Rain Garden Size Calculator: Area, Runoff Volume & Drain Time
1. Enter the basic design
2. Site suitability check
Record what you know. Unknown or adverse conditions prevent the tool from calling the site a pass.
EPA material flags slopes over 10% as challenging and over 12% as potentially unsuitable.
Penn State Extension recommends at least 10 ft.
One EPA guide identifies less than 24 in as unsuitable.
3. Advanced layout, storage, presets, and sharing
Credit only intentionally designed, initially empty drainable storage. Clean washed aggregate commonly has much larger connected voids than ordinary soil pores; do not credit soil or aggregate storage that may already contain water.
Example presets
Decision summary
Worked rain garden sizing calculation
Example: a 1,000 ft² roof, 1-inch storm, 0.95 runoff coefficient, 6-inch ponding depth, 10% area contingency, 0.5 in/hr infiltration, and a 24-hour target.
- Convert storm depth: 1 in ÷ 12 = 0.0833 ft.
- Runoff volume: 1,000 ft² × 0.0833 ft × 0.95 = 79.17 ft³ = about 592 gal.
- Storage area: 79.17 ft³ ÷ 0.5 ft = 158.3 ft²; × 1.10 contingency = 174.2 ft².
- Drain-safe area: 0.5 in/hr × 24 hr = 12 in maximum drainable depth, so the planned 6 in controls and the drain-safe area is also 174.2 ft².
- Event ponding and drain time: 87.08 ft³ adjusted runoff ÷ 174.2 ft² = 0.5 ft = 6 in; 6 in ÷ 0.5 in/hr = 12 hr.
Visible sizing workflow
- Build the catchment. Measure each roof, driveway, patio, or lawn section in plan view and enter its length, width, and runoff coefficient.
- Choose the design storm. Select a generic starting scenario or enter the location-specific water-quality storm required by your municipality, watershed authority, or stormwater manual.
- Measure infiltration. Pre-wet a test hole, measure the water-level drop over time, repeat the test, and use the slowest credible rate.
- Set depth and drain target. Enter the temporary ponding depth, area contingency, and required 24- or 48-hour drain-down target.
- Screen the site. Check slope, foundation clearance, utilities, septic systems, wells, groundwater, bedrock, and the overflow destination.
- Compare the two area constraints. Use the larger of the storm-storage area and drain-down area, then confirm the bottom dimensions, total footprint, excavation, and local requirements.
Methodology, sources, and limits
The calculation conservatively gives no infiltration credit while the design storm is filling the basin, assumes the entered runoff coefficients are constant, and assumes the measured infiltration rate remains constant after the storm. The reported required area is the level ponding-bottom area, not the whole landscaped or excavated footprint.
- US EPA, Soak Up the Rain: Rain Gardens — rain gardens collect runoff from roofs, driveways, and streets; more engineered systems may use amended soil and drains. Page updated April 27, 2026.
- US EPA, Saving the Rain — use the local design storm when criteria exist; 1 inch is described only as typical in many areas.
- Penn State Extension, How to Make a Rain Garden — field drainage test, rough sandy/silty/clay rates, 24-hour drainage guidance, 10-foot foundation distance, and a 10–20% impervious-area rule of thumb.
- EPA-hosted Cooperative Extension rain garden guide — 6-inch maximum ponding example, overflow, slope, septic, groundwater, and foundation screening.
- US EPA, Saving the Rain: Green Stormwater Solutions — measure roofs in overhead plan view, allocate roof area by downspout, and screen slopes and flow paths.
- UConn Extension CT NEMO sizing guidance — source for the named Connecticut 1.3-inch example, not a universal default.
Planning estimate only. Local rules may require setbacks, test methods, safety factors, overflow details, pretreatment, engineered media, an underdrain, permits, or professional design.
Frequently asked questions
How do I measure roof area for one downspout?
Use the roof's horizontal plan area, not its sloped surface area. If similar downspouts split the roof evenly, multiply total roof area by the number feeding the garden divided by total downspouts; trace unusual roof valleys separately.
What storm depth should I use?
Use the water-quality or design storm required by your municipality, watershed authority, or stormwater manual. One inch is only a generic starting scenario; requirements vary by location and project.
How do I perform an infiltration test?
Dig a test hole at the intended bottom elevation, pre-wet it, refill it, and measure the water-level drop over a timed interval. Repeat in several holes or cycles and design with the slowest credible rate; local manuals may require a specific test.
Can a rain garden work in clay soil?
Sometimes, but do not rely on the clay screening preset for construction. Test the site. Very slow drainage may require a larger or shallower basin, amended engineered media, an approved underdrain, another location, or a different practice.
What do 24- and 48-hour drainage targets mean?
They are selected maximum times for temporary event ponding to infiltrate after the storm. Local guidance may require one or the other; a shorter target needs shallower water, faster infiltration, or more bottom area.
Does increasing area fix slow drainage?
It can reduce event ponding depth and therefore calculated drain time, but it does not repair unsuitable saturated soil, high groundwater, bedrock, or a poor outlet. Those conditions need site review or a different design.
How does ponding depth differ from excavation depth?
Ponding depth is temporary water above the level basin bottom. Excavation may also include vertical freeboard, engineered media, aggregate, mulch, and grading; this calculator's geometric excavation estimate uses ponding depth plus freeboard only.
What happens in storms larger than the design event?
The garden can fill and overflow. Provide a stable, non-erosive overflow route that carries excess water away from buildings, septic areas, neighboring property, and unstable slopes.
Is this an engineering design?
No. It is a transparent planning estimate. It does not replace utility locating, local setbacks, permits, geotechnical review, detailed media and underdrain design, or professional stormwater engineering.