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Calcimator

Floor Area Ratio (FAR) Calculator

Calculate maximum building floor area from lot size and FAR zoning limits, accounting for setbacks and lot coverage requirements.

About this calculator

Floor Area Ratio is the zoning constraint that ties a building's total floor space to the size of the lot it sits on — a FAR of 2.0 on a 20,000-square-foot lot allows up to 40,000 square feet of total building floor area, however that's distributed across floors. This calculator checks your proposed building against that FAR ceiling, but it also runs a second, independent check most FAR calculations skip: whether setbacks alone would let you build that much floor area in the first place. It computes the buildable rectangle left after subtracting front, rear, and side setbacks from your lot's width and depth, then compares that setback-constrained footprint against your lot-coverage percentage limit and takes whichever is smaller as the actual buildable footprint — because a generous FAR allowance is meaningless if setback requirements physically don't leave enough room to build that much floor area within your planned number of floors.

Multiplying that real footprint by your number of floors gives the planned total floor area, and dividing that back by lot area gives your actual FAR, which the calculator then flags as compliant or non-compliant against your entered limit. The potential residential units figure applies a rough 900-square-foot-per-unit average, a reasonable planning placeholder but not a substitute for your actual unit mix, and the estimated building height assumes a flat 12 feet per floor, which will run short for a building with taller ground-floor retail or mechanical space.

Inputs

sq ft
ft
ft
ft
ft
ft

Results

Max Floor Area (by FAR)

40,000 sq ft

≈ 9 basketball courts

Planned Floor Area

48,000 sq ft

≈ 11 basketball courts

Actual FAR2.4
FAR ComplianceNo — exceeds FAR limit
Building Footprint12,000 sq ft
Open Space8,000 sq ft
Open Space %40%
Potential Residential Units53
Est. Building Height48 ft
Setbacks Fit Within Coverage Cap?No — the lot-coverage cap governs (setback envelope exceeds it)
How to Use This Calculator
  1. Enter Lot Area, FAR Limit, and Number of Floors.
  2. Set Max Lot Coverage, Lot Width, and Lot Depth.
  3. Adjust Front Setback, Side Setback as needed.
  4. Review Max Floor Area (by FAR) (sq ft) and Planned Floor Area (sq ft).
  5. Use Actual FAR and FAR Compliance to inform your decision.

How the result changes with Lot Area

Lot AreaMax Floor Area (by FAR)Planned Floor Area
10,00020,000 sq ft24,000 sq ft
15,00030,000 sq ft36,000 sq ft
30,00060,000 sq ft52,800 sq ft
50,000100,000 sq ft52,800 sq ft

What each input means

Lot Area
Total lot area in square feet.
FAR Limit
Maximum Floor Area Ratio allowed by zoning. Suburban: 0.3-0.5, Urban: 1.0-5.0, Downtown: 5-15+.
Number of Floors
Planned number of above-grade stories.
Max Lot Coverage
Maximum percentage of lot that the building footprint may cover.
Lot Width
Width of the lot at the building line.
Lot Depth
Depth of the lot from front to rear property line.
Front Setback
Required distance from front property line to building.
Side Setback
Required distance from side property line to building (each side).
Rear Setback
Required distance from rear property line to building.

What each result means

Setbacks Fit Within Coverage Cap?
Compares the setback-derived buildable rectangle against the lot-coverage cap. The calculator always builds to the smaller of the two, so either answer is compliant — this names which limit is binding.

How this is calculated

Worked example, using the default values

  1. Identify Input Parameters
    4 parameters
    Lot Area = 20000, FAR Limit = 2, Number of Floors = 4, Max Lot Coverage = 60 = 9 input(s) provided
  2. Calculate Max Floor Area
    Max Floor Area
    40000 = 40000
  3. Calculate Planned Floor Area
    Planned Floor Area
    48000 = 48000
  4. Calculate Actual FAR
    Actual FAR
    2.4 = 2.4
  5. Calculate FAR Compliance
    No — exceeds FAR limit = No — exceeds FAR limit

Engine last updated . Checked against 2 independently-derived tests — how we verify calculators. Built by Paul Gunder, a software engineer, not a licensed financial, medical, or legal professional.

Frequently Asked Questions

Why would a project be non-compliant even if it's under the FAR limit?

The calculator separately checks setback and lot-coverage constraints against FAR — a building that stays comfortably under the FAR limit can still fail because setbacks leave too little buildable footprint to fit the planned floor area across the planned number of floors. FAR compliance and physical buildability are two independent checks, and both need to pass.

What happens if my setback-constrained footprint is smaller than my lot-coverage limit allows?

The calculator uses whichever constraint is more restrictive — the smaller of the setback-derived footprint and the lot-coverage-derived footprint — as the actual buildable area. In practice, tight setback requirements on an oddly shaped or narrow lot often bind before the lot-coverage percentage ever does.

How can I increase my building's floor area without exceeding FAR?

Since total floor area is footprint times number of floors, and FAR caps the total regardless of shape, the same allowable floor area can be achieved with a larger footprint and fewer floors, or a smaller footprint and more floors. Going taller on a smaller footprint typically preserves more open space and can more easily clear setback constraints on a tight lot.

Is the potential residential units estimate reliable for my project?

It applies a flat 900-square-foot average per unit, which is a reasonable planning-level placeholder but won't match a project skewed toward studios, family-sized units, or a mixed-use building with significant non-residential floor area. For an accurate unit count, apply your project's actual unit size mix to the planned floor area instead.

Why does the calculator assume 12 feet per floor for building height?

Twelve feet per floor is a common reference for typical residential or light commercial floor-to-floor height, but it will understate real building height for a project with taller ground-floor retail, mechanical penthouse space, or higher ceiling residential units. Adjust the height estimate upward if your project includes any floors taller than the 12-foot assumption.

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