How this instrument works
Floor Area Ratio is a zoning-density measure: the total floor area of a building, summed across every level, divided by the area of the lot it sits on. Unlike lot coverage, which only measures the building's footprint on the ground, FAR adds up every floor — a two-story building covering half a lot has the same footprint as a one-story building covering half a lot, but roughly double the FAR, because the upper floor's area gets added to the total.
Municipalities set a maximum FAR for a zoning district as a way to control overall building bulk and density without dictating an exact building shape. A lot zoned for a FAR of 1.5, for example, could hold a single-story building covering 150% of the lot only if that were structurally sensible (it isn't, since a building can't exceed the lot itself in footprint) — in practice that FAR is achieved by stacking floors, such as a two-story building covering 75% of the lot, or a three-story building covering 50%.
One real-world wrinkle: not every zoning code defines 'floor area' identically. Some jurisdictions exclude basements, attics, mechanical rooms, or parking structures from the FAR calculation, while others count them fully. This instrument computes the arithmetic ratio from whatever total floor area figure you enter, so the accuracy of the result depends on using the floor-area definition your specific zoning ordinance actually applies — check the local code's definitions section before relying on a FAR figure for an actual permit application.
- Enter Total building floor area (sq ft) — the sum of every floor's area in the building, using whatever floor-area definition your local zoning code specifies.
- Enter Lot area (sq ft) — the total area of the parcel the building sits on.
- Read Floor Area Ratio (FAR) directly; it recalculates the instant either field changes.
- Compare the result against your zoning district's maximum permitted FAR, found in the local zoning ordinance, not derived from this calculation alone.
Worked example — 6,000 sq ft building on a 4,000 sq ft lot
Enter 6000 into Total building floor area (sq ft) and 4000 into Lot area (sq ft) — a building whose combined floor area across all its levels adds up to 6,000 sq ft, sitting on a 4,000 sq ft lot. Floor Area Ratio (FAR) reads 1.5: 6000 ÷ 4000 = 1.5 exactly.
A FAR of 1.5 on a 4,000 sq ft lot could be built as a two-story building covering 3,000 sq ft of the lot's footprint (75% lot coverage) with 3,000 sq ft on each floor, or as a three-story building covering just 2,000 sq ft (50% lot coverage) with 2,000 sq ft per floor — the same FAR supports many different building shapes.
Questions
What does a FAR of 1.5 actually mean for what can be built?
It means the total floor area across every level of the building can be up to 1.5 times the lot's area — not that the building covers 150% of the lot's footprint, which is physically impossible. A FAR of 1.5 is typically achieved by stacking floors: a two-story building covering 75% of the lot, or a three-story building covering 50%, both reach the same 1.5 total ratio.
What's the difference between FAR and lot coverage?
Lot coverage measures only the building's ground-level footprint as a percentage of the lot, ignoring how many stories tall it is. FAR instead sums the floor area of every level and divides by lot area, so a tall, narrow building and a short, wide building can have very different lot coverage figures while sharing the same FAR, if their total floor areas match.
Does floor area include basements, garages or mechanical rooms?
It depends entirely on your local zoning ordinance — some jurisdictions exclude basements, unfinished attics, parking structures or mechanical rooms from the FAR calculation, while others count some or all of them. This instrument just divides whatever total floor area figure you enter by the lot area, so check your specific zoning code's definitions section before using a FAR result for an actual permit or design decision.
Why do zoning codes use FAR instead of just capping building height?
FAR controls overall building bulk and density while leaving the architect flexibility in how that bulk is shaped — a fixed height cap alone doesn't limit how much floor area can be packed under that height, and a fixed footprint cap alone doesn't limit how tall a building can rise. Many zoning codes combine FAR with separate height and setback limits to control a building's shape from multiple angles at once.
Can I exceed my zoning district's maximum FAR?
Generally not without a variance, special permit, or other zoning relief process specific to your municipality, since the maximum FAR is a legal limit set in the zoning ordinance for that district. Some jurisdictions offer FAR bonuses in exchange for public amenities like open space or affordable housing units, but that's a separate process from the basic FAR calculation itself.