Floor area ratio & site coverage

What a floor area ratio calculator does

A floor area ratio calculator divides a building's gross floor area by the area of its lot to give the FAR, the main zoning measure of how much building a site can hold. This one also works out lot coverage, the share of the site covered by the building footprint, and shows how much floor area and footprint remain under the limits you enter.

FAR and lot coverage answer different questions. FAR controls total bulk: a FAR of 1.0 on a 10,000 ft² lot allows 10,000 ft² of floor area, whether that is one story covering the whole lot or four stories on a quarter of it. Lot coverage controls how much ground the building occupies, and so how much is left for yards, drainage and open space. Most zoning codes regulate both, alongside height and setbacks, and the tightest limit wins.

The calculator does not look up zoning. You enter the FAR and coverage limits for your own zoning district, one gross floor area per level, and either the lot area or the lot width and depth with its front, side and rear setbacks. It does the arithmetic consistently, works out the buildable envelope left by the setbacks, and draws a schematic envelope diagram you can export.

How to use the floor area ratio & site coverage

1. Choose Imperial or Metric. Areas switch between ft² and m², and your choice is remembered across the free tools.
2. Set Site input to Width × depth with setbacks and enter the frontage, depth and the front, side and rear setbacks, or choose Area only and enter the lot area directly.
3. Add one row per level with its gross floor area as counted toward FAR under your code; the FAR uses the sum. Enter the building footprint separately.
4. Enter the maximum FAR and the maximum lot coverage (%) for your zoning district. These are your values; the tool does not supply them, and the building-codes corpus is the place to find adopted code references.
5. Read the FAR, the site coverage, the buildable envelope area and dimensions, the footprint remaining within the envelope, and the floor area and footprint remaining to your limits. A negative remaining figure means the scheme is over the limit you entered.
6. Download a PDF, a CSV or the SVG envelope diagram, or copy the result. Save inputs stores a small file you can reopen later to compare options.

Worked example

Imperial: A 60 × 100 ft site with two 2,400 ft² levels (4,800 ft² gross) and a 2,400 ft² footprint has FAR 0.8 and 40% coverage. Setbacks of 25 ft front, 5 ft each side and 15 ft rear leave a 50 × 60 ft, 3,000 ft² buildable envelope.

Metric: A 600 m² site with 480 m² gross floor area and a 240 m² footprint has FAR 0.8 and 40% coverage.

What is floor area ratio (FAR)?

Floor area ratio is gross floor area divided by lot area. A 2,400 ft² house on a 6,000 ft² lot has a FAR of 0.4. A 60,000 ft² office building on a 20,000 ft² lot has a FAR of 3.0. The ratio has no units, so it works the same in square feet and square metres.

The same measure goes by other names. Plot ratio is common in the UK, Hong Kong and Singapore, and floor space ratio (FSR) in New South Wales, Australia. Some codes write FAR as a percentage, so a FAR of 0.8 may appear as 80%.

What counts as floor area is defined locally. Ordinances differ on basements, attics, parking, mechanical space and stairs, and on whether area is measured to the outside face of exterior walls. Measure gross floor area the way your code defines it before comparing it with a limit. For a longer explanation, see floor area ratio (FAR) explained.

FAR vs lot coverage: same floor area, different buildings

Because FAR measures total floor area and coverage measures only the footprint, one FAR can produce very different buildings. The table holds FAR at 1.0 on a 10,000 ft² lot and changes only the number of stories, assuming every floor has the same footprint.

This is why coverage, height limits or setbacks often bind before FAR does. On a 6,000 ft² lot with a FAR limit of 1.2 (7,200 ft²) and a coverage limit of 40% (a 2,400 ft² footprint), reaching the full FAR takes three full stories. If the height limit allows only two, the practical maximum is 4,800 ft², a FAR of 0.8.

Worked FAR and lot coverage scenarios

Each row was calculated with the same formulas the tool uses. Average stories is gross floor area divided by footprint, a quick check that the numbers describe a plausible building.

Take the ADU row, with user-entered limits of 0.6 FAR and 40% coverage. The existing 1,800 ft² house has a 1,200 ft² footprint. Adding a single-story 800 ft² accessory dwelling unit brings the totals to 2,600 ft² of floor area and a 2,000 ft² footprint. FAR becomes 0.3467 and coverage 26.7%. The tool reports 1,900 ft² of floor area remaining (4,500 − 2,600) and 1,000 ft² of footprint remaining (3,000 − 2,000). Whether an ADU counts toward FAR or coverage at all is a local rule, so check before you enter it.

How to measure site area and gross floor area for FAR

The ratio is only as good as its inputs. Before you enter numbers, confirm these definitions in your zoning code. To total floor areas from rectangular zones, use the floor and wall area calculator; to build up a brief room by room, use the room schedule and area program.

• Lot area: usually the area within the property lines, from a survey or plat. Some codes deduct land within a public right-of-way, easements or steep slopes.
• Gross floor area: which floors count (basements, attics, mezzanines), where area is measured (outside face of walls or inside), and whether parking, mechanical rooms or covered porches are excluded.
• Footprint or lot coverage: whether it includes decks, porches, eaves and accessory buildings. Some codes regulate impervious surface separately from building coverage.
• Bonuses and exceptions: some districts allow extra FAR for affordable housing, public space or other incentives. Enter the limit that actually applies to your scheme.

Who it is for and when to use it

Method, formulas & assumptions

FAR = sum of level gross floor areas ÷ site area. Site coverage = building footprint ÷ site area × 100. With setbacks modelled, site area = width × depth and the buildable envelope = (width − 2 × side setback) × (depth − front − rear). Remaining areas use the limits you enter.

Imperial inputs are converted with exact factors (1 ft = 0.3048 m, 1 lb = 0.45359237 kg, 1 US gal = 3.785411784 L) before the calculation runs in SI, then results are converted back.

Common mistakes to avoid

• Using net or usable area instead of gross floor area. FAR is almost always based on gross area as the local code defines it.
• Entering the footprint as the floor area of a multi-story building. Floor area adds up every floor; the footprint is the ground-level outline only.
• Assuming the full FAR is achievable. Height limits, setbacks, coverage and parking can cap floor area well below the FAR maximum.
• Mixing units, such as a lot area in acres with floor areas in square feet. One acre is 43,560 ft²; convert before you enter it.
• Ignoring exclusions and bonuses. Basement exemptions or density bonuses can change both the floor area that counts and the limit itself.
• Treating the result as a zoning approval. The tool only does arithmetic on the values you enter.

Scope & limits

Supply locally defined gross floor areas per level, site dimensions, footprint, setbacks and limits. This tool does not look up zoning or determine planning compliance; height limits, easements and encroachments are not modelled. Harth's building-codes corpus at /resources/building-codes/ collects adopted code references.

Frequently asked questions

Further reading

• Floor area ratio (FAR) explained
• Understanding zoning districts
• Residential setback requirements
• How to read a zoning map
• All site & solar tools
• All free architecture & construction tools
• Harth resource library

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