FAR Calculator — Floor Area Ratio & GFA

Zoning codes rarely cap a building by height alone. More often they cap how much floor space a lot may carry, as a multiple of the lot’s own area. A FAR of 2.5 on a 1,200 m² site allows 3,000 m² of floors, stacked however the design likes. This FAR Calculator adds up the floors you list and tells you where you stand against that number.

Each row is a floor type with an area and a count, so ten identical office floors take one line. A tick box decides whether a row counts toward gross floor area, because most codes leave out some spaces.

Worked Example: A Mixed-Use Block at FAR 2.25

The calculator opens on a 1200 m² site with a permitted FAR of 2.5 and four rows:

• Basement parking 1,100 m² and Roof plant 80 m², both unticked, so 1,180 m² is excluded
• Ground floor retail 750 m² plus Office floor 650 m² × 3, giving 2,700 m² of gross floor area
• FAR = 2,700 ÷ 1,200 = 2.25, using 90% of the 3,000 m² allowance
• The 300 m² left is flagged as less than one more 650 m² office floor

Load Sample moves to a US house lot in square feet, then to a tower at FAR 5.7 on a site zoned for 4. Over the limit, the tool says how far to cut the floor area and how large the site would have to be instead: 1,140 m² for that tower.

Formula and What Counts as Gross Floor Area

FAR = gross floor area ÷ site area, shown to two decimals and rounded half up. The maximum floor area is the limit times the site area, and the “more floors” line divides what is left by the area of your most repeated floor. The same ratio is called plot ratio in the UK and Hong Kong, and floor space index (FSI) in India.

The m² / ft² toggle converts every area already entered, to one decimal place. Rows need an area above zero and a whole-number count; a bad row is named, as in “Row 2: count must be a whole number from 1 to 200.” Up to 50 rows are allowed.

When Not to Use It as a Zoning Approval

• Whether stairs, lift shafts, balconies or basements count differs by jurisdiction; the tick boxes follow your call, not a code
• Some codes measure to the outside of walls and others to the inside, which can move GFA by several percent
• Bonuses for affordable housing or public space, and separate height or coverage limits, are not modelled