Calculate lot coverage percentage from building footprint and lot area, or find the maximum building footprint or required lot area for a coverage limit.
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Calculator Functions
| Function | What it does | What it requires |
|---|---|---|
| Lot coverage percentage | Finds the share of a lot that the counted footprint covers | Requires the total lot area and the total counted footprint |
| Maximum building footprint | Finds the largest footprint a lot allows at a coverage limit | Requires the total lot area and the coverage limit in percent |
| Required lot area | Finds the smallest lot that keeps a footprint within a coverage limit | Requires the total counted footprint and the coverage limit in percent |
Areas go in as square feet, square meters, acres or hectares, and the lot and the footprint can each use a different unit. The lot coverage percentage function has an optional coverage limit, which adds the footprint allowed at that limit and the amount left or the amount over. The area result unit and the display precision of 2, 4 or 6 decimals can be changed.
Lot Coverage Formulas
| LC | Lot coverage (%) |
| BA | Total counted footprint: the horizontal area the local rules count, added once, not the sum of every floor |
| LA | Total lot area, in the same unit as BA |
Lot coverage is the percentage of a lot's area that is occupied by buildings [1]. What counts as occupied is set by the local zoning code. One county, for example, counts the area under roof but leaves out the first 2 feet of roof overhang on the main building [1]. Whether decks, driveways or pools count depends on the code, so the local definition has to be read first. Mixed units are converted before dividing: 1 ft² is 0.09290304 m² and 1 hectare is 10,000 m² [2], and 1 acre is 43,560 ft².
Lot coverage percentage
The counted footprint is divided by the lot area and multiplied by 100. A 10,000 ft² lot with three buildings of 1,000, 4,000 and 1,200 ft² has a counted footprint of 6,200 ft², so LC = 6,200 ÷ 10,000 × 100 = 62%. With a coverage limit of 40% entered, the footprint at the limit is 4,000 ft² and the lot is 2,200 ft² over it. A footprint larger than the lot is rejected, since it usually means a unit mix-up or floor area entered as footprint.
Maximum building footprint
The lot area is multiplied by the coverage limit. A quarter-acre lot is 10,890 ft², and at a 35% limit the largest counted footprint is 10,890 × 0.35 = 3,811.5 ft².
Required lot area
The footprint is divided by the coverage limit as a fraction. A 2,400 ft² footprint under a 30% limit needs a lot of at least 2,400 ÷ 0.30 = 8,000 ft², which is 0.1837 acres.
Limits of This Estimate
- This is area arithmetic, not zoning approval. A result at or below the limit entered does not establish that a project complies.
- The local code decides what is counted and how the lot area is defined. Building coverage and impervious-surface coverage can be separate limits with separate percentages.
- Setbacks, easements, height limits and floor-area ratio can restrict a building further than the coverage limit does. Floor-area ratio counts the area of every floor, so it is a different measure from lot coverage.
Sources
- Maricopa County, Arizona, Planning and Development, Lot Coverage Calculations (DD-2021-01): a zoning ordinance's definition of lot coverage and an example of what a local code leaves out
- NIST, Guide for the Use of the International System of Units (SP 811), Appendix B.8: square foot and hectare in square meters
