What is this calculator?
In real estate, the same apartment can be quoted in three very different numbers: carpet area (the usable space inside your walls), built-up area (carpet plus the walls themselves), and super built-up area (built-up plus your share of common spaces like lobbies, staircases, lifts and amenities). This calculator converts a known carpet area into built-up and super built-up area so you can compare listings on a like-for-like basis.
How to use it
Enter the carpet area in square feet. Set the wall/loading factor — the percentage of the built-up area taken up by walls and ducts (commonly 8–15%). Then set the common-area loading applied by the developer to reach super built-up (commonly 20–35%). The calculator instantly shows the built-up area, the super built-up area, and how much area each step adds.
The formula explained
Because walls are inside the built-up area, you cannot simply add a percentage to carpet. Instead, carpet is a fraction of built-up, so we divide:
$$\text{Built-up} = \frac{\text{Carpet}}{1 - \text{loading\%}}$$
Super built-up then loads the common spaces on top of built-up:
$$\text{Super built-up} = \text{Built-up} \times (1 + \text{common\%})$$
Worked example
Carpet area = 600 sq ft, loading = 10%, common = 25%.
$$\text{Built-up} = 600 \div (1 - 0.10) = 600 \div 0.90 = 666.67 \text{ sq ft}$$
$$\text{Super built-up} = 666.67 \times 1.25 = 833.33 \text{ sq ft}$$
So a "833 sq ft" flat actually gives you only 600 sq ft of usable carpet area.
FAQ
Why is built-up calculated by dividing, not adding? The wall/loading factor is expressed as a share of the built-up area, not the carpet area, so the correct math is \(\text{carpet} \div (1 - \text{loading\%})\).
What is a typical loading factor? Wall thickness usually accounts for 8–15% of built-up; developer "super" loading typically adds 20–35% for shared facilities.
Which number should I trust when buying? Carpet area is the most meaningful for usability and is the legally mandated metric in many markets, including under India's RERA.
