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FeasibilityUrban PlanningZoningFree

FAR to GFA
Calculator

Enter your site area and the floor area ratio from your zoning designation — get the gross floor area implied by those two inputs in one click.

Who it's for: developers, architects, and owners checking how much floor area a site allows at pre-design, before designing. You need two numbers: the site (lot) area and the floor area ratio from your zoning designation.

Floor Area Ratio (FAR) — also called Floor Space Index (FSI) or Plot Ratio — is the ratio of total building floor area to the site area. A site of 1,000 m² with FAR 2.0 produces 2,000 m² GFA before local exclusions and other development controls are applied. This calculator does not supply the permitted FAR or determine which spaces count as GFA. Verify both against the zoning ordinance and official parcel information for the site's jurisdiction.

1
Enter site area
Total site area — enter in m² or use the toggle to enter in acres or sq ft
2
Enter FAR
The floor area ratio from your zoning code (e.g. 0.5, 2.0, 4.5)
3
Get FAR-derived GFA
Instant result in both m² and sq ft — no signup required
✓ Results in m² and sq ft✓ Accepts acres, sq ft, or m² input✓ No signup required
FAR-Derived GFA Calculator
Site area × entered FAR = gross floor area
Check your local zoning map or development plan

How to use this calculator

1
Enter the site area
Type the total site (lot) area. Use the unit toggle to switch between m², square feet, or acres — the tool converts to m² before calculating.
2
Choose the input unit
Pick m², sq ft, or acres to match the figure on your survey or zoning record. Results always come back in both m² and sq ft.
3
Enter the floor area ratio (FAR)
Type the FAR from your zoning designation or development plan, for example 0.5, 2.0, or 4.5. It is a plain number, not a percentage.
4
Read the FAR-derived GFA
The result is site area × entered FAR, shown in m² and sq ft. Confirm whether the local zoning instrument applies exclusions or additional controls.

The formula

The calculator applies the entered ratio to the entered lot area: FAR-derived GFA = Site Area × FAR.

FAR-derived GFA — Arithmetic gross floor area before local definitions, exclusions, and other controls are checked (m² or sq ft)
Site Area — Entered lot area, converted to m² when acres or sq ft are selected
FAR — Dimensionless ratio entered from the applicable zoning source; the calculator does not look it up or validate it

Worked example

Example
A 2,500 m² site with an entered FAR of 2.0 produces an FAR-derived gross floor area of 5,000 m² (2,500 × 2.0). The calculator converts that to about 53,820 sq ft. This is an arithmetic result, not an entitlement determination; the applicable zoning ordinance controls the FAR, GFA definition, exclusions, and other development limits.

When this estimate will be off

  • Does not account for setbacks, height limits, or lot coverage maximums — any of these can reduce the buildable envelope below the FAR ceiling.
  • The applicable zoning instrument defines the counted floor area, measurement boundary, and exclusions. Do not apply a generic parking, basement, balcony, or mechanical-space treatment.
  • Mixed-use or overlay zones may assign different FARs to different uses on the same parcel; this calculator applies one FAR uniformly across the whole site.
  • A bonus, transfer, overlay, or special-district modification changes the entered FAR only where the applicable local provisions authorize it and every condition is satisfied.

Frequently asked questions

A common mistake: Does not account for setbacks, height limits, or lot coverage maximums — any of these can reduce the buildable envelope below the FAR ceiling.

Floor area ratio expresses gross floor area relative to site area. In the calculator’s arithmetic, an entered FAR of 2.0 produces floor area equal to twice the entered site area. Local zoning determines the permitted FAR and which areas count in that ratio. Some jurisdictions use terms such as floor space index or plot ratio.

Multiply the site area by the entered FAR. A 2,500 m² site with a FAR of 2.0 produces 5,000 m² of gross floor area before jurisdiction-specific exclusions and controls are applied. The result does not determine the permitted footprint, height, storey count, or entitlement.

The effective local zoning instrument defines the floor area used in its FAR calculation, including the measurement boundary, included spaces, exclusions, and any special-district or use-specific modifications. Do not apply a generic list of parking, basement, mechanical-room, balcony, or circulation exclusions.

Planning authorities establish density controls through locally adopted zoning instruments, so permitted FAR values and calculation rules differ by jurisdiction, zone, overlay, and sometimes use. Read the effective zoning provisions and official parcel information for the specific site. Do not substitute a citywide or regional average.

Sources

  • Calculation basis — Ratio arithmetic applied to the user-entered lot area and FAR; the calculator does not choose or verify the applicable zoning ratio.
  • Required local authority — Verify the effective zoning instrument, official lot area, GFA definition, exclusions, overlays, bonuses, and other development controls for the parcel.

This free FAR Calculator is built and maintained by DataDrivenAEC, using the relevant codes and standards. It does not substitute for professional judgment.