Floor Space Index (FSI) is the ratio between the total built-up area and plot area available allowed by the government for a particular locality. Floor area ratio (FAR) also known as floor area index (FSI) is a ratio of building's total floor area to the size of the land upon which the building. FSI meaning is simple as it determines the ratio between total built-up area and plot area available allowed by the government for a particular area or locality. The difference between FAR and FSI is that the first is a ratio, while the latter is an index. What is FAR/FSI in India and why is it amended time-to-time? Floor Space Index ( FSI) or Floor Area Ratio (FAR) are significant words used in. Difference between FAI and FAR.