How to calculate floor area ratio
Floor area ratio compares a building’s total floor area with the size of its lot. Zoning codes use it to limit how much can be built.
The formula
FAR = total building floor area ÷ lot area
Worked example
On a 10,000 square foot lot with a zoning FAR of 2.0, you can build up to 20,000 square feet of floor area.
How to use this calculator
Enter the lot area and the total floor area of all stories, and the floor area ratio calculator divides one by the other. Add the FAR limit your zoning allows to see the floor area the limit allows, the floor area still available and how the building compares with the limit. Use the same unit for both areas. If your lot is measured in acres, convert it first: 1 acre is 43,560 square feet.
What the result means
FAR is a plain ratio with no unit. A FAR of 1.0 means the floor area equals the lot area. A FAR of 0.5 means the floor area is half the lot area. Any FAR above 1.0 needs more than one story, because a single floor cannot be larger than the lot it sits on.
The number you calculate for an existing building is its actual FAR. The number in the zoning code is the maximum FAR. Comparing the two shows how much room is left. A building that is already at the maximum cannot add floor area under that limit.
More examples
Finding the FAR of an existing building
A two-story building has 3,000 square feet on each floor, so its total floor area is 6,000 square feet. It sits on an 8,000 square foot lot. 6,000 ÷ 8,000 = 0.75, so the FAR is 0.75.
Finding the maximum buildable area
Multiply the lot area by the zoning FAR. The table uses the 10,000 square foot lot from the example above.
| Zoning FAR | Maximum floor area |
|---|---|
| 0.5 | 5,000 sq ft |
| 1.0 | 10,000 sq ft |
| 2.0 | 20,000 sq ft |
| 4.0 | 40,000 sq ft |
For a lot measured in acres, a 1 acre lot (43,560 square feet) with a FAR of 0.35 allows about 15,246 square feet of floor area.
Finding how much more you can build
A 10,000 square foot lot with a FAR limit of 2.0 allows 20,000 square feet. If the existing building has 12,000 square feet, its FAR is 1.2 and 8,000 square feet of floor area remain.
Same FAR, different building shapes
FAR sets the total floor area, not the shape. On the 10,000 square foot lot at a FAR of 2.0, the same 20,000 square feet can be arranged in several ways.
| Footprint | Share of lot covered | Floors needed |
|---|---|---|
| 10,000 sq ft | 100% | 2 |
| 5,000 sq ft | 50% | 4 |
| 2,500 sq ft | 25% | 8 |
In practice other zoning limits, such as height, setbacks and lot coverage, decide which of these shapes is allowed.
What changes the result
- Floor area. Adding a story or an addition raises the FAR.
- Lot area. A larger lot lowers the FAR for the same building. Combining two lots raises the floor area you can build in total.
- What your city counts. If the code leaves out space such as parking or basements, the counted floor area is smaller and so is the FAR.
Common mistakes
- Using the building footprint instead of the total of all floors.
- Mixing units, such as floor area in square feet and lot area in acres.
- Treating FAR as lot coverage. A one-story building covering half the lot and a two-story building covering a quarter of it both have a FAR of 0.5.
- Assuming FAR is the only limit. A project must meet every zoning rule that applies to the lot.
- Counting or leaving out space without checking how the local code defines floor area.
Related calculators
Use the square footage calculator to add up the floor area of each story, and the acres to square feet converter for the lot. Once you know how much you can build, the cost to build a house calculator gives a rough construction budget. Results are estimates, so confirm the limits for your lot with your local planning or zoning office before you design.
Frequently asked questions
Does FAR limit height?
Not directly. A FAR of 2.0 could be two floors covering the lot or four floors covering half of it.
What counts as floor area?
Rules vary by city. Parking, basements and mechanical space are often excluded.
Is FAR the same as lot coverage?
No. Lot coverage is the share of the lot covered by the building footprint.
What does a FAR of 1.0 mean?
The total floor area equals the lot area. On a 10,000 square foot lot that is 10,000 square feet of floor area, on one floor or spread over several.
How do I find the FAR limit for my lot?
It is set by the local zoning code and depends on the zone your lot is in. Your city or county planning office can tell you the zone and the limit.
Can I calculate FAR in square meters?
Yes. FAR is a ratio, so the answer is the same in any unit as long as the floor area and the lot area use the same one.
How do I work out how much more I can build?
Multiply the lot area by the maximum FAR, then subtract the floor area that already exists.