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%

    Frequently Asked Questions

    What areas count toward FAR?

    Generally, all enclosed habitable floor area counts, including each story of a multi-story building. Garages, basements, mechanical rooms, and parking structures may be partially or fully excluded depending on the local zoning code. Always check the specific definition in your jurisdiction.

    Can I increase the allowable FAR?

    Yes, through several mechanisms: rezoning to a higher-density zone, FAR bonuses for including affordable housing or public amenities, Planned Unit Development (PUD) approvals, or Transfer of Development Rights (TDR) programs. Each approach has different requirements and timelines.

    How does FAR affect property value?

    Unused FAR (the gap between current FAR and maximum FAR) represents development potential and adds value. Properties with significant unused FAR in desirable locations command premium prices because buyers can add floor area. This is sometimes called the "air rights" value.

    Ready to run the numbers?

    Open Floor Area Ratio (FAR) Calculator