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- Floor area ratio (FAR) = total floor area of all floors ÷ lot area.
- A 3,200 sq ft house on an 8,000 sq ft lot has a FAR of 0.40.
- Zoning often sets a maximum FAR, which caps the floor area you can build.
- Maximum floor area = lot area × allowed FAR.
How to use the floor area ratio calculator
Choose what to find
Pick FAR, maximum floor area, or the lot size you need.
Enter the lot size
Type the lot area in square feet or acres.
Enter the building or the limit
For FAR, enter the total floor area of every story. For the other options, enter the allowed FAR from your zoning rules.
Read the result
You get the ratio, the maximum floor area with a split by number of stories, or the lot size needed.
How floor area ratio is calculated
FAR compares how much floor space a building has with the size of the land it stands on. It counts the floor area of all stories, so a two-story house of the same footprint has twice the FAR of a one-story house. It does not describe how much of the lot the building covers.
FAR formulas
FAR = total floor area ÷ lot area
Maximum floor area = lot area × allowed FAR
Lot size needed = floor area ÷ allowed FAR
Convert acres to square feet first: 1 acre = 43,560 square feet.
Worked examples
A house on a lot
3,200 sq ft of floor area on an 8,000 sq ft lot.
3,200 ÷ 8,000 = 0.40.
Maximum floor area on a quarter acre
A 0.25 acre lot is 10,890 sq ft. The allowed FAR is 0.5.
10,890 × 0.5 = 5,445.
Lot size for a planned building
You want 3,000 sq ft of floor area where the allowed FAR is 0.5.
3,000 ÷ 0.5 = 6,000.
Splitting by stories
A 10,000 sq ft lot with an allowed FAR of 0.6.
The more stories you build, the smaller each floor must be.
Floor area by FAR chart
Maximum floor area on a 5,000 sq ft lot
By allowed FAR.
Higher FAR means denser building.
Where a FAR calculator helps
Home additions
Check if an addition keeps you within the zoning limit.
Buying a lot
See how much you could build on a plot.
Developers and planners
Compare the density of different projects.
Students
Practice urban planning and zoning calculations. Use the square footage calculator for areas.
What counts as floor area, for example basements, garages, attics or stair areas, depends on the local zoning code. Maximum FAR also varies by zone, and other limits such as height, setbacks and lot coverage may apply. Ask your local planning office for the exact rules before you plan a building.
Floor area ratio calculator FAQ
What is floor area ratio?
It is the total floor area of a building divided by the area of its lot. A FAR of 0.5 means the floor area equals half the lot size.
How do I calculate FAR?
Add the floor area of all stories and divide by the lot area. A 3,200 sq ft house on an 8,000 sq ft lot has a FAR of 0.40.
What is a good FAR?
There is no single good figure. Each zone sets its own maximum, and lower FAR means less dense building.
Does FAR include the garage and basement?
That depends on local rules. Some codes exclude garages or basements, others include them.
Is FAR the same as lot coverage?
No. Lot coverage is the share of the lot covered by the building footprint. FAR counts all floors.
How do I find the maximum floor area?
Multiply the lot area by the allowed FAR. A 10,000 sq ft lot with a FAR of 0.6 allows 6,000 sq ft of floor area.