What is the floor area ratio?
Type any two of the total floor area, the lot area and the floor area ratio. The floor area ratio calculator works out the third.
- Floor area ratio
- 2
A floor area of 20,000 ft² on a lot of 10,000 ft² is a floor area ratio of 2.
Floor area ratio: 2. A floor area of 20,000 ft² on a lot of 10,000 ft² is a floor area ratio of 2.
How to calculate
Computes the floor area ratio (FAR) of a building from its total floor area and the lot area, or the floor area a FAR allows on a lot.
Example with the default inputs (Total floor area 20,000 ft², Lot area 10,000 ft²): A floor area of 20,000 ft² on a lot of 10,000 ft² is a floor area ratio of 2.
Formula: FAR = total floor area ÷ lot area; floor area = FAR × lot area; lot area = floor area ÷ FAR.
- The floor area is the total the zoning code counts: many codes leave out cellars, mechanical space and some parking.
- Both areas are in the same unit; the ratio has no unit.
Worked examples
Each example is checked against the calculator on every build.
- Lot area 10,000 ft², Total floor area 20,000 ft² gives Floor area ratio (FAR) 2.Source: NYC Zoning Resolution §12-10, definition of floor area ratio (a 10,000 ft² lot with 20,000 ft² of floor area has a FAR of 2.0; a 20,000 ft² lot with 40,000 ft² of floor area also has 2.0), https://zr.planning.nyc.gov/article-i/chapter-2/12-10 (retrieved 2026-10-18)
- Lot area 20,000 ft², Total floor area 40,000 ft² gives Floor area ratio (FAR) 2.Source: NYC Zoning Resolution §12-10, definition of floor area ratio (a 10,000 ft² lot with 20,000 ft² of floor area has a FAR of 2.0; a 20,000 ft² lot with 40,000 ft² of floor area also has 2.0), https://zr.planning.nyc.gov/article-i/chapter-2/12-10 (retrieved 2026-10-18)
- Lot area 5,000 ft², Floor area ratio (FAR) 0.5 gives Total floor area 2,500 ft².Source: NYC Zoning Resolution §12-10, definition of floor area ratio (a 10,000 ft² lot with 20,000 ft² of floor area has a FAR of 2.0; a 20,000 ft² lot with 40,000 ft² of floor area also has 2.0), https://zr.planning.nyc.gov/article-i/chapter-2/12-10 (retrieved 2026-10-18)
- Total floor area 32,292 ft², Floor area ratio (FAR) 1.5 gives Lot area 21,528 ft².
How it works
The floor area ratio (FAR) compares the floor area of the buildings on a lot with the area of the lot:
- FAR = total floor area ÷ lot area
- Floor area = FAR × lot area
- Lot area = floor area ÷ FAR
Type any two of the three; the page works out the third. If two or more buildings stand on the same lot, their floor areas are added before dividing. Both areas can be typed in square feet, square meters or acres; they are converted to the same unit (1 ft² = 0.09290304 m² exactly, 1 acre = 43,560 ft² = 4,046.8564224 m², NIST's current acre) before dividing, so the ratio has no unit.
Rounding. The FAR shows to 3 significant digits (2.35, 12.5, 0.0007), rounded half up, trailing zeros left off. Areas show in the unit of their box.
Assumptions
- The total floor area is the area your zoning code counts. Codes differ on cellars, attics, mechanical space and parking.
- Every value is more than 0. Areas are at most 1,000,000,000 in their base unit (square meters), and the FAR is at most 1,000.
Worked examples by hand
A 10,000 ft² lot with a 20,000 ft² building. 20,000 ÷ 10,000 = 2.0 (the NYC Zoning Resolution's own example).
A 20,000 ft² lot with two buildings, 40,000 ft² of floor area in all. 40,000 ÷ 20,000 = 2.0.
How much floor area a FAR of 0.5 allows on a 5,000 ft² lot. 0.5 × 5,000 = 2,500 ft².
How big a lot a 3,000 m² building needs at a FAR of 1.5. 3,000 ÷ 1.5 = 2,000 m².
Other questions people ask
How do I calculate the floor area ratio?
Add up the floor area of every story of every building on the lot, then divide by the lot area. A 10,000 ft² lot with a 20,000 ft² building has a FAR of 20,000 ÷ 10,000 = 2.0.
How much can I build with a given FAR?
Multiply the FAR by the lot area. A FAR of 0.5 on a 5,000 ft² lot allows 2,500 ft² of floor area, which could be one story of 2,500 ft² or two stories of 1,250 ft² each.
Does a FAR of 2 mean a two-story building?
Not necessarily. A FAR of 2 allows floor area equal to twice the lot area. That could be two stories covering the whole lot, four stories covering half of it, or eight stories covering a quarter of it.
What counts as floor area?
It depends on the zoning code. New York City, for example, leaves out most cellar space, attic space under 8 feet of headroom, space for mechanical equipment, open balconies within limits, and some accessory parking. Check your local code for what counts before you add up the stories.
Is FAR the same as lot coverage?
No. Lot coverage is the share of the lot a building covers when seen from above. FAR counts the floor area of every story, so a tall building on a small footprint can have low lot coverage and a high FAR.
What is a floor space index (FSI)?
Floor space index (FSI) is another name for the floor area ratio. It is worked out the same way: total floor area divided by the plot area.