FSI is the ratio of a building's total floor area to its plot size, limiting the maximum construction on land. Know its full form , meaning and how to calculate in construction & real estate. The FSI full form is Floor Space Index, the maximum permitted floor area that a developer can build on. Read about what is FSI, FSI Meaning, FSI Calculation Formula, and more. FSI Full form is Floor Space Index or Floor Area Ratio (FAR), is a critical parameter in urban planning and zoning regulations. It determines the maximum allowable construction area on given plot of land relative to its size. This metric helps regulate the density of development in urban areas, ensuring infrastructure efficiency and maintaining environmental sustainability. FSI full form is Floor Space Index, the maximum authorized floor area that a developer can construct on. Learn more about floor space index meaning, formula, calculation, and importance.