FSI stands for Floor Space Index. It is the same as Floor Area Ratio of buildings.

Floor space index

The illustration above shows a Floor Area Ratio (FAR) of 1.0. This simply means that, if the area of the plot is 100 square meters, then 100 square meters of gross floor area has been built on the plot. The illustration above shows a 4-story building covering 1/4 of the site, giving a FAR of 1.0. Four floors of 25 square meters each are built on a site of 100 square meters.

The reference design for carfree cities is based on a FAR of 1.5. Here are some ways to get to a FAR of 1.5:

Build a 2-story building on 75% of the site (2 x 0.75 = 1.5)

Build a 3-story building on 50% of the site (3 x 0.5 = 1.5)

Build a 4-story building on 37.5% of the site (4 x 0.375 = 1.5)
It will be noted that a FAR of 1.5 is quite high, although this density is not unusual in Venice or central Paris, and is considerably exceeded in most of Manhattan. It requires 4-story buildings and narrow streets with modest interior courtyards. (Higher buildings would leave more room for streets and gardens, ...more

Floor Space Index is called FSI. It is the ratio between the extent of land to the plinth area. To calculate FSI, FSI = Plinth area / Extent of Land.

Floor space Index