How to Use This Calculator
- Measure your drainage area — enter the width and length of the roof section, driveway, or hard surface that will drain into the rain garden. Use the horizontal footprint (plan view), not the sloped surface area.
- Select your surface type — this sets the runoff coefficient (how much rainfall becomes runoff vs. soaking into the surface itself).
- Enter design rainfall — 1 inch (25 mm) captures about 90% of typical storm events and is the standard for residential rain gardens in most of the US.
- Set ponding depth — the depth of standing water in the garden after a storm. Use 3–4 inches for clay sites; 5–6 inches for loam; up to 8 inches for fast-draining sandy soils.
- Pick your native soil type — this drives the drainage time check and whether you need to amend or replace the soil.
- Review results — check that your drainage time is under 24 hours. If not, increase garden area or improve soil drainage.
- Use the amendment table to know exactly how many cubic yards (or cubic meters) of each component to order, and how many bags to buy.
What Is a Rain Garden and Why Size It Correctly?
A rain garden is a shallow, planted depression that captures stormwater runoff from impervious surfaces like roofs and driveways. Instead of flowing into storm drains, the water sits in the basin and slowly infiltrates through the amended soil, filtering pollutants and recharging groundwater.
Sizing matters because an undersized garden overflows during every storm — defeating the purpose — while an oversized garden wastes yard space and planting budget. The goal is a garden that fills during the design storm and empties completely within 24 hours, keeping the soil healthy for plants and preventing mosquito breeding.
Sizing Methods Explained
Volume Method (this calculator's primary method)
Calculate the volume of runoff = Drainage Area × Rainfall Depth × Runoff Coefficient. Then divide by the ponding depth to get the required garden area. This method accounts for how much rainfall actually becomes runoff based on the surface type.
Formula: Garden Area = (Drainage Area × Rainfall × Runoff Coefficient) / Ponding Depth
Quick Rule: The 10/20 Rule (Alabama Extension)
For fast estimation: garden area = drainage area ÷ 10 for a 3-inch ponding depth, or ÷ 20 for a 6-inch ponding depth. This assumes a 0.90 runoff coefficient and is a useful cross-check against the volume method result.
Drainage Time Check
Hours to drain = Ponding Depth ÷ Infiltration Rate. For the garden to pass the 24-hour rule, your infiltration rate must exceed: Ponding Depth (in inches) ÷ 24. In clay soil at 0.3 in/hr with 6 inches of ponding, that's 20 hours — borderline acceptable. If the native soil can't drain fast enough, you either amend it, reduce ponding depth, or increase garden area.
The Standard Rain Garden Soil Mix
Most cooperative extension guidelines recommend replacing (or heavily amending) the top 8–18 inches of soil in the basin with a blend designed for both infiltration and plant health:
- Coarse sand (50%) — provides drainage. Use horticultural or concrete sand; avoid fine play sand, which compacts.
- Compost (25%) — feeds plants and hosts the microbial community that filters pollutants. Well-aged compost only.
- Topsoil or native soil (25%) — provides structure and minerals. If native soil is clay-heavy, skip it and use more compost.
This blend targets an infiltration rate of 1–2 in/hr, well within the acceptable window for most residential rain gardens. Mulch the surface (2–3 inches of shredded hardwood bark) to suppress weeds, prevent erosion, and further slow surface runoff into the basin.
Common Mistakes to Avoid
- Placing the garden in a perpetually wet spot — if an area is always damp, the water table is too high for a rain garden to drain properly.
- Measuring sloped roof area instead of the horizontal footprint. Rainfall is measured vertically, so you use horizontal area.
- Skipping the drainage time check — using the garden area formula without verifying whether the native soil can actually drain the water in 24 hours.
- Using fine play sand instead of coarse or horticultural sand. Fine sand compacts into a nearly impermeable layer over time.
- Too close to the foundation — keep the garden at least 10 feet (3 m) from any basement wall or crawl space to avoid directing water toward the structure.
- No overflow route — plan a safe overflow path (a grass swale or level spreader) for storms that exceed the design size.
Frequently Asked Questions
How big should a rain garden be?
It depends on how much impervious area drains into it, the design storm depth, and your soil type. For a 1-inch design storm and a standard roof-to-garden setup, rain garden area typically works out to 5–20% of the drainage area. Clay soils push toward 20%; fast sandy soils need only 5–8%. This calculator gives you the volume-method result alongside the standard 10/20 quick-check so you can cross-verify.
What is the best soil mix for a rain garden?
The most widely recommended blend is 50% coarse sand, 25% compost, and 25% topsoil. This achieves good drainage (1–2 in/hr), supports native plants, and filters common stormwater pollutants including nitrogen, phosphorus, and heavy metals. For very clay-heavy sites, replace the topsoil fraction with additional compost to get adequate drainage. Always use coarse or horticultural sand, not fine play sand, which compacts over time and dramatically reduces infiltration.
How long should a rain garden take to drain?
A properly designed rain garden should drain completely within 24 hours after a storm event. This is the standard used by most state extension guidelines and stormwater programs. If it takes longer, you risk plant stress, mosquito breeding, and soil compaction. To improve drainage: increase garden area (spreads the same water over more soil), amend the native soil with coarse sand to boost infiltration, or install a gravel/perforated-pipe underdrain.
How deep should a rain garden be?
Ponding depth (the depth of standing water above grade) is typically 3–6 inches for most residential gardens. Use 3 inches in clay-heavy soils where drainage is slow. Use 4–6 inches in loam or sandy loam. Up to 8 inches is acceptable in fast-draining sandy or gravelly soils. The total excavation depth is deeper: ponding depth plus the thickness of your amended soil layer (usually 8–18 inches), so total excavation is often 12–24 inches below existing grade.
Where should I put a rain garden?
Position the garden at least 10 feet from your house foundation, 25 feet from a septic system, and 50–100 feet from a drinking water well. Avoid areas that stay permanently wet or have bedrock within 2 feet of the surface — both indicate poor drainage potential. The best location is naturally downslope from impervious surfaces (roof, driveway), on level or gently sloped ground (under 12% slope), with reasonable sun exposure to support plant growth. Check your municipality for any permit requirements before digging.
Can I use this calculator for a driveway runoff rain garden?
Yes. Select "Asphalt driveway (0.90)" or "Gravel / packed dirt (0.70)" for the surface type. Driveways contribute significant runoff — a 20 × 40 ft asphalt driveway sheds roughly 600 gallons per inch of rain. Note that driveway runoff often carries oils and sediment, so using a pretreatment swale or sediment forebay before the main garden basin is recommended to prevent clogging the amended soil layer.
Do I need a permit to build a rain garden?
Requirements vary by jurisdiction. Most municipalities don't require a permit for small residential rain gardens built on private property, but some stormwater districts require notification or review if the system is connected to a drainage easement or within a regulated floodplain. Always check with your local public works or stormwater authority before construction. Many municipalities actively encourage rain gardens and may offer rebates or free plants to homeowners who install them.