Carpet area is the usable floor inside the flat's walls; built-up area adds the thickness of the walls and, depending on who measures, the balcony. Carpet area has a statutory definition under RERA; built-up area does not, so compare homes on carpet area.
Compare price per square foot on carpet area, check it in the agreement for sale, and measure it at handover.
When Built-up area matters
Built-up area appears in older resale deeds, some municipal tax assessments and approved plans; know what it includes before you rely on it.
In more detail
There is no fixed ratio between the two: it depends on wall thickness and on what the seller included, especially the balcony. Ask for the carpet area in writing rather than converting.
The full guide covers super built-up area, loading and a worked example of comparing two flats on the same basis.