How do you size a rain garden?

A common planning method estimates runoff volume as drainage area multiplied by design storm depth and runoff coefficient, then divides that volume by the planned ponding depth to estimate the required rain garden surface area.

What design storm should I use?

Many homeowner guides use roughly one inch of runoff, while UConn Extension describes a 1.3 inch runoff sizing example for residential rain gardens in Connecticut. Use the depth required by your local stormwater guidance if available.

How fast should a rain garden drain?

Home rain garden guides commonly expect standing water to infiltrate within about 24 hours. This calculator flags slower drawdown based on your ponding depth and measured infiltration rate.

Is this an engineering design?

No. It is a planning estimator for residential-scale rain gardens. It does not replace local code, utility marking, geotechnical review, overflow design, underdrains, or professional stormwater engineering.

Are my inputs uploaded?

No. The calculation runs locally in your browser and does not send your site measurements anywhere.

Rain Garden Sizing Calculator for Stormwater Runoff

Estimate rain garden area, runoff volume, drawdown time, and layout dimensions. Private by design: runs locally in your browser.

Inputs

Units Drainage surface Runoff coefficient

Drainage area

Plan-view area draining to the rain garden, in square feet.

Design storm depth

Rainfall or runoff depth to capture, in inches.

Target capture

Percent of the design-storm runoff you want to store.

Ponding depth

Temporary storage depth in the garden basin, in inches.

Measured infiltration rate

How quickly water drops in a test hole, in inches/hour.

Safety / freeboard factor

Adds extra surface area for uncertainty and minor sediment.

Advanced layout options

Oval length:width ratio Extra storage layer depth

Optional stone/soil pore storage depth, in inches.

Storage layer void ratio

Optional subsurface storage is treated as depth times void ratio. Use zero unless you are intentionally adding engineered storage.

Planning-level estimator only. Keep rain gardens away from foundations, septic systems, wells, steep slopes, and buried utilities; provide a safe overflow route for larger storms.

Results

What This Calculator Estimates

Runoff volume: drainage area x design storm depth x runoff coefficient x target capture.
Required surface area: runoff volume / effective storage depth, then multiplied by your safety factor.
Effective storage depth: ponding depth plus optional subsurface storage depth times void ratio.
Drawdown time: ponding depth / measured infiltration rate. This is a simple vertical infiltration check, not a full hydraulic model.

In imperial mode, 1 inch of rain over 1 square foot is about 0.623 US gallons. In metric mode, 1 mm over 1 square meter is 1 litre.

Assumptions, Sources, And Limits

Last reviewed: June 30, 2026

This tool is for early residential planning. It does not design underdrains, overflow structures, erosion protection, soil media, pollutant treatment, setbacks, permits, or utility-safe excavation.

EPA rain garden overview: Soak Up the Rain: Rain Gardens describes rain gardens as depressed landscape areas that collect runoff from roofs, driveways, or streets, allow water to soak into the ground, reduce runoff, and help filter pollutants.
Residential sizing guidance: UConn Extension CT NEMO Siting and Sizing describes sizing many residential gardens for about one inch of runoff, gives a 1.3 inch by 6 inch depth sizing example, and expects water to infiltrate within 24 hours.
Editable defaults: The default 1.3 inch design storm, 6 inch ponding depth, and 10% safety factor are starting points. Replace them with local stormwater guidance, measured infiltration, and actual site constraints.
Overflow: Even a correctly sized rain garden can overflow during larger storms. Route overflow away from foundations, neighboring property, septic systems, and erosion-prone slopes.

FAQ

What runoff coefficient should I use?

Use high values for impervious surfaces: about 0.90 to 0.95 for roofs, concrete, and asphalt. Use lower values for gravel, permeable pavers, turf, or mixed drainage areas. If in doubt, calculate a conservative high-flow case and a lower-flow case.

Is a deeper rain garden always better?

No. Deeper ponding reduces the surface area required, but it can be unsafe, unattractive, harder on plants, and slower to drain. Many residential guides use shallow basins around 3 to 8 inches deep.

Why does the calculator warn about drawdown?

Long-standing water can stress plants and create nuisance conditions. If the drawdown time is over about 24 hours, consider a shallower basin, a better-draining location, amended soil, an underdrain where allowed, or a different stormwater practice.

Can I put a rain garden next to my house?

Avoid placing infiltrating water near foundations. UConn Extension recommends keeping a rain garden at least 10 feet away from a house with a basement. Check local guidance for your site and building type.

Does this replace local stormwater rules?

No. Local rules may specify design storms, setbacks, overflow details, soil testing, and professional design requirements. This calculator is only a transparent planning estimate.

Rain Garden Sizing Calculator for Stormwater Runoff

Inputs

Results

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What This Calculator Estimates

Assumptions, Sources, And Limits

FAQ

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