Tool

Calculating stormwater runoff

Enter your catchment area and soil type. This returns the runoff volume from a design storm and the rain garden surface area needed to infiltrate it within the drawdown period.

Runoff and sizing

Catchment area Square feet of the surface that drains to the garden. For a roof, use the footprint, not the sloped area.

Surface type Runoff coefficient: the fraction of rainfall that leaves the surface.

Design storm Inches of rainfall. 1.0 in is the common water-quality storm; check your local code.

Soil type Use a measured rate if you have one. A shovel-and-bucket perc test takes an afternoon.

Ponding depth Inches of standing water at full design depth. 6 in is typical; over 9 in raises safety and code issues.

Drawdown time How long the basin is allowed to hold water. Under 48 hours avoids mosquito breeding.

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Runoff, gallons

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Runoff, cubic feet

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Rain garden area, sq ft

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Sizing factor

Suggested shape	Dimensions	Area

Method: runoff volume = area × runoff coefficient × (storm depth ÷ 12). Design ponding depth = the lesser of your entered depth and (infiltration rate × drawdown hours), so the basin always empties within the window. Required surface = volume ÷ (design ponding depth ÷ 12). Infiltration during the storm is deliberately not credited against the design volume — this is the conservative convention used by most stormwater manuals. General reference only; confirm against your local standard before construction.

How the numbers work

Runoff volume

Rainfall depth times catchment area gives the total water that lands on the surface. The runoff coefficient subtracts what never leaves — absorbed by the surface, held in surface depressions, or evaporated. A roof releases almost everything, which is why roofs are the best catchment to work with and the easiest to plumb.

Why soil sets the size

A rain garden is a temporary reservoir plus a drain. The reservoir is the ponded water. The drain is the soil underneath. The basin has to hold the whole design storm at once, because the storm arrives far faster than the ground can absorb it — so the reservoir does the catching and the soil only has to empty it before the next one.

That is why soil controls the footprint indirectly. Slow soil cannot empty a deep basin inside the drawdown window, so the design depth gets capped at whatever the soil can actually drain in that time. A shallower basin holding the same volume has to be wider. Clay sites end up at 40 percent or more of the catchment for exactly this reason, and no amount of digging deeper fixes it — deeper water just sits longer.

What to do about clay

Three options, in order of cost. Reduce the catchment by splitting it across two gardens. Excavate 18 to 24 inches and backfill with an engineered mix — roughly 60 percent coarse sand, 30 percent topsoil, 10 percent compost — which behaves like a much faster soil. Or add a perforated underdrain to a storm sewer or daylight outlet, which converts the garden from an infiltration basin into a filtration basin. The third option needs a permit in most jurisdictions.

Run a percolation test before you trust a soil-type estimate. Dig a hole 12 inches deep at the proposed location, fill it with water, let it drain completely, then refill and measure the drop over one hour. That number, in inches per hour, goes into the measured-rate field above. Soil-type defaults can be off by a factor of five on a disturbed suburban lot.

Where this garden should not go

Within 10 feet of a foundation, or further if your soil is expansive.
Over a septic leach field or within its setback.
Where the seasonal high water table is within 2 feet of the basin bottom.
On a slope over about 12 percent without terracing.
Anywhere you have not called for utility locates first.