Zoning fundamentalsExplainer

Floor area ratio, explained

Where a zoning code uses it, FAR is the number that caps how much floor area a piece of land can hold. It is also the one most often mistaken for a height limit — and often not the constraint that actually caps the building.

FARtotal floor arealot area

So: floor area allowed = lot area × maximum FAR

One worked example runs through this page: a 50 × 125 ft lot at FAR 1.2, with 5 ft side, 15 ft front and 30 ft rear setbacks and a 38 ft height limit. The Chicago figures are read from the same rule table the LotGrade analyzer uses; how floor area is measured varies by city, as the “Elsewhere” note explains.

Definition

What FAR means

Floor area ratio is a building’s total floor area divided by the area of the lot it sits on. A zoning district states a maximum FAR; multiply it by the lot area and you have the floor area you are permitted to build.

FAR 1.0 on a 5,000 sq ft lot allows 5,000 sq ft of building. That could be one story covering the entire lot, two stories covering half of it, or four covering a quarter. FAR fixes the total. It says nothing about the shape. Height limits, setbacks and lot coverage rules decide that — which is why a district with generous FAR can still produce a small building.

Chicago’s measurement rule §17-17-0305 divides the building’s floor area by the gross area of the zoning lot; planned developments and townhouse developments use net site area instead.

Method

How to calculate FAR

Take the worked example: a 50 × 125 ft lot in a district with a maximum FAR of 1.2.

Lot area50 ft × 125 ft
6,250 sq ft
Maximum FARset by the zoning district
1.2
Floor area allowed6,250 × 1.2
7,500 sq ft

Going the other way, a 7,500 sq ft building on that lot is FAR 1.2 — 7,500 ÷ 6,250.

The arithmetic is easy. The hard part is what counts as floor area, which varies by jurisdiction: basements, parking, mechanical space and balconies may be included, partly excluded, or excluded outright. Those definitions routinely move the answer by a whole story, so check the local rule before trusting a figure.

The binding constraint

FAR is rarely the only limit — and often not the binding one

A building has to satisfy every rule at once, so the real ceiling is whichever limit runs out first. Setbacks decide the footprint, the height limit decides how many floors fit above it, and FAR caps the total. Work them in that order and the binding constraint reveals itself.

Step 1

Setbacks set the footprint

A 50 by 125 foot lot with the setbacks shaded, leaving a 40 by 80 foot buildable footprint.

5 ft sides, 15 ft front and 30 ft rear leave 40 × 80 ft — a 3,200 sq ft footprint.

Step 2

Height sets the floors

The same footprint extruded to the 38 foot height limit — 3 stories.

A 38 ft limit fits 3 stories, so the envelope holds 9,600 sq ft.

Step 3

FAR caps the total

The same envelope cut down to what floor area ratio allows: 2 full floors covering the whole footprint, plus a set-back top floor holding the leftover 1,100 square feet. The unbuildable remainder of the envelope is shown as a dashed outline.

Binding limit
FAR 1.2 allows only 7,500 sq ft: here, 2 full floors plus a set-back top floor of 1,100 sq ft. The dashed outline is envelope you may not fill.

Same 6,250 sq ft lot at each step. The envelope would hold 9,600 sq ft, but FAR stops at 7,500 sq ft, so FAR is the binding limit and 2,100 sq ft of envelope goes unused. Dividing that allowance by the footprint gives 2.3 floors, which is not a building — what it really means is 2 full floors and a smaller 1,100 sq ft floor on top. Raise FAR and the height limit binds instead. Heights are drawn exaggerated ×1.6 so the massing reads at this scale.

A binding FAR is a choice, not a fraction

Floor area does not arrive in fractions of a floor. Once FAR is the binding limit you are choosing how to spend the allowance — and the two obvious ways give very different buildings on the same 7,500 sq ft.

Option A

Full footprint, set-back top

Option A: 2 full floors of 3,200 square feet, with a 1,100 square foot top floor set back toward the rear.

Keep the whole 3,200 sq ft footprint for 2 floors, then put the remaining 1,100 sq ft in a smaller floor above — the set-back top story you see all over the city.

Floors
2 + partial
Footprint
3,200 sq ft
Total
7,500 sq ft

Option B

Every floor, smaller footprint

Option B: 3 full floors of 2,500 square feet on a shorter footprint, leaving 700 square feet of the buildable pad as open ground at the rear.

Spread the same allowance across all 3 floors the height limit permits. More floors, and 700 sq ft of the buildable pad left as open ground.

Floors
3
Footprint
2,500 sq ft
Total
7,500 sq ft

Same FAR, same lot, same height limit — different unit mix, different construction cost, different yard. That is the decision FAR actually hands you, and it is why “what is the FAR?” is the wrong first question on a specific site. The useful question is which of the three limits runs out first, because that is the one worth trying to change.

Scope

What FAR does and doesn’t control

It controls

  • The total floor area you may build on the lot.
  • Indirectly, the practical unit count — though many cities also cap units directly: Chicago with a minimum lot area per unit, New York City by dividing floor area by a unit factor.

It does not control

  • Height. A separate limit, in feet or stories.
  • Footprint or coverage. Setbacks and lot-coverage rules decide those.
  • Use. What may operate in the building is the district’s use table.
  • Shape. Two buildings at identical FAR can look nothing alike.

Chicago

FAR in Chicago

Chicago sets a maximum FAR for each zoning district in Title 17 of the Municipal Code, from well under 1.0 in the detached-house districts to double digits downtown. Height is governed separately, so the two columns below have to be read together — that is the whole point of the diagram above.

DistrictTypical useMax FARHeight limit
RS-1 Detached houses, largest lots 0.5 30 ft
RS-3 Detached houses, standard city lot 0.9 30 ft
RT-4 Two-flats and townhouses 1.2 38 ft
RM-5 Low-rise apartments 2 45-47 ft
B3-2 Shops with apartments above 2.2 Varies (§17-3-0408)
C1-2 Commercial, neighborhood scale 2.2 Varies (§17-3-0408)
M1-1 Light manufacturing 1.2 none_far
DX-5 Downtown mixed-use 5 No limit (PD review)
DX-12 Downtown, high density 12 No limit (PD review)

A selection of districts. Figures come from the same rule table the LotGrade analyzer uses; each district links to its full reference page.

The Zoning Field Guide lists every district with its FAR, height, setbacks and permitted uses.

Bonus FAR

Several Chicago districts allow more than the base figure in specific circumstances — among them transit-served locations and neighborhood programs that let a project buy additional floor area. The conditions are district-specific and change, so each district’s reference page carries its own bonus figure rather than a single rule of thumb.

Questions

Common questions

What does a FAR of 1.0 mean?

It means you may build total floor area equal to the lot area. On a 5,000 sq ft lot, FAR 1.0 allows 5,000 sq ft of building — as one story covering the whole lot, two stories covering half of it, or four covering a quarter. FAR fixes the total, not the shape.

How do you calculate floor area ratio?

Divide the building’s total floor area by the area of the lot it sits on. A 7,500 sq ft building on a 6,250 sq ft lot is FAR 1.2. To go the other way, multiply the lot area by the district’s maximum FAR to get the floor area you are allowed.

Does FAR include the basement, garage or balconies?

It depends on the jurisdiction, and this is where most FAR estimates go wrong. Chicago measures floor area to the outside of the exterior walls and excludes some space — parking and certain below-grade area among it. Check the local definition before trusting a number, because the exclusions can move the result by a full story.

Is a higher FAR always better?

Only if you can actually use it. FAR is one of several limits acting at once. If setbacks and the height limit already cap the building below what FAR permits, extra FAR buys nothing — the binding constraint is somewhere else.

What is the FAR in Chicago?

There is no single figure — it is set per zoning district, from 0.5 in the lowest-density residential districts to 12 and above downtown. Some districts also offer a bonus FAR above the base figure.

Keep reading

For a real address

Check the real number for a real parcel

Reading about FAR only gets you so far — the figure that matters is the one for the parcel in front of you, with its actual lot area, district, setbacks and height limit resolved together. LotGrade does that for any Chicago address and shows which limit binds.

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