Floor Area Ratio Calculator

How to use the floor area ratio calculator

What this calculator does. This tool links six project inputs to a consistent set of area metrics: carpet area, wall or non-carpet area, super built-up area, loading factor, open space ratio, total built-up area, and floor area ratio. It is useful for early feasibility checks, reviewing a property brochure, comparing alternative unit mixes, and checking whether stated area relationships are internally consistent. It is not a zoning approval tool because local codes may define countable floor area, lot area, exemptions, and bonuses differently.

When to use it. Use it when comparing apartment schemes with different efficiency ratios, testing how common areas affect saleable or allocated area, estimating project-wide floor area from a typical unit, or checking how much total construction is represented relative to a site. The planning concept is commonly expressed as total floor area divided by lot area; the American Planning Association's floor area ratio explanation provides useful background.

How to calculate. The calculator opens with a complete demonstration based on a 1,400 sq ft built-up unit, a 60% carpet ratio, 100 units, 28,000 sq ft of common area, 60,984 sq ft of open space, and 174,240 sq ft of land. The example workbook is ready immediately. To use your own figures:

1. Replace each sample value with project data in square feet, using a period as the decimal separator and optional commas for thousands.
2. Read the live FAR first, then review carpet area, super built-up area, loading factor, open space ratio, and project total.
3. Use the breakdown table to confirm that per-unit and project-level figures align.
4. Select Download Excel to export the current validated model. Select Reset to clear all demonstration data; export stays disabled until every required field is complete and valid again.

Input guide. Built-up area per unit is a required positive decimal in square feet; 1,400 is a realistic example. Raising it increases carpet area, wall area, super built-up area, total built-up area, and FAR. Do not confuse it with carpet area. Carpet-to-built-up ratio is a required percentage above 0 and at most 100; 60 means 60%, not 0.60. A higher ratio increases usable carpet area and reduces the implied wall/non-carpet share. Number of units is a required positive whole number; 100 is the example. It scales total built-up area but does not change per-unit carpet area. Common area is a required nonnegative square-foot amount shared across all units; 28,000 adds 280 sq ft per unit in the demonstration. Excluding corridors, stairs, or shared amenities understates super built-up area. Open space area is a required nonnegative square-foot amount and should not exceed total land area; 60,984 produces a 35% open space ratio. Total land area is the required positive site area; 174,240 sq ft equals four acres. Confirm that the land definition matches the zoning authority's rules.

Output guide. Floor area ratio (FAR / FSI) is total built-up area divided by land area and is an exact identity within this model. A value of 0.80 means total built-up floor area equals 80% of the site area; it does not mean only 80% of the ground is covered. Carpet area per unit estimates usable area from the stated ratio. Walls / non-carpet area is built-up area minus carpet area. Super built-up per unit adds each unit's share of common area to built-up area. Loading factor is the common-area allocation relative to carpet area; high values can signal a large gap between usable and allocated area. Open space ratio is open space divided by land area. Total built-up area is built-up area per unit multiplied by unit count.

Worked example. With 1,400 sq ft built-up area and a 60% carpet ratio, carpet area is 1,400 × 0.60 = 840 sq ft and wall/non-carpet area is 560 sq ft. A 28,000 sq ft common area divided among 100 units adds 280 sq ft per unit, so super built-up area is 1,680 sq ft. Total built-up area is 1,400 × 100 = 140,000 sq ft. Dividing 140,000 by 174,240 gives FAR = 0.8035, displayed as 0.80. Open space ratio is 60,984 ÷ 174,240 = 35.00%.

Learn more. Local zoning controls can set district-specific maximum FAR values and special counting rules. See the New York City residential bulk regulations for an example of how FAR is embedded in a legal zoning framework, and review the UK government's gross internal area definition for a contrasting measurement convention.