Estimate your
impervious-area total.
Add roofed areas, paving and other counted surfaces to estimate your property’s existing and proposed impervious percentage.
Educational estimates only. Check local classification and source measurements before using results for a permit.
Impervious Surface Calculator
Replace the sample inputs with your own measurements. Calculations update as you type; nothing is transmitted or stored.
Enter your areas
Use square feet. Zero means none. Negative values aren’t accepted.
YOUR ESTIMATED COVERAGE
Enter valid figures to see an estimate.
Request a Site Plan Quote ↗Not a certified calculation or determination of regulatory compliance.
Understand the percentage.
On a hypothetical 10,000 sq ft property with 2,000 sq ft of roofs, 900 sq ft of driveway and 300 sq ft of patio, the estimated current total is 32.0%.
Proposed counted area = existing counted area + proposed additions – existing counted area removed.
Estimated coverage (%) = proposed counted area ÷ lot area × 100.
What counts as a building or impervious surface is determined by the relevant local ordinance, not this page. Do not assume this percentage establishes permit eligibility.
Check the result before relying on it.
Verify lot size
Use an appropriate property-area source and local definitions.
List counted surfaces
Separate buildings, paving, decks, patios and other areas that the local rule classifies as impervious.
Add proposed changes
Enter added and removed surfaces, then confirm the resulting percentage with the reviewing jurisdiction.
What the result means.
These estimates are useful for planning and discussion with your permitting authority, not a substitute for a project-specific professional review.
Can I exclude permeable pavers?
Not automatically. Check the governing ordinance and any conditions for approved permeable surfaces.
Is impervious coverage the same as building coverage?
No. Impervious area may include paving and other surfaces beyond building footprints, depending on local definitions.
Does the calculator provide stormwater engineering?
No. It performs basic area arithmetic and does not design drainage, determine detention needs or replace professional engineering.
Need a site plan for your real property?
Send the address, property information and proposed improvements. We can discuss the drawings and coverage schedules appropriate to your project.