Total of Floor space of each flat in an apartment divided by Total plot size. E.g. If there are 48 flats each with a carpet area of 400 sq. ft in a 6 storey apartment then total floor space is 48*400 = 19,200. If the plot area is also 19,200 sq. ft. then the FSI is 1. In Mumbai Island city (Colaba to Mahim), the FSI allowed is 1.33 while for Mumbai suburbs (Bandra to Dahisar & Kurla to Mulund) the FSI allowed is 1.