A rain garden sizing calculator helps you design a bioretention garden that captures and filters stormwater runoff from your roof, driveway, or other hard surfaces. Properly sized rain gardens prevent localized flooding, remove pollutants before they reach streams, and recharge groundwater. Size depends on drainage area, soil infiltration rate, and local rainfall intensity.
Site Parameters
e.g., 1,200 sq ft = one side of a typical 2,400 sq ft ranch roof
Use 1" for typical design; 1.5" for heavier events
How to Size a Rain Garden
A properly sized rain garden can manage the stormwater from your entire roof or driveway, reducing flooding and sending that water back into the groundwater system. The design process starts with measuring your drainage area and understanding your soil's natural infiltration rate.
Step 1: Measure Your Drainage Area
The drainage area is the total impervious surface (roof, driveway, patio) that drains to the point where your rain garden will be located. For a roof, measure the footprint area in square feet — not the actual roof surface area. One side of a 2,400 sq ft single-story ranch is about 1,200 sq ft of drainage area per downspout.
Step 2: Identify Your Soil Type
Your native soil's drainage rate determines both the required garden size and depth. Do a simple percolation test: dig a 1-foot hole, fill with water, and measure how quickly it drains. Sandy soil drains 1+ inch per hour (fast), loam drains 0.5-1 inch per hour, and clay drains less than 0.5 inch per hour. Clay soils require larger, shallower rain gardens or soil amendment.
Step 3: Choose Your Design Storm
The design storm is the rainfall event your garden is designed to handle. A 1-inch storm is the standard for most residential rain gardens and covers the 90th percentile of storm events in most US regions. For areas with intense rainfall events (Southeast, Gulf Coast), designing for 1.5-2 inches provides better protection.
Step 4: Review the Sizing Results
The calculator provides garden area, recommended depth, soil amendment mix, plant count estimate, and a rough cost estimate. The garden area is typically 20-30% of the drainage area for loam soils, larger for clay, smaller for sandy soils. The depth is the maximum ponding depth before overflow occurs — usually 6-12 inches.
Planting and Maintenance
Plant rain gardens with native plants tolerant of both wet and dry conditions. The center zone (lowest point) should have the most flood-tolerant plants (sedges, rushes, swamp milkweed). The middle zone can have plants that tolerate occasional wet feet. The outer rim zone should have drought-tolerant natives that handle normal garden conditions. Mulch with 3 inches of shredded hardwood bark (not stone) to reduce erosion and retain moisture between storms.
FAQ
Is this rain garden sizing calculator free?
Yes, the rain garden sizing calculator is completely free with no limits. Size rain gardens for any drainage area and soil type. No signup or account required, and all calculations run locally in your browser.
Is my data private?
Yes. Every calculation runs locally in your browser. No site dimensions, drainage areas, or personal information are ever sent to a server. Your data stays completely private.
What is a rain garden?
A rain garden is a shallow planted depression designed to collect and absorb stormwater runoff from roofs, driveways, and other hard surfaces. Water flows into the garden, pools temporarily (12-24 hours), and slowly infiltrates into the soil rather than running off into storm drains. Rain gardens reduce flooding, filter pollutants, and recharge groundwater.
How big should a rain garden be?
The general rule is that a rain garden should be 20-30% of the drainage area it receives runoff from, adjusted for soil infiltration rate. Clay soils need larger gardens; sandy soils can handle smaller ones. This calculator gives you the precise recommended area based on your specific soil and rainfall inputs.
Where should a rain garden be located?
Place rain gardens at least 10 feet from building foundations and 25 feet from septic systems. They should be at least 10 feet from property lines. Ideal placement is at the bottom of a gentle slope, directly in the path of roof downspout or driveway runoff. Avoid low spots that already hold water — those areas may have high water tables.
Do rain gardens attract mosquitoes?
No. Rain gardens are designed to drain completely within 24-48 hours, which is not long enough for mosquitoes to breed (which requires 7-10 days of standing water). A properly designed and maintained rain garden should never hold standing water for more than 48 hours. If it does, the infiltration rate or overflow needs to be improved.
What plants work in a rain garden?
The best rain garden plants are native species that tolerate both wet and dry conditions, since rain gardens alternate between flooding and drought between rain events. Good choices include native sedges, rushes, switchgrass, swamp milkweed, Joe-Pye weed, blue lobelia, and buttonbush. Use the Native Plant Finder tool to identify species for your region.
Does the calculator support metric units?
Yes. Toggle between square feet/inches and square meters/millimeters. All dimensions and volumes update automatically when you switch units.