Floor Area Ratio Calculator — FAR and Buildable Area

Zoning codes ration bulk with a single ratio: the floor area ratio, FAR — total floor area divided by lot area. It decides how much building a parcel may carry before height limits, setbacks or parking even enter the conversation, which is why every development appraisal and most extension arguments begin with this one division and its result. Enter your lot and your building figures, and this calculator returns your FAR, the maximum floor area a stated FAR allows, and the floors that makes at your footprint.

One Ratio, Every Skyline

FAR is the zoning code’s volume knob. A FAR of 1.0 permits floor area equal to the lot — a one-storey building wall-to-wall, or two storeys on half of it, or ten on a tenth. Residential districts commonly run 0.3–1.0 (the worked example’s 0.5 is a classic suburban figure); downtown commercial zones run 5, 10, sometimes 15 and above, which is why skyscrapers are a legal phenomenon before they are an engineering one. The ratio’s democratic property is what made it the twentieth century’s favourite control: it rations total bulk while leaving its shape — squat or slender — to the designer, within the separate fences of height and coverage limits. This calculator handles the ratio in both directions, as owners actually meet it: what is my FAR, and what does the cap let me build?

Worked Example: The 6,000-Square-Foot Lot

The defaults price the textbook parcel: a 6,000 square foot lot carrying a building of 2,400 square feet gross (two floors of 1,200). FAR = 2,400 ÷ 6,000 = 0.40. At the district’s 0.5 limit the lot may carry 0.5 × 6,000 = 3,000 square feet — 2.5 floors at the 1,200 footprint, in practice two full floors plus a half-storey or a footprint conversation — leaving 600 square feet of headroom under the cap. The headroom figure is the one that launches a thousand extension projects, and also the one most often disappointed by the other controls (the extension fits the FAR and fails the rear setback), which is why the calculator’s note names them every time: FAR compliance is necessary, never sufficient.

What Counts as Floor Area (and What Doesn’t)

The ratio is simple; its numerator is a legal document. Jurisdictions differ on the inclusions, and the differences are where projects live: basements (often excluded if substantially below grade), garages (excluded, capped, or counted in various cities), mezzanines and double-height spaces (counted once, or by formula), balconies and terraces (usually out), mechanical and circulation space (usually in). Many codes also grant bonuses — extra FAR in exchange for plazas, affordable units, transit improvements — making the effective limit a negotiation as much as a number in exactly the districts where the numbers are largest. The calculator computes with the gross figure you supply and the limit you state; the definitions that produce those two numbers for a real parcel live in the municipal code and, at any serious scale, in a zoning attorney’s letter.

FAR in the Feasibility Sketch

Developers run this page’s division before almost any other, because it sizes the entire proposition: a parcel’s value is roughly its buildable floor area times the margin per square foot, and the FAR cap sets the first term before design begins. The same logic inverted prices residential extensions — the headroom output is the extension’s maximum programme, against which the architect’s first sketch is measured. Two professional footnotes belong here: the stated FAR is the zoning limit, and existing nonconforming buildings (built past today’s cap under yesterday’s) complicate everything downstream of a purchase; and bonuses, transfers and special-district overlays can move the effective cap in both directions. The calculator gives the clean ratio around which those negotiations orbit — deliberately, and with the caveat printed where the number appears.

Three Controls Walk Onto a Lot

FAR never binds alone, and a combined example shows the pecking order. Take the worked example’s 6,000-square-foot lot and add its other likely controls: a 40% lot-coverage cap (footprint at most 2,400 square feet) and a 35-foot height limit (about three residential floors). Coverage and height together would permit 2,400 × 3 = 7,200 square feet of bulk — but the FAR of 0.5 caps the total at 3,000, so on this lot FAR is the binding control and the building must shrink its floors, not its height. Move the same building to a FAR 2.0 district and the arithmetic inverts: the FAR would permit 12,000, coverage and height hold it to 7,200, and the envelope binds instead. Feasibility is the minimum of the three computations, always — which is why this calculator prints the FAR result with its rivals named in the note, and why no single ratio, however elegantly it divides, zones a city by itself.

A Brief History of the Ratio

FAR entered the codes with New York’s 1916 zoning resolution — the first comprehensive American ordinance, provoked by the Equitable Building’s sheer cliff of offices darkening whole streets — and matured in the 1961 revision that made the ratio, rather than the wedding-cake setback envelope alone, the primary ration of bulk, sweetened with plaza bonuses that traded floor area for public open space at grade. The concept has since zoned most of the urbanised world, under its own name and under cousins like plot ratio and floor space index. Knowing the lineage explains the instrument’s character: it was born as a compromise between development’s appetite and the street’s light, and it has been renegotiated, by amendment and variance, ever since — the calculator divides by the current terms of that century-old negotiation. Every contemporary argument about housing supply, shadows, parking and neighbourhood character is, underneath its vocabulary, an argument about where this one ratio should sit on a given street.

Related Tools

More from the Construction Calculators hub: the Gambrel Roof Calculator for the top of whatever the FAR allows, and the Heat Loss Calculator for the energy bill of the floor area once built.

Frequently Asked Questions

How is floor area ratio calculated?

FAR = gross floor area of the building (all floors summed) ÷ lot area, in the same units. A 2,400-square-foot building on a 6,000-square-foot lot has a FAR of 0.40; a tower with 150,000 square feet on a 15,000-square-foot lot has a FAR of 10. The ratio is dimensionless and indifferent to the building’s shape — which is its purpose and, critics add, its blindness: bulk is rationed, form is left to other controls.

What is a typical FAR for a house?

Single-family districts most often cap FAR between 0.3 and 0.6 — the worked example’s 0.5 sits mid-band — a control aimed at preventing mansionisation of modest lots: it limits the house’s total bulk relative to its parcel regardless of height. Two-storey houses on small lots press against it routinely, which is why so many extension assessments begin with exactly this page’s division, run on survey figures rather than estate-agent ones.

Does FAR include the garage and basement?

By local definition — there is no universal answer, which is why the calculator takes the gross area as given. Many codes exclude below-grade basement area and a standard garage allowance; many others count both. The swing can be a fifth of a house’s nominal area, enough to decide compliance. Read your code’s “floor area, gross” definition before trusting any FAR figure for a filing; for a feasibility sketch, run the calculator both ways and treat the truthful answer as the range between them.

What is the difference between FAR and lot coverage?

Coverage is the footprint’s share of the lot (one storey deep); FAR is all floors’ share. A building can satisfy a 40% coverage cap and wildly breach FAR by stacking floors, or satisfy FAR and breach coverage with a vast single storey. Codes deploy both because they govern different things: coverage preserves open ground, light and drainage at the surface; FAR rations total bulk and the population it brings. This calculator’s footprint input lets the floors-at-footprint reading bridge the two intuitions, but the compliance checks remain separate.

Can FAR be transferred or bought?

In many cities, yes — transferable development rights (TDRs) let a landmark or a low-rise parcel sell its unused FAR to a neighbour, which then builds beyond its own cap by the purchased amount. It is how some famous towers stand over small landmarks that never used their sky. The calculator cannot price your parcel’s market in air rights — a broker’s and attorney’s trade — but its headroom output is exactly the quantity such markets transact in, which is a fair measure of the ratio’s real-world weight.

Does a higher FAR raise a property’s value?

Usually, because it prices development potential into the land: the same lot at FAR 2.0 carries twice the buildable programme of FAR 1.0, and land value follows the programme in functioning markets, step for step, parcel for parcel, which is why assessors and appraisers watch rezoning dockets the way traders watch earnings calendars every quarter. Rezonings that lift FAR revalue whole districts overnight (and downzonings the reverse) — the political heat around zoning reform is, at bottom, an argument over the number this page divides by. Valuation, though, belongs to appraisers working the local market; the calculator’s contribution is the verified ratio itself, computed cleanly and without an agenda of its own.