Floor Area Ratio (FAR) Calculator Formula

Understand the math behind the floor area ratio (far) calculator. Each variable explained with a worked example.

Use the Floor Area Ratio (FAR) Calculator

Formulas Used

Current Floor Area Ratio

far_value = current_far

Maximum Building Area Allowed

max_allowed_sqft = max_floor_area

Remaining Buildable Area

remaining_buildable = remaining_area > 0 ? remaining_area : 0

Average Floor Plate (footprint)

building_footprint = footprint

Implied Lot Coverage

lot_coverage_from_far = lot_area_sqft > 0 ? (footprint / lot_area_sqft) * 100 : 0

FAR Utilization

far_utilization = max_far > 0 ? (current_far / max_far) * 100 : 0

Variables

VariableDescriptionDefault
lot_area_sqftTotal Lot Area (sq ft)(sq ft)15000
building_floor_areaTotal Building Floor Area (sq ft)(sq ft)6000
num_storiesNumber of Stories2
max_farMaximum Allowed FAR0.5
current_farDerived value= lot_area_sqft > 0 ? building_floor_area / lot_area_sqft : 0calculated
max_floor_areaDerived value= lot_area_sqft * max_farcalculated
footprintDerived value= num_stories > 0 ? building_floor_area / num_stories : 0calculated
remaining_areaDerived value= max_floor_area - building_floor_areacalculated

How It Works

Floor Area Ratio (FAR)

FAR is a zoning measurement that compares the total floor area of a building to the area of the lot it sits on. It controls building bulk and is especially important in urban zoning.

Formula

FAR = Total Building Floor Area / Lot Area

For example, a 6,000 sq ft building on a 15,000 sq ft lot has a FAR of 0.40.

How FAR Controls Development

  • FAR of 0.25-0.50: Low-density residential (single-family)
  • FAR of 0.50-1.0: Medium-density residential (townhomes, duplexes)
  • FAR of 1.0-3.0: Mixed-use, multifamily, low-rise commercial
  • FAR of 3.0-10.0: Urban commercial, mid-rise to high-rise
  • FAR of 10.0+: Dense urban core (Manhattan-type development)

    • FAR vs. Lot Coverage

  • Lot coverage limits the footprint; FAR limits total floor area
  • A tall, narrow building can have a high FAR with low lot coverage
  • Both must be satisfied for zoning compliance

    • FAR Bonuses

    Many cities offer FAR bonuses for public plazas, affordable housing, LEED certification, or transit-oriented development.

    Worked Example

    A 15,000 sq ft lot with a 2-story building totaling 6,000 sq ft, and a maximum allowed FAR of 0.50.

    lot_area_sqft = 15000building_floor_area = 6000num_stories = 2max_far = 0.5
    1. 01Current FAR: 6,000 / 15,000 = 0.40
    2. 02Maximum allowed floor area: 15,000 x 0.50 = 7,500 sq ft
    3. 03Remaining buildable: 7,500 - 6,000 = 1,500 sq ft
    4. 04Average floor plate: 6,000 / 2 = 3,000 sq ft
    5. 05Implied lot coverage: 3,000 / 15,000 = 20.0%
    6. 06FAR utilization: 0.40 / 0.50 = 80.0%

    Ready to run the numbers?

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