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Floor Space Index Calculator

Work out floor space index instantly with clear inputs, formula shown and shareable results.

About Floor Space Index Calculator

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.

Formulas

Floor space index

FSI = total built-up area / plot area

Permitted area

Permitted built-up = plot area x permitted FSI

Worked example

With the default values below, Floor Space Index Calculator returns Floor space index achieved of 2.5000. Change any input above and every figure here is recalculated in your browser.

Inputs used

Plot areaminimum 0.1 m²
500 m²
Total built-up area proposedminimum 0 m²
1,250 m²
Permitted floor space indexminimum 0.1
2.5

Results

Floor space index achieved
2.5000
Built-up area permitted
1,250.0 m²
Unused development potential
0.0 m²
Utilisation of permitted FSI
100.0% %

Step by step

  1. 1Values used
  2. 2Floor space index
  3. 3Permitted area
  4. 4Floor space index achieved= 2.5000
  5. 5Built-up area permitted= 1,250.0 m²
  6. 6Unused development potential= 0.0 m²
  7. 7Utilisation of permitted FSI= 100.0 %

Assumptions and limits

  • The result is derived only from the 3 inputs listed above. Anything not among them is not part of the calculation.
  • In particular, this calculator does not take inflation, tax, fees or charges or compounding frequency into account.
  • Figures are rounded for display to 4 decimal places, so totals built from several rounded parts can differ slightly from the unrounded arithmetic.
  • Preliminary estimate only. Countable area definitions and permitted FSI must be confirmed against the local development control regulations.

How these calculators are built and tested is described on the Methodology page. If a figure looks wrong, please report it.

Preliminary estimate only. Countable area definitions and permitted FSI must be confirmed against the local development control regulations.

Frequently Asked Questions

How is the Floor Space Index figure worked out?

FSI = total built-up area / plot area. Floor space index, also called floor area ratio, is total built-up area divided by plot area.

Is every floor counted?

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.

What values do I need to enter?

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.

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