) calculator

Frequently asked questions

Is FAR the same as lot coverage?

No. Coverage is the footprint as a share of the lot (a flat, 2-D limit); FAR is the total floor area on all storeys as a share of the lot. A single-storey building at 50% coverage has FAR 0.5; ten storeys at 50% coverage is FAR 5.

How do I calculate the FAR of a building?

Add up the gross floor area of every floor that counts under your code and divide by the lot area. A 30,000 ft² building on a 12,000 ft² lot has FAR 2.5. Use the same units for both.

What is the difference between FAR and FSI?

They are the same ratio. FSI (floor space index), used in India and elsewhere, is often written as a percentage or a decimal: FSI 1.5 or 150% means the floor area may be 1.5 times the plot area.

Do parking garages and basements count toward FAR?

Often not. Many codes exclude below-grade floors, structured parking, mechanical rooms, stair and elevator shafts, but the rules vary widely. Enter excluded areas separately to see the total building size.

How does FAR relate to housing density?

Units ≈ FAR × lot ÷ average gross floor area per unit. FAR 2.5 with 1,000 ft² gross per unit gives about 109 units per acre, before parking, open-space and unit-mix rules bite. Enter an average unit size under More options to see it.

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Planning arithmetic using the definitions most zoning codes share; the actual FAR, setbacks, height, coverage, parking ratios and density rules are set by the local ordinance and overlay districts, and many cities exclude or bonus certain areas. Confirm with the zoning code and a land-use professional before relying on a yield.