Zoning codes limit how much building can go on a lot with a floor area ratio – the total floor area of all storeys divided by the lot area. A FAR of 1.0 allows a one-storey building covering the whole lot, a two-storey building covering half, and so on. In India and elsewhere the same idea is called the floor space index (FSI).
How to use the Floor Area Ratio Calculator
- Lot area
- Building footprint
- Number of floors
- Allowed FAR (zoning)
- Every result field is editable: type into Floor area ratio and Number of floors plus every other field update to match.
Results update as you type, and every field accepts the units shown in its menu.
How it works
Enter the lot area, the building footprint, the number of floors and the FAR permitted by zoning. The calculator returns the actual FAR, the lot coverage and how much floor area and how many floors are allowed.
Tips for an accurate estimate
- Take measurements at floor level and at the ceiling – walls and rooms are often slightly out of square.
- Use the manufacturer’s coverage, spacing and installation data when it differs from the typical values here.
- Keep a little extra of every finish material for touch-ups and repairs later.
- Check local building codes and permits before structural, electrical, plumbing or gas work.
Common mistakes to avoid
- Sizing heating or cooling equipment by area alone when insulation, windows and climate make a big difference.
- Buying too little paint, stain or flooring and ending up with mismatched batches.
- Overlooking the extra material for steps, railings, gables and trim.
Inputs explained
| Field | What to enter |
|---|---|
| Lot area | Units: ft², m², yd², acre |
| Building footprint | Units: ft², m², yd², acre |
| Number of floors | A number |
| Allowed FAR (zoning) | A number |
What each result means
- Gross floor area = [Building footprint]×[Number of floors]
- Floor area ratio = [Gross floor area]÷[Lot area]
- Lot coverage = [Building footprint]÷[Lot area]×100
- Maximum floor area allowed = [Allowed FAR (zoning)]×[Lot area]
- Floors allowed at this footprint = [Allowed FAR (zoning)]×[Lot area]÷[Building footprint]
Example values
How the main result changes when Number of floors changes and everything else stays at the example values above:
| Number of floors | Floor area ratio |
|---|---|
| 1 | 0.3 |
| 1.5 | 0.45 |
| 2 | 0.6 |
| 2.5 | 0.75 |
| 3 | 0.9 |
| 4 | 1.2 |
Formula
Gross floor area = footprint × floorsFAR = gross floor area ÷ lot areaLot coverage = footprint ÷ lot areaMaximum floor area = allowed FAR × lot areaWorked example
Inputs — Lot area: 5,000 ft²; Building footprint: 1,500 ft²; Number of floors: 2; Allowed FAR (zoning): 0.75.
Results — Gross floor area = 3,000 ft²; Floor area ratio = 0.6; Lot coverage = 30 %; Maximum floor area allowed = 3,750 ft²; Floors allowed at this footprint = 2.5.
Key terms
- FSI
- Floor space index – another name for floor area ratio.
Frequently asked questions
What does a FAR of 0.5 mean?
The total floor area may be half the lot area – e.g. 2,500 ft² on a 5,000 ft² lot.
Do basements count toward FAR?
It depends on the local code; many exclude basements, garages or areas below a certain ceiling height.
What is the difference between FAR and lot coverage?
FAR counts all floors; lot coverage only measures the footprint on the ground.
What formula does the Floor Area Ratio Calculator use?
Gross floor area = footprint × floors.