Last updated: February 16, 2026
One Ratio Decides How Much Building Your Land Can Hold
A developer buys a 10,000 sq ft corner lot, hires an architect, and walks into the planning counter with drawings for a five-story mixed-use building. The planner checks thefloor area ratio and says “your FAR is 2.0, so total gross floor area caps at 20,000 sq ft—your drawings show 28,000.” Eight thousand square feet of leasable space vanish in one sentence. FAR (called FSI in parts of South Asia) is the multiplier that converts land area into maximum buildable floor area, and getting it wrong means your pro forma and your loan application are based on a building that can never be approved.
This calculator takes your lot size, the permitted FAR or FSI, and optional constraints like setbacks and height caps, then returns gross floor area, implied floor count, and remaining capacity. It does not look up your zoning district—you enter the correct ratio from your local code.
FAR and FSI in Plain Language
Floor Area Ratio (FAR) divides total built-up floor area by lot area. A FAR of 2.0 on a 10,000 sq ft lot means you can build up to 20,000 sq ft of gross floor area across every storey combined. Floor Space Index (FSI) is the same metric under a different name, common in India and parts of Southeast Asia. The formula is identical:
FAR = Total Floor Area ÷ Lot Area
Rearranged: Max Floor Area = FAR × Lot Area
FAR is dimensionless—both areas must be in the same unit. A FAR of 1.0 means the total floor space equals the lot. A FAR below 1.0 restricts you to less floor space than the lot itself, typical for low-density suburban zones. A FAR of 5.0 or higher appears in dense urban centres and lets you stack five or more lot-equivalents of floor space—provided height, coverage, and parking rules also cooperate.
What Counts Toward Gross Floor Area—and What Doesn’t
The biggest source of FAR miscalculation is the definition of gross floor area (GFA). Not every square foot inside a building necessarily counts. Jurisdictions diverge on several categories:
| Element | Often included | Often excluded |
|---|---|---|
| Residential & commercial floors | Almost always | — |
| Below-grade parking | Some codes | Many US & Canadian codes |
| Above-grade structured parking | Most codes | Select transit-oriented zones |
| Mechanical rooms & elevator shafts | Most codes | NYC, some Indian DCRs |
| Balconies & terraces | If enclosed | If open or < 50 % enclosed |
| Basements used for habitation | Usually | Only if below a depth threshold |
If your code exempts parking from GFA, a four-level garage adds zero to your FAR tally—a huge advantage that can flip a project from infeasible to profitable. Always read your local zoning ordinance’s GFA definition before entering numbers into any calculator.
10,000 Square Feet, FAR 2.0, Four-Story Cap: The Full Arithmetic
Site: 10,000 sq ft lot. Zoning allows FAR 2.0, four stories max, 60 % ground coverage, 10 ft floor-to-floor height.
- Max GFA from FAR: 2.0 × 10,000 = 20,000 sq ft
- Max footprint from coverage: 10,000 × 0.60 = 6,000 sq ft
- GFA at full coverage × 4 floors: 6,000 × 4 = 24,000 sq ft
- Binding constraint: FAR caps you at 20,000, not the 24,000 the coverage and height would allow. You are FAR-limited.
- Floor plate per level: 20,000 ÷ 4 = 5,000 sq ft (50 % coverage, below the 60 % cap)
With a 5,000 sq ft floor plate, each storey is smaller than the maximum footprint, leaving 5,000 sq ft of open space at grade—room for a courtyard, landscaping, or outdoor dining if the ground floor is retail. If the coverage limit were 40 % instead of 60 %, max footprint drops to 4,000 sq ft, and four floors yield only 16,000 sq ft—now coverage binds before FAR and you leave 4,000 sq ft of entitlement on the table.
Five Traps That Shrink Your Buildable Area
- Parking garages that count after all. You assumed parking was exempt, but your code only exempts below-grade parking. Two above-grade levels at 3,500 sq ft each eat 7,000 sq ft of your FAR allowance—more than a full floor of leasable space gone.
- Balcony rules that flip mid-project. Open balconies are typically excluded from GFA. Enclose them with glass during design development and they become countable floor area, pushing you over the FAR cap by the area of every balcony on every floor.
- The podium-tower split nobody models. A three-storey retail podium at 80 % coverage contributes 24,000 sq ft of GFA on a 10,000 sq ft lot. If total FAR is 4.0 (40,000 sq ft), only 16,000 remains for the tower. At a 30 % tower footprint (3,000 sq ft), that is roughly five residential floors—not the twelve the developer sketched on a napkin.
- Basement area that suddenly counts. A basement used for storage or mechanical equipment may be exempt. Convert part of it to a commercial gym or habitable workspace and some codes reclassify the entire level as countable GFA. One tenant improvement can retroactively violate your FAR.
- Bonus FAR with a hidden ceiling. Your code offers +0.5 FAR for affordable units and +0.3 for green certification, but the combined bonus cap is +0.5—not +0.8. Stacking bonuses past the cap yields zero additional floor area and a misleading pro forma.
FAR Is the Ceiling, Not the Whole Blueprint
Staying under the FAR cap does not guarantee an approvable building. Setbacks shrink the buildable envelope, height limits cap the number of floors, and lot coverage restricts footprint area at grade. A project can be well within FAR and still fail because the parking layout does not fit or the open-space requirement eats into a floor of revenue.
Use FAR as the first capacity check: multiply the lot by the ratio, compare to your program, and see whether the building you need is even theoretically possible. If FAR passes, move on to density (units per acre) if you are doing residential, then to coverage and height. The plot division planner helps when the project is a subdivision rather than a single building—lot yield depends on net area after roads, not on FAR.
What the Calculator Leaves Out
The tool multiplies lot area by FAR and divides by floor count. It does not interpret overlay districts, apply bonus-density programs, exclude parking or mechanical space, or verify whether your floor-to-floor height is structurally realistic. Conversions use standard factors (1 acre = 43,560 sq ft). For anything on a planning application, verify the FAR definition and GFA inclusions with your local planning department and have a licensed architect confirm the basis.