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Rain Garden and Swale Calculator
Size a rain garden or swale from roof runoff and soil infiltration, with required bed area, overflow, drain time and a planting count.
Rain Garden and Swale Calculator
Chlorova · updated for 2026
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This is a storage-and-drainage sizing estimate, not a permit design. Infiltration figures are typical saturated rates by texture class, and a real percolation test on your own soil, taken at the depth you intend to dig, is the number to design with. Local rules often govern rain gardens near a septic field, a well, a property line or a slope above a road, and many municipalities require an overflow path that does not load a neighbour. Get a percolation reading, check setbacks, and have a professional design any basin over about 500 sq ft or deeper than 2 ft.
A rain garden is a shallow basin that pretends to be a wetland for one afternoon. Its size falls out of two numbers: how much roof you ask it to drink and how fast your soil drains. A thousand square feet of roof sheds over six hundred gallons in one inch of rain, and soil infiltration, not rainfall, is what decides whether the basin empties by morning or breeds mosquitoes for a week. Enter your catchment, your soil and a bed size, and it returns the area you need, the overflow and the drain time.
Sizing a Basin to Its Soil
- Choose your soil texture honestly, since a percolation test beats guessing.
- Enter the roof or paved area that will drain into the bed.
- Set the design storm depth your region plans for, often one inch.
- Enter the basin depth you can dig, usually 4 to 6 in.
- Read the required bed area before you shape anything on the ground.
- Enter the area you actually have and check the overflow line.
- Compare drain time in native soil against the amended-soil line.
- Keep the bed well away from foundations and from any well.
- Set plant spacing to what your chosen species really needs.
- Plan an overflow path to lawn or barrel before the first storm.
Rainfall Volume, Storage and Infiltration
Capture follows a simple conversion: one inch of rain on one square foot of roof is 0.623 gallons, so runoff in gallons is catchment area times storm depth times 0.623. Storage works the same way in reverse, since one inch of water standing over a square foot of bed equals the same 0.623 gallons. Dividing runoff by depth and that factor gives the bed area a full-depth storm requires, so 800 sq ft of roof and a 5 in basin needs about 160 sq ft of garden.
Drain time is the real design test. Multiply bed area by the soil's infiltration rate in inches per hour and by 0.623 to get gallons leaving each hour, then divide stored volume by that figure. A rain garden should empty within 24 hours, and ideally far sooner, because water standing longer than a day is mosquito habitat and root rot. Where native clay fails that test, the engineered-mix line shows what a backfill of sand, compost and topsoil would buy you.
Worked Example: An 800 Square Foot Roof
- Select loam, which infiltrates at roughly 0.6 in per hour.
- Enter 800 sq ft of roof and a 1 in design storm.
- Runoff comes to 800 times 1 times 0.623, or 498 gallons.
- A 5 in deep bed stores 3.1 gallons for every square foot.
- Dividing shows the required bed area is about 160 sq ft.
- Suppose you can dig only 150 sq ft, which stores 467 gallons.
- The overflow line therefore reports 31 gallons to spill safely.
- Native loam drains that stored volume in roughly 8.3 hours.
- An engineered mix at 1.5 in per hour empties it in about 3.3.
- At 18 in spacing, the planting count lands near 67 plugs.
Infiltration by Soil Texture
Saturated infiltration rates used by the selector, in inches per hour. Your own percolation test beats any table, and clay ground needs roughly 0.5 in per hour to function.
| Texture | Infiltration | Bed size | Notes |
|---|---|---|---|
| Sand | 2.5 | Smallest | Retain water with compost |
| Sandy loam | 1.5 | Small | Usually no amendment |
| Loam | 0.6 | Moderate | The comfortable middle |
| Silt loam | 0.3 | Large | Amend or enlarge |
| Clay loam | 0.15 | Very large | Backfill advised |
| Clay | 0.05 | Impractical | Dig out and refill |
Key Terms
- Design storm
- The rainfall depth a basin is built to absorb, commonly one inch.
- Catchment area
- Roof or paving that sheds water into the garden, in square feet.
- Infiltration rate
- Inches of water a saturated soil accepts per hour.
- Berm
- The raised, compacted low edge that holds water inside the basin.
- Overflow
- The controlled escape path once stored water exceeds design depth.
- Engineered mix
- Sand, compost and topsoil blended to drain faster than native clay.
Details That Decide Whether It Works
- Shape matters less than level, because a basin dug on an unlevel floor holds water at one end and dries at the other, so screed the bottom before planting.
- Rain gardens are not ponds, and plants chosen for permanent wet feet will die in a bed that drains in eight hours, so pick species tolerant of both pulse and drought.
- Keep the bed at least 5 ft from a foundation on clay ground and 10 ft from a private well, since the whole point is to move water where soil will accept it.
- Connect the downspout before enlarging the bed, because many failed rain gardens are simply an unconnected roof with a decorative planting sitting above it.
- Mulch with shredded hardwood rather than rock, since rock stores heat and a mulched basin holds moisture through the dry weeks between storms.
Common Mistakes & Fixes
- Sizing from rainfall alone and forgetting the soil cannot drain it in time.
- Digging a very deep basin, which only moves the ponding problem underground.
- Planting bog species in a bed that is dry eight days out of ten.
- Loose uncompacted berm soil that erodes away in the very first heavy storm.
- Routing the overflow across a neighbour's grade or toward a foundation wall.
- Skipping the percolation test before you dig, then blaming plants for a slow drainage problem.
When to Reach for This
Use it when a downspout or paved strip discharges somewhere it should not, when a soggy low spot keeps forming after every storm, when a driveway or patio needs its runoff managed on site, and when a swale along a slope is being planned rather than guessed. Pair it with the soil percolation test for your real infiltration number, the rain barrel calculator for roofs you cannot drain into a bed, and the slope terrace tool on hilly ground.
Rain Garden and Swale Calculator: Frequently Asked Questions
How big should a rain garden be?
Start from the roof: multiply catchment area by storm depth and 0.623 for gallons, then divide by bed depth times the same factor. A 5 in basin needs roughly 3.1 gallons of storage per square foot. Most home downspouts land between 100 and 300 sq ft of garden.
Will it work in heavy clay?
Not in native clay, which drains far too slowly to empty a basin within a day. Either dig the garden out and refill with an engineered sand, compost and topsoil mix, or enlarge the area substantially. A percolation test tells you which of those you actually need.
How deep should the basin be?
Four to six inches is the practical range for a home garden, deeper only where you have a safe overflow. Too deep and the bottom stays wet while the edges dry, which stresses plants and invites mosquitoes. A shallow, wide, level basin does the same job more reliably.
Does a rain garden attract mosquitoes?
Not if it drains within 24 hours, since mosquito larvae need several days of standing water. A properly sized garden is wet for a morning or two after a storm and dry afterwards. If your soil fails that test, amend the soil rather than accepting a pond.
How far from the house should it sit?
Five feet is a common minimum on stable ground, and more on expansive clay or where a foundation is old, because the rule exists to keep added water from saturating bearing soil. Keep it at least 10 ft from a private well.
What differs between a swale and a rain garden?
A swale is a shallow vegetated channel that moves water along a slope, while a rain garden is a basin that soaks water in on level ground. Good designs often use both, with the swale feeding the garden. Sizing here is storage-based, so a long swale needs its volume divided into segments.
Sources & Further Reading
- Rain Gardens: Site Assessment, Design and Installation — Washington State Department of Ecology. Storage sizing method and the 24-hour drainage rule.
- Rain Gardens for Homeowners — Iowa State University Extension and Outreach. Bed depth, plant selection and foundation setback guidance.
- Soil Texture and Saturated Infiltration — USDA Natural Resources Conservation Service. Infiltration rate ranges by texture class used in the selector.
Chlorova Gardening Team
Builds, checks and documents every tool we publish
Chlorova editorial review
Checked against extension-service publications
Formula verified against published references · Updated
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