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Building Floor & Land Area Ratios Calculator

Calculate floor-area, open-space and usable-area ratios from entered areas.

  • Formula & worked example
  • Private in your browser
  • No signup
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Calculator inputs

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How to use this calculator

  1. Enter the known values in the units shown. Results update as you type.
  2. Edit a calculated value to solve backwards, or lock a value to hold it fixed.
  3. Use the worked example to check the method. Reset restores the starting fields.

Private by default

Inputs and results stay in this browser tab. Bookify does not upload or store the values you enter.

Formula and method

Ten identical units with 80 m² of carpet area and 20 m² of wall area each have 1,000 m² of total built-up area. On a 2,000 m² plot, FAR is 0.5. With 500 m² of open space, the open-space ratio is 0.25.

FAR = total built-up floor area ÷ land area; open-space ratio = open-space area ÷ land area; carpet ratio = carpet area ÷ built-up unit area

Worked example

Enter these known values and leave the other values blank.

Carpet area
80 m²
Walls area
20 m²
Number of units
10
Common area
200 m²
Open space area
500 m²
Total land area
2000 m²
Built-up area
100 m²
Carpet to built-up area ratio
80 %
Super built-up area
120 m²
Total built-up area
1000 m²
Indoor area
100 m²
Loading factor (LF)
0.5
Open space ratio (OSR)
0.25
Floor area ratio (FAR) or floor space index (FSI)
0.5

Assumptions and limitations

  • It computes ratios from your areas. It does not determine zoning permissions or decide which spaces count under a local definition. Units are modeled as identical, and shared floor area is allocated equally when calculating the super built-up area.
  • Enter dimensions, densities and prices that describe the actual project. Quantities absent from the formula are not added automatically.

Common questions

Does this determine the permitted floor area for a property?

It computes ratios from your areas. It does not determine zoning permissions or decide which spaces count under a local definition. Units are modeled as identical, and shared floor area is allocated equally when calculating the super built-up area.

References

The calculation equations, inverse formulas, units, and input rules were imported from this source. Bookify provides the interface and equation solver.

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