Coverage formula
Impervious percent = roof, driveway, patio, and other hard surfaces divided by total lot area.
RainYard Tools
Lot coverage
Impervious Area Percent Calculator
Calculate the share of a lot covered by roof, driveway, patio, and other hard surfaces.
Impervious share36.9%Share of lot area.
Hard surface2,950 sq ftTotal measured impervious area.
Pervious area5,050 sq ftRemaining lot area.
Example
On an 8,000 sq ft lot with 2,950 sq ft of hard surface, impervious cover is about 36.9%.
Why this helps
Impervious area is one of the fastest ways to screen how much runoff a lot creates and how a local fee might be calculated.
Questions
Common checks.
Does gravel count?
Some cities count compacted gravel as impervious or semi-impervious. Check local definitions.
Do permeable pavers count?
Rules vary. Some programs give partial credit if the installation meets standards.