What this calculator does
Floor area ratio is total floor area divided by lot area, summed across every storey. A 3,000 square foot building on a 6,000 square foot lot has a FAR of 0.50.
The ratio says nothing about how the building sits on the lot, which is the point. A FAR of 1.0 permits a single-storey building covering the entire lot, or a two-storey building covering half, or a four-storey tower on a quarter. Zoning usually pairs the FAR cap with setback and height limits precisely because FAR alone does not control form.
The formula
The total floor area across all storeys is divided by the lot area. The maximum permitted floor area is the local FAR limit multiplied by the lot area, and the remaining capacity is the difference. A negative remaining figure means the building already exceeds the cap.
| Term | Meaning |
|---|---|
| FAR | Floor area ratio: total floor area over lot area. Called plot ratio in some jurisdictions. |
| Gross floor area | What counts toward the total. Definitions vary, and basements, garages and balconies are often excluded. |
| Site coverage | The lot footprint covered by building, which is a different and separate control. |
| Setback | Minimum distance from boundaries, which shapes the building alongside the FAR limit. |
The inputs explained
| Field | What to enter |
|---|---|
| Lot area (sq ft) | Lot area in square feet. |
| Total building floor area (sum of all floors) (sq ft) | Total floor area summed across all storeys, using your local definition of what counts. |
| Local FAR limit (zoning cap) | The local FAR cap from the zoning code. |
When to use it
Checking development capacity
The remaining floor area shows how much more can be built under the cap.
Assessing a site before purchase
FAR limit times lot area gives the buildable floor area, which drives site value.
Testing a design against zoning
A proposed scheme can be checked against the cap before detailed design.
Worked examples
Every figure in the tables below is produced by this page’s own calculator at build time, so the numbers and the tool always agree. Select any row to load that scenario.
What does each FAR limit permit?
The same building and lot against a range of zoning caps.
| Local FAR limit | Maximum floor area allowed under the FAR limit | Remaining floor area available | Floor area ratio |
|---|---|---|---|
| FAR 0.35 | 2,100 sq ft | -900 sq ft | 0.50 |
| FAR 0.5 | 3,000 sq ft | 0 sq ft | 0.50 |
| FAR 0.75 | 4,500 sq ft | 1,500 sq ft | 0.50 |
| FAR 1 | 6,000 sq ft | 3,000 sq ft | 0.50 |
Questions
What counts toward floor area?
It depends entirely on the local definition, which is why the figure must come from your zoning code rather than from a general rule. Basements, garages, balconies, attics and plant rooms are variously included or excluded, and the differences are large enough to change a scheme.
What is a typical FAR limit?
Suburban residential zones are often 0.3 to 0.6, urban residential 1.0 to 3.0, and dense commercial districts can exceed 10. There is no general answer: it is set locally and is one of the main levers planners use to control density.
Does FAR control building height?
No, and that is the point. A FAR of 1.0 could be one storey covering the whole lot or ten storeys covering a tenth. Height, setback and site coverage limits are applied alongside FAR precisely because the ratio alone does not determine the shape.
What if my building exceeds the limit?
An existing building over the cap is usually a legal non-conforming use, permitted to remain but not to expand. Extending it typically requires a variance. This is a planning matter and the calculation only tells you where you stand, not what is permitted.
For measuring the areas, see the square footage calculator. For the structure itself, see the wall stud calculator.