He Floor Area Ratio (FAR) or Floor Space Index (FSI) is the ratio of the total floor area of buildings on a certain location to the size of the land of that location, or the limit imposed on such a ratio.
The Floor Area Ratio is the total building square footage (building area) divided by the site size square footage (site area).
As a formula: Floor Area Ratio = (Total covered area on all floors of all buildings on a certain plot)/(Area of the plot)
Thus, an FSI of 2.0 would indicate that the total floor area of a building is two times the gross area of the plot on which it is constructed, as would be found in a multiple-story building
Your vote on this answer has already been received
There is no formula as such.FSI depends upon the area of land to get constructed.If land is 10000sq.ft. & allowed FSI is 1 then u construct max.10000 sq.ft only,& if FSI is 0.5 then only 5000sq.ft. & so on.