Floor Area Ratio (Far) Calculator

What Is Floor Area Ratio (FAR)?

Floor Area Ratio (FAR) — also called Floor Space Ratio (FSR) or Plot Ratio in some jurisdictions — is a foundational zoning metric that expresses the total floor area of a building as a proportion of the land parcel it occupies. Urban planners, architects, real estate developers, and municipal governments use FAR to control building density, preserve neighborhood character, and ensure that roads, utilities, and public services can adequately support new construction.

The FAR Formula

The calculation follows a simple division:

FAR = Gross Floor Area (GFA) ÷ Lot Area

Both values must share the same unit of measurement — typically square feet (sq ft) in the United States or square meters (m²) internationally. The result is a dimensionless ratio. According to Investopedia's Floor Area Ratio guide, a FAR of 2.0 means the total floor area of all building levels is exactly twice the area of the land beneath it.

Understanding Each Variable

Gross Floor Area (GFA)

Gross Floor Area is the sum of the enclosed floor areas across every level of a building, measured from the exterior face of the outer walls. It includes the ground floor, all upper stories, mezzanines, and any habitable basement levels. Parking garages, open-air terraces, mechanical penthouses, and uncovered exterior walkways are typically excluded — though definitions vary by local code. For a 4-story building where each floor measures 5,000 sq ft, the GFA totals 20,000 sq ft.

Lot Area

Lot Area (also called Site Area) is the total horizontal area of the land parcel as defined by the legal property boundaries. This figure appears on the deed, survey plat, or county tax assessor records. For a rectangular lot measuring 100 ft × 200 ft, the Lot Area is 20,000 sq ft.

Worked Calculation Example

A developer plans a 5-story office building on a 15,000 sq ft downtown lot. Each floor has a gross area of 12,000 sq ft:

• Total GFA: 5 floors × 12,000 sq ft = 60,000 sq ft
• Lot Area: 15,000 sq ft
• FAR: 60,000 ÷ 15,000 = 4.0

The resulting FAR of 4.0 means the building contains four times the floor area of the land it sits on. If the local zoning code caps FAR at 3.5, this project would require a variance before permits could be issued.

Typical FAR Ranges by Zone Type

Permitted FAR values differ widely by jurisdiction and zoning classification. As documented in the Portland.gov Zoning Code Guide on Floor Area Ratio, common ranges include:

• Single-family residential: 0.3 – 0.6
• Low-density multi-family: 0.6 – 1.5
• Mixed-use urban corridor: 1.5 – 4.0
• Downtown commercial core: 4.0 – 15.0+

Midtown Manhattan historically permitted FARs exceeding 15.0 for commercial towers, while many U.S. suburban residential zones cap FAR at 0.4 or 0.5. Always verify the applicable FAR limit with the local planning or zoning department before finalizing any development program.

Using FAR to Determine Maximum Buildable Area

Developers invert the formula to find the largest building a site can legally support:

Maximum GFA = FAR Limit × Lot Area

On a 25,000 sq ft lot with a permitted FAR of 2.0, the maximum allowable floor area is 50,000 sq ft — distributable across any number of stories, subject to applicable height limits, setback requirements, and lot coverage restrictions.

FAR vs. Building Coverage Ratio

FAR is frequently confused with Building Coverage Ratio (BCR), which measures only the ground-floor footprint relative to lot area. FAR accounts for all floors, making it the more comprehensive density measure. A slender 10-story tower and a wide 2-story podium building can share the same FAR while having dramatically different BCRs and street-level impacts.

Regulatory Context

The City of San Pablo Density and FAR Calculations handbook notes that FAR limits are codified in local zoning ordinances, specific area plans, and general plans. Exceeding the permitted FAR requires a variance, conditional use permit, or density bonus — all of which add review time and cost. Confirming the applicable FAR limit before site acquisition or schematic design is standard practice in responsible real estate development.