Impervious Surface Calculator

Enter lot size and surface areas to see coverage analysis.

What impervious surface means

Impervious surfaces are any materials that prevent water from soaking into the ground -- concrete driveways, asphalt, rooftops, patios, and similar hard surfaces. When rain hits these areas, it runs off instead of absorbing into the soil.

Why municipalities limit it

Local zoning ordinances cap impervious coverage (typically 20-40% for residential lots) to control stormwater runoff. Too much hard surface strains municipal storm drains, causes flooding, reduces groundwater recharge, and degrades water quality in nearby streams.

Typical limits by zone

Residential lots commonly allow 25-40% impervious coverage. Smaller urban lots may allow 50-60%. Rural and large-lot zones may restrict coverage to 20% or less. Always verify with your local planning department.

How appraisers use this

Appraisers analyze lot coverage as part of the site description on the URAR. A property exceeding zoning limits creates a non-conforming condition that may affect marketability and value. Compliance with impervious surface limits is also verified during environmental reviews for FHA and VA loans.

What counts as impervious surface?

Rooftops, concrete or asphalt driveways, concrete or brick patios, sidewalks, pool decks, and any other hard, non-permeable material. Some jurisdictions also count compacted gravel. Lawns, gardens, mulch beds, and uncompacted gravel are generally considered pervious.

What is a typical impervious surface limit?

Most residential zones allow 25-40% lot coverage. A standard 10,000 sqft lot at 35% allows 3,500 sqft of hard surface. Dense urban single-family zones sometimes go higher (50-60%), while low-density suburban and rural zones may be stricter (20-25%).

Does a gravel driveway count as impervious?

It depends on your local ordinance. Loose, uncompacted gravel is usually considered pervious because water seeps through. Heavily compacted gravel or gravel over a solid base is often treated as impervious. When in doubt, check with your local zoning office or use permeable gravel pavers to stay clearly on the pervious side.