Floor Area Ratio Calculator: Formula and Example

A floor area ratio calculator divides a building’s total counted floor area by the area of its site. Enter both areas in the same units to get the floor area ratio (FAR), a dimensionless value that helps describe how much building floor space sits on a parcel.

Formula

FAR = total counted floor area ÷ site area

• Total counted floor area (A) is the sum of the building areas included under the applicable measurement rules, in square feet, square metres, or another area unit.
• Site area (S) is the land area used as the denominator, measured in the same area unit as A.
• Floor area ratio (FAR) has no unit. The percentage output is FAR multiplied by 100.

For example, an FAR of 2.00 means counted floor area is twice the site area. It does not mean the building covers twice the site at ground level: floor area may be distributed over several storeys.

Worked Example

Using the calculator’s default inputs:

1. Total counted floor area: A = 20,000 sq ft.
2. Site area: S = 10,000 sq ft.
3. FAR = A ÷ S = 20,000 ÷ 10,000 = 2.00.
4. As a percentage of site area: 100 × 20,000 ÷ 10,000 = 200.0%.

The result means the counted floor area is twice the lot area. One possible arrangement would be two floors of 10,000 sq ft each, but that is only an illustration; FAR alone does not specify a building’s shape, layout, or number of floors.

What Floor Area Ratio Measures

Floor area ratio, sometimes called floor space index or floor space ratio in particular places, compares a building’s floor area with the land parcel beneath or associated with it. It is a planning and development measure, not a direct measure of building height, density of residents, or ground coverage.

Consider two sites, each 10,000 sq ft. A building with 20,000 sq ft of counted floor area on either site has an FAR of 2.00. One might be a two-storey building covering much of the lot; another might have more floors and a smaller footprint. Setbacks, open space, height limits, lot shape, and other rules can affect what can actually be built, even when the FAR is the same.

The term is used in planning and zoning, but its precise definition is not universal. A local code may define the numerator, the denominator, or both in a specific way. The calculator performs the arithmetic; it does not determine how a particular authority classifies a project.

How to Choose the Areas

The most important step is deciding what the two area figures are supposed to represent. Use the definitions required for your project rather than assuming that every floor and every piece of land is counted in the same way everywhere.

• Find the site area: use the parcel or site boundary specified for the calculation. Check whether the relevant instructions include or exclude access strips, shared land, easements, or land dedicated for public use.
• Find the counted floor area: add the areas of the floors that the applicable method includes. Plans, surveys, or a code definition may specify whether measurement is to the inside face, outside face, or another boundary.
• Check special spaces: basements, parking, balconies, mechanical rooms, atriums, covered areas, and voids may receive special treatment. Do not assume they are automatically included or exempt.
• Keep units consistent: square feet divided by square feet works, as does square metres divided by square metres. Convert one figure before dividing if the units differ.

For conversion, 1 square metre is approximately 10.7639 square feet. Convert area to area: a length conversion factor must be squared when converting square units. For instance, square metres cannot be converted to square feet by multiplying by the metres-to-feet factor just once.

How to Read and Use the Result

FAR is a ratio, so a result such as 1.25 is read as 1.25 units of counted floor area per unit of site area. The percentage output expresses the same comparison in another form: 1.25 is 125% of the site area. Neither figure means that 125% of the ground is covered.

To estimate a floor-area total from a known site area and a stated target ratio, rearrange the formula: total floor area = FAR × site area. For a 12,000 sq ft site and an illustrative FAR of 1.5, that multiplication gives 18,000 sq ft of floor area under the assumed definition. Whether that amount is permitted or physically feasible depends on the relevant rules and site conditions.

Use enough precision for the task. Rounding an FAR to two decimal places is often convenient for an informal comparison, but a project review may require greater precision or prescribed rounding. Keep the unrounded inputs during calculations and round only the displayed result when possible.

Common Mistakes and Limits

• Mixing units: dividing 20,000 sq ft by 1,000 sq m produces a meaningless result unless one area is converted first.
• Using lot coverage instead of FAR: lot coverage compares a building footprint with site area. FAR compares total counted floor area with site area.
• Assuming every floor counts: exclusions and measurement rules depend on the applicable definition. A generic calculator cannot decide them.
• Treating a ratio as a permission: an FAR calculation does not establish allowable development. Local requirements may impose other controls, and definitions can vary.
• Rounding inputs too early: rounded site or floor areas can change the final ratio, especially for close comparisons.

This calculator assumes that the two entered values already match the relevant definition and use the same units. It does not account for zoning, building codes, easements, design constraints, or exemptions. For a compliance question, check the governing planning documents or ask the relevant local authority or a qualified professional to confirm which areas count.

Frequently Asked Questions

How do I calculate floor area ratio?
Divide total counted floor area by site area. If both are measured in square feet, for example, divide square feet by square feet; the result is a unitless ratio.

What does an FAR of 2 mean?
It means the counted floor area is two times the site area. It does not, by itself, say how many storeys the building has or how much of the ground it covers.

Is floor area ratio the same as lot coverage?
No. Lot coverage generally compares the building footprint with the site area, while FAR compares counted floor area across the building with the site area.

Do basements count in floor area ratio?
That depends on the measurement rules that apply to the site. Some definitions treat particular basement areas differently, so check the relevant local documents rather than relying on a universal rule.

Can I calculate FAR in square metres?
Yes. Divide floor area in square metres by site area in square metres. The result is the same kind of unitless ratio as a calculation using square feet.

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in a | Total counted floor area | sq ft | 20000 | 0
in s | Site area | sq ft | 10000 | 0
out far | Floor area ratio | - | a / s | 2
out pct | Floor area as percent of site | % | 100 * a / s | 1