Building Coverage Ratio Calculator
Calculate the building coverage ratio (%) from site area and building footprint. Reverse-calculate the maximum buildable area from a zoning limit, with corner-lot relaxation explained.
Last updated:
Input
What to calculate
Result
Coverage ratio
50%
Within the allowed coverage ratio of 60%
Site area
150 m²
Building footprint
75 m²
Coverage ratio (limit)
60 %
| Item | Value |
|---|---|
| Site area | 150 m² |
| Building footprint | 75 m² |
| Coverage ratio | 50 % |
| Buildable area at the allowed limit | 90 m² |
Coverage ratio = building footprint ÷ site area × 100. The building footprint is the horizontal projected area of the building seen from directly above. For corner lots and other sites that meet conditions set by the local authority, the limit may be relaxed by 10% (corner-lot relaxation). Always confirm whether a relaxation applies and the exact figures with your municipality's city-planning rules and ordinances.
How it works
- Building coverage ratio is found with "building footprint ÷ site area × 100". The building footprint is the horizontal projected area of the building seen from directly above (roughly the area enclosed by the centerlines of the first-floor exterior walls and columns).
- In "Find the coverage ratio" mode, enter the site area and building footprint to display the coverage ratio (%). Enter the allowed coverage ratio limit as well, and the tool checks whether you are within or over the limit.
- In "Find the buildable area" mode, enter the site area and the zoning coverage ratio to reverse-calculate the maximum buildable footprint (m²). Use it as a guide when searching for land or planning a floor layout.
- The coverage ratio limit is set by city planning for each zoning district. On sites that meet conditions designated by the local authority, such as corner lots, a "corner-lot relaxation" raising the limit by 10% may apply.
- Other relaxations exist as well, for example when building a fire-resistant structure within a fire-prevention district. Always confirm the figures and conditions that actually apply with your municipality's city-planning or building-guidance department.
- Results from this tool are estimates based on general principles. For official decisions such as building confirmation applications or purchase contracts, rely on professionals and the latest information from the authorities.
Frequently asked questions
- How is the building coverage ratio calculated?
- The building coverage ratio (kenpeiritsu) is building footprint area ÷ site area × 100. The footprint is the horizontal projection seen from directly above, roughly the area enclosed by the first-floor outer walls or column center lines.
- How does this differ from the floor area ratio?
- The coverage ratio uses the building footprint (the area seen from above) ÷ site area, showing how much of the land is covered. The floor area ratio instead uses total floor area of all stories ÷ site area — use the floor area ratio calculator when you need the total floor coverage.
- Can it reverse-calculate the buildable area or account for corner-lot relaxation?
- In the buildable-area mode it works backward from the site area and the designated coverage ratio to give the maximum footprint you may build. Corner lots that meet the conditions set by the local authority may receive a 10% relaxation, so confirm the actual figure with your municipality.
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