Carpet area is usually about 70% to 80% of built-up area
Carpet area = built-up area × 0.70 to 0.80
If built-up area is 1,000 sq ft, carpet area is about 700 to 800 sq ft
Built-up area includes carpet area plus wall thickness and balconies
Carpet area excludes walls, balconies, and common areas
Use the exact ratio provided by the builder or local rules if available