Impervious Area (sq ft) Rainfall Depth (inches) Runoff Coefficient (0-1) Ponding Depth (inches) Calculate Area

The Power of Rain Gardens

Rain gardens are shallow, planted depressions designed to capture stormwater runoff from roofs, driveways, and other hard surfaces. Instead of sending polluted water rushing into storm drains, the garden temporarily holds it, allowing soil and plants to filter pollutants before the water slowly soaks into the ground. They help prevent flooding, recharge groundwater, and create habitat for birds and beneficial insects. Whether you live in a dense city or a suburban neighborhood, installing a rain garden is a beautiful way to manage water responsibly.

Determining the ideal size ensures your garden can handle typical rainfall events without overflowing. This calculator relies on a straightforward formula that factors impervious surface area, rainfall depth, the runoff coefficient, and your planned ponding depth. The runoff coefficient represents how much rainfall actually becomes runoff. Roofs and pavement have coefficients close to 1, meaning almost all rainfall flows off. Lawns or gravel surfaces might be 0.2 or 0.3 since they absorb more water. Ponding depth is the depth of temporary standing water your garden will hold before soaking in.

The Basic Calculation

To estimate the volume of runoff to capture, multiply the impervious area by the rainfall depth (converted from inches to feet) and the runoff coefficient. Dividing this volume by your ponding depth gives the minimum garden footprint:

Here A is the impervious area in square feet, C is the runoff coefficient, D is the rainfall depth in feet, and P is the ponding depth in feet. This simplified approach assumes the garden drains within a day or two, making it suitable for most residential scenarios.

Plant Selection Tips

Zone	Suggested Plants
Bottom (Wet)	Blue Flag Iris, Swamp Milkweed
Middle	Joe Pye Weed, Coneflower
Edge (Dry)	Black-Eyed Susan, Little Bluestem

Choose native plants with varying moisture tolerances. Species that thrive in your region will require less maintenance and provide food for pollinators. Mulch the surface after planting to retain moisture and suppress weeds while the garden establishes itself.

Maintaining Your Garden

For the first year, water new plants during dry spells to help them develop deep roots. Weed regularly and top up mulch as needed. Once established, rain gardens typically need minimal care beyond seasonal clean-up of debris. Check after heavy rains to ensure water drains within 24 hours—if not, adjust soil mix or increase the garden’s depth slightly.

By diverting runoff from storm drains, you reduce the burden on municipal systems and help keep local waterways cleaner. When neighbors see your lush rain garden brimming with flowers and butterflies, they may be inspired to plant their own, leading to a network of small but effective stormwater filters across your community.

Testing Soil and Expanding Capacity

Before finalizing your design, dig a small test hole and fill it with water. Measure how long it takes to drain completely. If it remains for more than twenty-four hours, amend the soil with compost or sand to boost infiltration. You can also enlarge the garden area so the captured water spreads out and soaks in more gradually.

Some homeowners add an underdrain—a gravel-filled trench with perforated pipe—to move excess water away during severe storms. It is not required for every location, but it can prevent prolonged saturation in dense soils.

Community Incentives and Resources

Check with local water agencies for rebates or neighborhood initiatives that help cover the cost of plants and soil amendments. Many cities distribute free native plants in spring or offer site evaluations by master gardeners.

If you belong to a homeowner association, review its landscaping rules before you dig. Providing photos of successful rain gardens can build support and demonstrate that these features enhance property values while reducing runoff.