In the series of Real-Estate Jargon , This is second blog, you might interested in first blog of the series abut definition of different kind of area  (Definition Of Carpet Area , Built Up Area , Super Built Up Area)

Floor area ratio is very commonly known as floor space index (FSI) also in India. In laymen language it is indicator that shows how many numbers of floor can be build in certain area, it is the ratio of the total floor area of buildings to the size of the land of that location.

Municipal corporation or urban development centra set the norms for FSI and builders/constructor has to follow them strictly. so anyone can not construct building which is very tall and does not provide enough space for surrounding buildings.

In nutshell FSI restricts that taller buildings should have better space at ground so it does not impact surroundings.

As a formula: Floor Area Ratio = (Total covered area on all floors of all buildings on a certain plot)/(Area of the plot).

Let’s take an example, if FSI is 3 for any city, and plot area is 3000 sq. meter,In this case builder can construct

• 5  floor of  1800 sq meter (60% of 3000)  covered area in each floor  = 5*1800 = 9,000 /3 = 3,000
• 10 floor of 900 sq meter (30% of 3000) covered area in each floor = 10*900 = 9,000/3 = 3,000
• 20 floor of 450 sq meter (15 % of 3000) covered area in each floor = 20*450 = 9,000/3 = 3,000

Normally FSI varies according to city and area, In vadodara it is between 2 -3 .

Reference:

http://www.carfree.com/far.html

http://en.wikipedia.org/wiki/Floor_Area_Ratio