The Situation
You have a downspout that drains about 1,200 square feet of roof area. Currently, it dumps water right next to the foundation, creating a soggy spot and potential water damage. Instead of running a pipe to the street, you want to build a rain garden about 15 feet from the house. The garden will absorb the roof runoff, look attractive, and support local pollinators. Your soil is sandy loam, and you live in a region that gets about 3.5 inches of rain in a typical heavy storm.
Which Calculator to Use
The Rain Garden Calculator determines the right garden size (area and depth) based on your contributing drainage area, soil type, and local rainfall.
Walking Through the Inputs
Drainage Area (A = 1,200 sq ft)
This is the roof area that drains to this single downspout. You can estimate this by looking at your roof from above — the footprint of the section that slopes toward this downspout. In this case, it is roughly a 30-foot by 40-foot section of roof, and this downspout handles one side: 1,200 square feet.
Impervious Percentage (100%)
The contributing area is entirely roof, which is 100% impervious. We set this to 100% (the calculator may express this as a percentage or as a coverage type).
Rainfall Depth (P = 3.5 inches)
This is the design storm depth. We want the rain garden to handle a typical heavy rain event. A 1-year, 24-hour storm of 3.5 inches is a reasonable design target for a residential rain garden. This means the garden will handle most everyday storms completely and overflow only during larger events.
Soil Class (Sandy Loam)
Sandy loam has an infiltration rate of about 1 to 3 inches per hour. This is good news — the rain garden does not need to be as large as it would in clay soil because the water soaks in relatively quickly.
Maximum Ponding Depth (dp = 8 inches)
This is how deep the water will temporarily pool in the garden during a storm. 8 inches is a common maximum for residential rain gardens. Deeper ponding means a smaller garden footprint but also means the garden takes longer to drain. Most plants in a rain garden can tolerate 6-12 inches of temporary ponding.
Target Drawdown Time (td = 24 hours)
The garden should drain completely within 24 hours after a storm. This prevents mosquito breeding and keeps the garden healthy. With sandy loam soil, 24-hour drawdown is easily achievable.
The Results
The calculator determines:
- Rain garden area: Approximately 80-100 square feet (roughly a 10-foot by 10-foot garden)
- Amended soil depth: 18-24 inches of amended planting soil over 6-12 inches of gravel
- Storage volume: About 50-65 cubic feet
What This Means for Your Project
A 100-square-foot rain garden is a very manageable weekend project:
- Shape: An oval or kidney shape about 10 feet by 10 feet looks natural and is easy to dig
- Location: 15 feet from the house is ideal — far enough to protect the foundation, close enough to reach with a pipe from the downspout
- Pipe: Run a 4-inch PVC pipe from the downspout to the garden inlet
- Overflow: Include an overflow outlet (6-inch pipe) at the maximum ponding depth that directs excess water away from the house
- Plants: Choose native plants that tolerate both wet and dry conditions: black-eyed Susan, blue flag iris, switchgrass, Joe-Pye weed, or local native alternatives
Estimated Cost
- Excavation (DIY): Free (your labor)
- Amended soil mix (2 cubic yards): $80-120
- Gravel for base (1 cubic yard): $40-60
- Native plants (15-20 plants): $150-300
- PVC pipe and fittings: $40-60
- Total: $310-540
Try This Scenario
Try this scenario in the Rain Garden CalculatorNext Steps
- Read our Rain Garden Design for Beginners guide for complete planting and construction details
- Test your soil infiltration rate with the home percolation test to verify it matches “sandy loam”
- Check your local extension service for native rain garden plant lists specific to your region