Impervious Surface Ratio Calculator
Check the impervious surface ratio of a lot against stormwater limits, combining building footprint, driveways and hard landscaping.
- Typical Cap
- 45%
- Stormwater limit
- Driveway
- 600–900 sqft
- Two-car
- Patio
- 300–500 sqft
- Hard surface
Formula & Statutory Rule
Formula
impervious_pct = (footprint + hardscape) / lot_area × 100
Statutory Rule
typical_cap = 45%; permeable_pavers = excluded or discountedRegional Benchmark Reference
| Jurisdiction / Tier | Rate or Limit | Statutory Note |
|---|---|---|
| Typical cap | 45% | Stormwater |
| Driveway | 600–900 sqft | Two-car |
| Patio | 300–500 sqft | Hard surface |
Frequently Asked Questions
What is an impervious surface?
Any hard surface that prevents water infiltration, including roofs, driveways, patios and walkways.
Why is impervious ratio regulated?
To limit stormwater runoff and protect drainage systems and water quality.
What is a typical cap?
Many jurisdictions cap impervious coverage at 45% of the lot area.
Do permeable pavers count?
Permeable pavers may be excluded or discounted from the impervious calculation.