Carpet Area is essentially the area within the walls of your home that you can actually "carpet" or furnish.
Formula
$$
\text{Carpet Area} = \text{Total Built-Up Area} \times 0.70
$$
Example
If your total built-up area is 1500 ft²:
$$
\text{Carpet Area} = 1500 \times 0.70 = 1050 \text{ ft}^{2}
$$