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Floor Area Ratio Calculator

Enter the site area and total floor area to instantly get the floor area ratio (%). You can also work backward from a permitted ratio to find how much floor area you can build, plus an explanation of road-width limits.

Input

Calculation direction

m²
m²

Result

Floor area ratio

150%

Site area

120 m²

Total floor area

180 m²

Floor area ratio

150 %

Site area120 m²
Total floor area180 m²
Floor area ratio150 %
FormulaFloor ÷ Site × 100

* The floor area you can actually build is limited not only by the permitted floor area ratio for the zoning district but also by the width of the front road. When the front road is less than 12 m wide, the applicable cap is the smaller of the permitted ratio and the value derived from the road width: "width (m) × 0.4" for residential zones and "width (m) × 0.6" for other zones (multiplied by 100 to convert to %). If the site faces more than one road, use the width of the widest one.

How it works

  • In "Areas to ratio" mode, the total floor area (the sum of every floor's area) is divided by the site area and multiplied by 100 to get the floor area ratio (%). The formula is FAR = total floor area ÷ site area × 100.
  • In "Ratio to floor area" mode, the site area is multiplied by the permitted floor area ratio (%) and divided by 100 to estimate the maximum floor area you can build on that site.
  • The permitted floor area ratio is the upper limit set by city planning for each zoning district. Check your local government's zoning information to find the value for your land.
  • Total floor area is the sum of the floor area of every story. Lofts, basements meeting certain conditions, parking areas, and similar spaces may be partially excluded from the FAR calculation under relief provisions.
  • Beyond the zoning district's permitted ratio, when the front road is less than 12 m wide an additional road-width limit applies, and the smaller of the two becomes the actual cap.
  • The displayed figures are rough estimates. For an official determination, including building coverage ratio and height limits, consult an architect or your local building guidance office.