Floor Area Ratio Calculator
Calculate floor area ratio by comparing total building floor area to lot size.
What Is Floor Area Ratio?
Floor area ratio (FAR) is a zoning metric that compares a building's total floor area to the size of the lot it sits on. It is calculated by dividing the sum of all floor areas by the total lot area. FAR is used by planners and developers to regulate building density, control urban form, and ensure development aligns with local zoning codes.
A higher FAR indicates more built space relative to the lot, while a lower FAR means less density. Understanding FAR helps you evaluate whether a proposed building complies with local regulations before committing to a design or purchase.
How to Calculate Floor Area Ratio
The formula is straightforward:
FAR = Total Building Floor Area ÷ Lot Area
Total building floor area includes all habitable and usable floors, measured from the exterior walls. It typically excludes basements, parking garages, mechanical rooms, and unenclosed spaces like balconies or patios, though definitions vary by jurisdiction. Lot area is the total land area of the parcel, usually expressed in square feet or square meters.
To use this calculator, enter your total building floor area and lot area in the same unit. The tool returns the FAR as a decimal number, which you can compare against your local zoning ordinance.
Example Calculation
Suppose you have a lot measuring 5,000 square feet. You plan to build a three-story building with 1,500 square feet per floor. The total floor area is 4,500 square feet. Divide 4,500 by 5,000 to get an FAR of 0.9. If your local zoning allows a maximum FAR of 1.0, your design complies. If the limit is 0.75, you would need to reduce floor area or increase lot size.
Understanding Your FAR Result
The FAR value itself is a ratio, not a measure of height or number of stories. Two buildings with the same FAR can look very different depending on lot size and floor configuration. For example, an FAR of 1.0 could mean a single-story building covering the entire lot, or a two-story building covering half the lot.
Local zoning codes often specify maximum FAR values for different zones. Exceeding the allowed FAR may require a variance, rezoning, or redesign. Always verify your result against the specific regulations for your property's zoning district.
Common Mistakes When Using FAR
- Including excluded areas: Basements, attics, parking structures, and mechanical spaces are often excluded from floor area calculations. Including them inflates your FAR and may lead to incorrect compliance checks.
- Mixing units: Ensure both floor area and lot area use the same unit. Mixing square feet with square meters produces an incorrect ratio.
- Confusing FAR with coverage: Floor area ratio is not the same as lot coverage. Lot coverage measures the footprint of the building, while FAR accounts for all floors.
- Ignoring local definitions: Municipalities define floor area differently. Some include mezzanines, stairwells, or elevator shafts. Always check your local zoning ordinance for the exact definition.
Practical Use Cases
- Pre-purchase feasibility: Before buying land, calculate the maximum buildable area based on the lot size and allowed FAR to determine if your project fits.
- Design validation: During schematic design, check that your proposed floor area stays within zoning limits before investing in detailed drawings.
- Property valuation: Real estate appraisers use FAR to compare development potential across similar lots, which influences land value.
- Zoning compliance review: Architects and planners use FAR calculations to verify that building plans meet municipal density requirements.
Limitations of FAR
FAR is a density control, but it does not regulate building height, setbacks, or open space. A building may comply with FAR limits while still violating height restrictions or setback requirements. Additionally, FAR does not account for floor-to-floor heights, so two buildings with the same FAR may have different total volumes. Always use FAR alongside other zoning metrics like lot coverage, height limits, and floor area bonuses to get a complete picture of development potential.
FAQ
What is a good floor area ratio?
There is no universal "good" FAR. Acceptable values depend entirely on local zoning codes and the intended use. Residential zones often allow FARs between 0.5 and 1.5, while commercial districts in dense urban areas may allow 5.0 or higher. Check your municipality's zoning ordinance for the specific maximum FAR in your zone.
Does FAR include basement and garage?
Typically no, but definitions vary. Most zoning codes exclude basements, parking garages, mechanical rooms, and unenclosed spaces from floor area calculations. However, some jurisdictions include finished basements or above-grade parking. Always verify with your local planning department.
Can FAR be greater than 1?
Yes. An FAR greater than 1 means the total floor area exceeds the lot area. This is common in multi-story buildings on small lots. For example, a four-story building on a 2,000-square-foot lot with 1,500 square feet per floor has an FAR of 3.0.
How is FAR different from lot coverage?
Lot coverage measures the percentage of the lot covered by the building's footprint. FAR measures the total floor area across all stories relative to the lot. A building can have low lot coverage but high FAR if it is tall, or high lot coverage but low FAR if it is single-story.
What happens if my FAR exceeds the limit?
Exceeding the maximum FAR typically means your building does not comply with zoning regulations. You may need to reduce floor area, apply for a variance, or seek a zoning amendment. Non-compliance can result in permit denial, fines, or required modifications after construction.