# What is the floor area ratio?

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.

- Page: https://www.acalculator.org/construction/floor-area-ratio-calculator
- JSON spec: https://www.acalculator.org/construction/floor-area-ratio-calculator.json
- Version: 6e031b7e408c

## Default answer

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.

## Inputs

| Key | Label | Description |
| --- | --- | --- |
| floor | Total floor area | The floor area of every story of every building on the lot, added up, as the zoning code counts it. |
| lot | Lot area | The area of the lot (the zoning lot). |
| far | Floor area ratio (FAR) | The total floor area divided by the lot area. |

## Outputs

| Key | Label | Description |
| --- | --- | --- |
| floorArea | Total floor area | The floor area of every story of every building on the lot, added up, as the zoning code counts it. |
| lotArea | Lot area | The area of the lot (the zoning lot). |
| far | Floor area ratio | The total floor area divided by the lot area. |

## Method

FAR = total floor area ÷ lot area; floor area = FAR × lot area; lot area = floor area ÷ FAR.

## Assumptions

- 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

1. lot = 929.0304, floor = 1,858.0608 gives 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).
2. lot = 1,858.0608, floor = 3,716.1216 gives 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).
3. lot = 464.5152, far = 0.5 gives floor = 232.2576. 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).
4. floor = 3,000, far = 1.5 gives lot = 2,000.

## FAQ

### 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.

## Sources

- NYC Zoning Resolution, Section 12-10 Definitions: "floor area ratio" (total floor area on a zoning lot divided by the lot area, with worked examples), "floor area" (and what it leaves out) and "lot coverage". https://zr.planning.nyc.gov/article-i/chapter-2/12-10 (retrieved 2026-10-18)
- NIST SP 811 (2008), Appendix B: 1 ft = 0.3048 m exactly, so 1 ft² = 0.09290304 m². https://www.nist.gov/pml/special-publication-811 (retrieved 2026-10-18)
- NIST, U.S. Survey Foot: Revised Unit Conversion Factors: 1 acre = 43,560 ft² = 4,046.856 422 4 m² with the international foot; units based on the U.S. survey foot are to be avoided after December 31, 2022. https://www.nist.gov/pml/us-surveyfoot/revised-unit-conversion-factors (retrieved 2026-10-18)