Floor Area Ratio Calculator
Calculate buildable floor area from plot size and the permitted ratio, then see the footprint and number of storeys the site coverage limit allows.
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Two rules usually govern how much can be built on a plot. The floor area ratio caps total floor space as a multiple of the plot area, and site coverage caps how much of the plot the building may occupy at ground level. Together they determine the shape as well as the size of what is possible.
Names differ — floor space index, plot ratio, floor area ratio — but the arithmetic is the same everywhere. What changes is which areas count toward it, which is set locally and worth confirming before designing anything.
The formulas
Buildable floor area = plot area × floor area ratio. Maximum ground footprint = plot area × site coverage. Storeys needed = buildable area ÷ footprint, which tells you whether the two rules are compatible with the height limit.
Worked example
A 10,000 square foot plot with a ratio of 1.5 permits 15,000 square feet of floor space. With site coverage capped at 40%, the footprint is 4,000 square feet, so the scheme needs about 3.8 storeys — in practice four, with the top one smaller. At 80% efficiency, about 12,000 square feet is usable.
Check what counts toward the ratio
Basements, parking, balconies, plant rooms and circulation are included in some jurisdictions and excluded in others, and the difference can be a whole floor. Confirm the local definition before assuming a site works.
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Create your free property pageFrequently asked questions
What is the difference between floor area ratio and site coverage?
Floor area ratio limits total floor space across all storeys; site coverage limits how much of the plot the building occupies at ground level. One controls volume, the other controls footprint, and both apply at once.
Does the basement count?
It depends entirely on local rules. Many jurisdictions exclude basement parking and plant from the ratio, which is why underground parking is so common where land is expensive. Confirm the local definition before designing.
What is a realistic efficiency ratio?
Usable area is typically 75 to 85 percent of gross floor area once cores, corridors, walls and plant are deducted. Tall buildings and heavily serviced ones sit at the lower end because cores and risers take more space.
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