Building Coverage Ratio: How to Calculate Kenpeiritsu

Calculates the building coverage ratio as building area ÷ site area × 100 (%). Each zoning district has its own upper limit.

Explanation

The building coverage ratio, kenpeiritsu in Japanese, is the share of a plot that a building covers when seen from directly above. Filling a site edge to edge would block light and air and let fire spread from roof to roof, so every zoning district caps the ratio. It is usually the first constraint a design runs into.

\text{coverage} = \dfrac{\text{building area}}{\text{site area}} \times 100\ (\%)

Building area is the footprint: the outline of the outer walls projected straight down. It is normally the ground floor, but where an upper storey overhangs, the wider outline governs.
Site area is the plot, also measured as a horizontal projection.

Example

Take the defaults, a building area of 60 m² on a site of 150 m².

\dfrac{60}{150} \times 100 = 40\ (\%)

The coverage ratio is 40%. If this plot happened to be capped at 60%, the footprint could grow to 150 × 0.6 = 90 m², so there would be 30 m² of headroom left.

Notes

The limit is set plot by plot, through the zoning district and the city plan. There is no single national figure, so check the municipal zoning map before you rely on a number.
Corner plots, and fire-resistant buildings in designated fire-prevention districts, can qualify for a relaxed limit.
Do not confuse building area with total floor area. A two-storey house does not have twice the building area, because only the footprint counts. Dividing total floor area by site area gives the floor area ratio, a separate limit that applies at the same time.
Deep eaves and balconies that project well beyond the wall begin to count towards the building area. The rules on what is and is not included are worth checking with an architect.