To calculate floor area ratio, divide a building’s total floor area by the area of the lot it sits on: FAR = Total Floor Area ÷ Lot Area. Both numbers use the same unit, usually square feet, and the result is a plain decimal. A 15,000-square-foot building on a 10,000-square-foot lot produces a FAR of 1.5. The arithmetic is straightforward. What takes care is knowing exactly which spaces count toward floor area and which portions of the parcel count toward lot area, because your local zoning code, not a national standard, defines both.
The Formula and What the Number Means
A FAR of 1.0 means the building’s total floor area equals the lot area. A FAR of 2.0 means you can build twice the lot area in total floor space, spread across whatever number of stories your other zoning limits allow. A FAR of 0.5 caps total floor space at half the lot’s square footage.
The ratio doesn’t dictate a building’s shape. A FAR of 1.0 on a 10,000-square-foot lot could be a single story covering the entire lot, two stories of 5,000 square feet each, or four stories of 2,500 square feet each. FAR caps the total interior volume relative to the land underneath. Height limits, lot coverage, and setbacks decide the shape.
What Counts as Total Floor Area
Total floor area generally includes all enclosed space within a building, measured to the outside face of the exterior walls. Every floor level with usable headroom counts: living areas, offices, retail space, storage rooms, closets, and mezzanines. Some jurisdictions also count enclosed porches with solid roofs.
Exclusions are where the definition gets tricky, and they vary by jurisdiction. Spaces commonly left out of the FAR calculation include:
- Basements. Unfinished or non-habitable basements are often excluded, though a finished basement used as living space typically counts.
- Parking. Structured parking garages and underground parking are frequently excluded, which is partly why cities can encourage structured parking without penalizing a project’s buildable floor area.
- Mechanical space. Floors or rooms dedicated entirely to building systems like HVAC, electrical switchgear, or elevator machinery are excluded in many codes.
- Vertical circulation. Elevator shafts and enclosed stairwells sometimes get excluded, though some codes count them. This is one of the items most likely to differ between jurisdictions.
- Open features. Unenclosed balconies, open porches, and canopies without walls are typically excluded because they lack full enclosure.
The precise list of inclusions and exclusions lives in your local zoning ordinance. Two cities in the same state can draw the line differently. Before running your calculation, pull the floor area definition from the zoning code that governs your parcel. Getting this wrong by a few hundred square feet can push a project over the limit.
How to Measure the Lot Area
Lot area is the total area of the land parcel your building sits on. The critical question is whether your jurisdiction uses gross lot area or net lot area in the FAR formula. Gross lot area is the entire parcel as shown on the plat. Net lot area subtracts portions the owner can’t actually build on: land dedicated to public streets, rights-of-way, wetlands, flood zones, and sometimes utility easements. Some municipalities exclude land encumbered by utility easements from the calculation, which effectively reduces how much you can build.
For most development projects beyond a straightforward single-family home, you’ll need a professional boundary survey to pin down the lot area. An inaccurate lot measurement throws off every downstream calculation, so a current survey is worth the cost.
Worked Examples
Three scenarios show the formula at work.
- Single-story commercial building. A one-story retail building with 8,000 square feet of floor area sits on a 20,000-square-foot lot. FAR = 8,000 ÷ 20,000 = 0.4.
- Multi-story office building. A four-story office building with 5,000 square feet per floor sits on a 10,000-square-foot lot. Total floor area is 20,000 square feet. FAR = 20,000 ÷ 10,000 = 2.0.
- Checking against the limit. A zoning district allows a maximum FAR of 1.5 on a 12,000-square-foot lot. Maximum buildable floor area = 1.5 × 12,000 = 18,000 square feet. A proposed three-story building at 6,500 square feet per floor totals 19,500 square feet, exceeding the cap by 1,500. The project would need to shrink or seek a variance.
Running the formula in reverse, as in that third example, is often more useful than calculating the FAR of an existing building. Multiply the allowed FAR by your lot area, and you know your ceiling before you start design.
How to Find Your Property’s Maximum FAR
Knowing the formula is only half the job. You also need the FAR your zoning district allows.
- Identify your zoning district. Most municipalities publish zoning maps online. Search your city or county’s planning department website for an interactive zoning map, then look up your parcel by address or parcel number. The map will show a designation like R-1, C-2, or MU-3.
- Pull up the zoning code. Once you have the district designation, find the bulk regulations or development standards table for that district in the municipal code. The maximum FAR is usually listed alongside height limits, setbacks, and lot coverage.
- Call the planning department. If the online code is hard to navigate, a quick call or email can confirm the FAR limit, any overlay districts that modify it, and any pending code changes.
Don’t skip the overlay step. Many cities layer special overlay zones on top of base zoning districts. A historic district overlay, transit corridor overlay, or environmental protection overlay can raise or lower the allowed FAR beyond what the base district shows.
How FAR Interacts With Lot Coverage and Height
FAR is one of several bulk controls, and the number alone won’t tell you what you can build. The two most commonly confused are FAR and lot coverage.
Lot coverage measures the percentage of the lot that a building’s footprint covers at ground level. A 50% lot coverage limit on a 10,000-square-foot lot means the footprint can’t exceed 5,000 square feet, regardless of how many stories you add. FAR controls total floor area across all stories. You can comply with a generous FAR but still violate lot coverage if the ground floor is too large, or vice versa.
Height limits add a third constraint. Even if your FAR and lot coverage numbers check out, you can’t exceed the district’s maximum building height. A low lot coverage limit combined with a high FAR pushes the design toward a taller, slimmer tower; a high lot coverage limit with a low FAR encourages a shorter, wider building. There’s no point sketching a four-story building if the height limit caps you at three, even though the FAR would theoretically permit it.
Calculating FAR in Mixed-Use Buildings
Many zoning codes assign separate FAR limits to different uses within the same district. A commercial district might allow a FAR of 2.0 for retail and office space but only 0.75 for any residential component in the same building. When your project combines uses, you typically calculate each use’s floor area independently and check it against that use’s individual cap.
Shared spaces like lobbies, hallways, and mechanical rooms need to be allocated between uses. A common approach is proportional: if residential space accounts for 60% of the non-shared floor area, 60% of the shared space is attributed to the residential FAR and 40% to the commercial FAR. The total of all floor area ratios for all uses generally cannot exceed the highest single-use FAR permitted on the lot, though some codes set an explicit combined cap. Check your local code carefully because the allocation method can determine whether your project fits.
When the Calculation Comes Out Over the Limit
A calculated FAR that exceeds the district maximum means the project as drawn can’t be permitted. Two situations behave differently.
An existing building that exceeds the current FAR limit is typically classified as a legally nonconforming structure, assuming it was built in compliance with the rules that existed at the time. It can continue operating as-is. Restrictions kick in when you want to expand or substantially alter it, and if the building is damaged beyond a certain threshold, some codes require rebuilding in compliance with the current FAR rather than the original footprint.
New construction that would exceed the limit requires a variance. Obtaining one means convincing a zoning board that strict application of the rule creates a hardship tied to the physical characteristics of the property, not a financial inconvenience. Common qualifying hardships include unusual lot shape, severe topography, or soil conditions that make the buildable area smaller than the lot dimensions suggest. The process typically involves filing an application, paying a fee, documenting the hardship, and presenting at a public hearing. Approval is never guaranteed. If you’re close to the limit, redoing the calculation with a careful reading of the local floor area definition, especially the exclusions for basements, parking, mechanical space, and stairwells, sometimes brings a project into compliance without a variance at all.