Impervious Use Calculator

Accurately calculate your property’s impervious surface area and percentage to understand stormwater runoff implications and comply with local regulations.

Calculate Your Impervious Use

What is Impervious Use?

Impervious use refers to any surface that prevents or significantly impedes the natural infiltration of water into the soil. These surfaces, commonly found in urban and suburban environments, include rooftops, driveways, sidewalks, patios, compacted gravel areas, and even highly compacted lawns. When rain falls on an impervious surface, it cannot soak into the ground; instead, it becomes stormwater runoff.

Understanding and calculating your property’s impervious use is crucial for several reasons, primarily related to environmental impact and compliance with local land development regulations. Excessive impervious surfaces contribute to increased stormwater runoff, which can lead to flooding, erosion, and pollution of waterways as it picks up contaminants like oil, chemicals, and sediment.

Who Should Use an Impervious Use Calculator?

• Property Owners: To understand their property’s environmental footprint and comply with local zoning or stormwater management ordinances.
• Developers and Builders: Essential for planning new construction or renovations, ensuring projects meet site coverage and stormwater runoff requirements.
• Architects and Landscape Designers: To design sustainable sites that balance functional needs with environmental responsibility, often incorporating permeable solutions.
• Real Estate Professionals: To inform clients about potential regulatory limitations or environmental considerations of a property.
• Environmental Consultants: For assessing site impact and recommending mitigation strategies.

Common Misconceptions About Impervious Use

• “Only concrete and asphalt count.” Many people mistakenly believe that only hard, paved surfaces are impervious. However, compacted gravel, some types of pavers with minimal gaps, and even highly compacted soil can significantly reduce water infiltration and are often classified as impervious.
• “It’s just about aesthetics.” While impervious surfaces are part of a property’s design, their primary impact is environmental and regulatory, not just visual.
• “My small patio won’t make a difference.” While individual small surfaces might seem insignificant, the cumulative effect of many small impervious areas across a neighborhood or city contributes substantially to stormwater issues.
• “Permeable pavers are 100% permeable.” While much better than traditional paving, permeable pavers still have some degree of imperviousness due to the paver material itself, though the joints allow infiltration. They are often treated differently in regulations but are not entirely “invisible” to water.

Impervious Use Formula and Mathematical Explanation

The calculation of impervious use is straightforward, focusing on the total area of non-permeable surfaces relative to the total property area. The goal is to determine the percentage of your land that contributes to stormwater runoff rather than absorption.

Step-by-Step Derivation

1. Identify all Impervious Surfaces: Begin by listing every surface on your property that prevents water infiltration. Common examples include building rooftops, driveways, patios, walkways, and any other paved or compacted areas.
2. Measure Each Impervious Area: For each identified surface, measure its area (e.g., length × width for rectangular shapes).
3. Sum Individual Impervious Areas: Add up all the measured impervious areas to get the “Total Impervious Area.”
4. Determine Total Lot Area: Obtain the total area of your property from your property deed, survey, or local assessor’s office.
5. Calculate Impervious Coverage Percentage: Divide the Total Impervious Area by the Total Lot Area and multiply by 100 to express it as a percentage.
6. Calculate Permeable Area and Percentage: Subtract the Total Impervious Area from the Total Lot Area to find the Permeable Area. Then, divide the Permeable Area by the Total Lot Area and multiply by 100 for the Permeable Coverage Percentage.

Variable Explanations

Variables for Impervious Use Calculation

Variable	Meaning	Unit	Typical Range (Residential)
Total Lot Area	The entire area of the property parcel.	Square feet (sq ft) or Square meters (sq m)	5,000 – 20,000 sq ft
Building Footprint Area	The ground area covered by the main building(s).	Square feet (sq ft) or Square meters (sq m)	1,000 – 4,000 sq ft
Driveway Area	Area of paved or compacted driveways and parking.	Square feet (sq ft) or Square meters (sq m)	300 – 1,500 sq ft
Patio/Deck Area	Area of solid patios, decks, or concrete slabs.	Square feet (sq ft) or Square meters (sq m)	100 – 800 sq ft
Walkway Area	Area of paved paths and sidewalks.	Square feet (sq ft) or Square meters (sq m)	50 – 300 sq ft
Other Impervious Area	Any additional non-permeable surfaces (e.g., pool decks, compacted gravel).	Square feet (sq ft) or Square meters (sq m)	0 – 500 sq ft
Total Impervious Area	The sum of all individual impervious surfaces.	Square feet (sq ft) or Square meters (sq m)	Calculated
Impervious Coverage Percentage	The proportion of the lot covered by impervious surfaces.	Percentage (%)	15% – 60%
Permeable Area	The total area of the lot that allows water infiltration.	Square feet (sq ft) or Square meters (sq m)	Calculated
Permeable Coverage Percentage	The proportion of the lot covered by permeable surfaces.	Percentage (%)	40% – 85%

Practical Examples of Impervious Use Calculation

Let’s walk through a couple of real-world scenarios to illustrate how the impervious use calculator works and what the results mean.

Example 1: Standard Suburban Home

A homeowner wants to understand their property’s impervious coverage before adding a new patio. Their property details are:

• Total Lot Area: 8,000 sq ft
• Building Footprint Area: 1,800 sq ft
• Driveway Area: 700 sq ft
• Patio/Deck Area: 300 sq ft (existing)
• Walkway Area: 100 sq ft
• Other Impervious Area: 0 sq ft

Calculation:

1. Total Impervious Area = 1,800 + 700 + 300 + 100 + 0 = 2,900 sq ft
2. Impervious Coverage Percentage = (2,900 / 8,000) × 100 = 36.25%
3. Permeable Area = 8,000 – 2,900 = 5,100 sq ft
4. Permeable Coverage Percentage = (5,100 / 8,000) × 100 = 63.75%

Interpretation: This property has 36.25% impervious coverage. If local zoning limits impervious surfaces to 40%, the homeowner has 3.75% (or 300 sq ft) remaining capacity for new impervious additions like a larger patio, without needing special permits or mitigation.

Example 2: Urban Property with Extensive Paving

A small urban lot owner is concerned about stormwater fees and wants to assess their impervious use. Their property details are:

• Total Lot Area: 4,500 sq ft
• Building Footprint Area: 1,500 sq ft
• Driveway Area: 600 sq ft
• Patio/Deck Area: 400 sq ft
• Walkway Area: 150 sq ft
• Other Impervious Area: 200 sq ft (compacted gravel parking)

Calculation:

1. Total Impervious Area = 1,500 + 600 + 400 + 150 + 200 = 2,850 sq ft
2. Impervious Coverage Percentage = (2,850 / 4,500) × 100 = 63.33%
3. Permeable Area = 4,500 – 2,850 = 1,650 sq ft
4. Permeable Coverage Percentage = (1,650 / 4,500) × 100 = 36.67%

Interpretation: With 63.33% impervious coverage, this property likely exceeds typical residential limits, especially for a smaller lot. The owner might face higher stormwater utility fees or be required to implement stormwater mitigation measures if they plan any further development. Reducing some of the “Other Impervious Area” by replacing compacted gravel with permeable pavers could be a good strategy. For more information on reducing runoff, consider our Green Infrastructure Benefits Guide.

How to Use This Impervious Use Calculator

Our impervious use calculator is designed for ease of use, providing quick and accurate results to help you understand your property’s stormwater impact. Follow these simple steps:

Step-by-Step Instructions

1. Gather Your Property Information: You’ll need your total lot area (from your property deed or survey) and the approximate areas of all impervious surfaces on your property. This includes your home’s footprint, driveway, patios, decks, and walkways.
2. Enter Total Lot Area: Input the total area of your property in square feet into the “Total Lot Area” field.
3. Enter Individual Impervious Areas: For each category (Building Footprint, Driveway, Patio/Deck, Walkway, Other Impervious), enter the corresponding area in square feet. If a category doesn’t apply to your property, enter ‘0’.
4. Real-time Calculation: As you enter values, the calculator will automatically update the results. There’s also a “Calculate Impervious Use” button if you prefer to click after entering all data.
5. Review Results: The “Calculation Results” section will display your primary result (Impervious Coverage Percentage) prominently, along with intermediate values like Total Impervious Area and Permeable Area.
6. Reset if Needed: If you want to start over or test different scenarios, click the “Reset” button to clear all fields and restore default values.
7. Copy Results: Use the “Copy Results” button to quickly save the calculated values and formula explanation to your clipboard for documentation or sharing.

How to Read Results and Decision-Making Guidance

The most critical result is the Impervious Coverage Percentage. This number directly indicates how much of your property contributes to stormwater runoff. Local regulations often set limits on this percentage, especially for new construction or significant renovations. For example, a municipality might cap impervious coverage at 35% for residential zones.

• Below Limit: If your percentage is below the local limit, you likely have room for additional impervious surfaces or are in good standing regarding stormwater management.
• Near Limit: If you are close to the limit, any new impervious additions will require careful planning, potentially involving permeable alternatives or stormwater mitigation. Our Permeable Pavement Guide can offer solutions.
• Above Limit: If your percentage exceeds local limits, you may face fines, be required to implement stormwater best management practices (BMPs), or be denied permits for further development. This often necessitates exploring options to reduce existing impervious surfaces or install green infrastructure.

Understanding your impervious use is the first step towards responsible land management and compliance. For more details on local rules, consult your city’s zoning ordinances, often found on municipal planning department websites. You can also check our Zoning Ordinances Explained resource.

Key Factors That Affect Impervious Use Results

Several factors significantly influence a property’s impervious use and its implications. Understanding these can help property owners and developers make informed decisions.

• Total Lot Size: Smaller lots often have a higher impervious coverage percentage simply because essential structures (house, driveway) take up a larger proportion of the total area. Larger lots offer more flexibility for permeable landscaping.
• Building Footprint: The size and shape of the primary structure directly contribute to imperviousness. Multi-story homes can offer more living space with a smaller footprint, potentially reducing impervious coverage compared to sprawling single-story designs.
• Driveway and Parking Requirements: Local zoning often dictates minimum driveway lengths and parking spaces, which can significantly increase impervious area. The choice of materials (asphalt vs. permeable pavers) also plays a huge role.
• Patio and Deck Design: Large, solid concrete or wood decks contribute to imperviousness. Incorporating permeable patio materials or elevated decks that allow water flow underneath can reduce this impact.
• Local Zoning and Stormwater Ordinances: These are paramount. Municipalities set limits on maximum impervious coverage to manage stormwater runoff. Exceeding these limits can lead to permit denials, fines, or requirements for costly mitigation measures. Our Site Coverage Rules article provides more context.
• Soil Type and Permeability: While not directly calculated by the impervious use calculator, the underlying soil’s ability to absorb water (its permeability) is crucial. Even with some permeable surfaces, poor soil drainage can exacerbate runoff issues.
• Landscaping Choices: Extensive hardscaping (retaining walls, large rock gardens with weed barriers) can contribute to imperviousness or reduce effective permeable area. Green infrastructure like rain gardens and bioswales can help manage runoff from impervious surfaces. Learn more about this in our Sustainable Landscaping Guide.
• Climate and Rainfall Patterns: Areas with heavy or frequent rainfall have a greater need to manage impervious surfaces to prevent flooding and erosion. Regulations in such areas are often stricter.

Each of these factors plays a role in determining a property’s overall impervious use profile and its environmental and regulatory consequences. Proactive planning and the use of permeable alternatives can significantly mitigate negative impacts.

Frequently Asked Questions (FAQ) about Impervious Use

Q1: What is the difference between impervious and permeable surfaces?

A: Impervious surfaces, like concrete or asphalt, prevent water from soaking into the ground, leading to runoff. Permeable surfaces, such as lawns, gardens, or permeable pavers, allow water to infiltrate the soil, reducing runoff and recharging groundwater.

Q2: Why is calculating impervious use important?

A: It’s important for environmental protection (reducing stormwater runoff, pollution, and flooding), compliance with local zoning and stormwater regulations, and potentially reducing stormwater utility fees in some areas. Understanding your impervious use helps manage your property’s environmental impact.

Q3: Are all decks considered impervious?

A: Generally, solid decks (e.g., concrete slabs, solid wood decks directly on the ground) are considered impervious. Elevated decks with gaps between boards that allow water to pass through to permeable ground below might be treated differently, sometimes as partially impervious or even permeable, depending on local regulations. Always check your local ordinances.

Q4: Does compacted gravel count as impervious?

A: Yes, typically. While gravel itself is porous, when it’s compacted for driveways or paths, the spaces between the stones are significantly reduced, severely limiting water infiltration. Most jurisdictions classify compacted gravel as an impervious surface for stormwater management purposes.

Q5: What are common impervious coverage limits for residential properties?

A: Limits vary widely by municipality and zoning district, but residential properties often have impervious coverage limits ranging from 20% to 60% of the total lot area. Urban areas with smaller lots might have higher limits, while suburban or rural areas might have lower ones. Always consult your local planning department for specific regulations regarding impervious use.

Q6: How can I reduce my property’s impervious use?

A: You can reduce imperviousness by replacing traditional paving with permeable pavers, gravel, or grass pavers; installing rain gardens or bioswales; using green roofs; or reducing the size of unnecessary paved areas. Directing downspouts to permeable areas instead of driveways also helps manage runoff.

Q7: Does a swimming pool count as an impervious surface?

A: Yes, the surface area of a swimming pool is generally considered impervious because it prevents water infiltration. The water in the pool itself does not soak into the ground, and the pool structure contributes to the overall impervious footprint.

Q8: Can I get a permit if my impervious use exceeds the limit?

A: It depends on local regulations. Some jurisdictions offer variances or allow exceeding limits if you implement approved stormwater mitigation measures, such as installing significant green infrastructure (e.g., large rain gardens, underground detention systems) that offset the impact of the excess impervious area. This often requires professional engineering and planning. For more on managing stormwater, see our Stormwater Management Calculator.

Related Tools and Internal Resources

Explore our other valuable tools and articles to further enhance your understanding of property development, environmental impact, and sustainable practices:

• Stormwater Management Calculator: Calculate runoff volumes and design basic stormwater solutions.
• Permeable Pavement Guide: Learn about the benefits and types of permeable paving options.
• Site Coverage Rules Explained: Understand how local ordinances define and regulate site coverage.
• Green Infrastructure Benefits Guide: Discover the advantages of incorporating natural systems into your property.
• Sustainable Landscaping Guide: Tips for designing eco-friendly and water-efficient outdoor spaces.
• Zoning Ordinances Explained: A comprehensive overview of how zoning impacts property use and development.