Work out floor space index instantly with clear inputs, formula shown and shareable results.
Floor space index, also called floor area ratio, is total built-up area divided by plot area. It is the primary density control in most planning codes because it caps the intensity of development independently of how the building is shaped. Unused potential is the permitted area less what is proposed.
Floor space index
FSI = total built-up area / plot area
Permitted area
Permitted built-up = plot area x permitted FSI
Preliminary estimate only. Countable area definitions and permitted FSI must be confirmed against the local development control regulations.
FSI = total built-up area / plot area. Floor space index, also called floor area ratio, is total built-up area divided by plot area.
Not always. Many codes exclude basements used only for parking, service floors, stilt parking and certain balconies, so the countable area can be well below the physical built area.
This calculator takes 3 inputs: Plot area, Total built-up area proposed, Permitted floor space index. The pre-filled defaults are a realistic worked example — replace them with your own site or project figures.