Rain garden sizing calculator

The core formula is drainage area times a runoff coefficient, scaled by your regional design storm, then multiplied by a soil-and-depth sizing factor: roof area × 0.95, plus pavement area × 0.90, plus lawn area × 0.20, gives the effective runoff-generating area. Roofs and driveways shed nearly all the rain that falls on them. Lawn sheds much less, because grass and topsoil soak up a lot of it first. Those coefficients are the same values reproduced in Rational Method runoff tables used across state and municipal stormwater manuals.

That effective area gets multiplied by your state's design storm depth (in inches, relative to a 1-inch baseline), then by a soil-and-depth sizing factor taken directly from the University of Wisconsin-Madison Arboretum's rain garden worksheet, itself built on Wisconsin DNR guidance. That factor already does the work of dividing by a soil's infiltration rate: sandy soil at 8 inches deep uses a factor of 0.08, clay at 3 to 5 inches deep uses 0.43. Slower-draining soil and a shallower garden both push that number up, which spreads the same runoff over more square footage.

The state dropdown sets two starting defaults: a design storm depth and a soil type. Neither one is a precise site-specific measurement, that would need your street address and a percolation test, not a state name. Where a state publishes its own sizing figure we used it directly (Wisconsin and Iowa both do). Everywhere else we used NOAA's published U.S. climate regions to set a reasonable regional rainfall intensity, and USDA NRCS soil region descriptions (Piedmont clay, Coastal Plain sand, glacial till loam, Blackland Prairie clay, and so on) to set a reasonable regional soil default. Every state entry in the calculator lists its specific basis. Switch off "use state defaults" any time you have a real percolation test or a county soil survey result, that beats any default on this page.

UConn CLEAR's Rain Garden App is a genuinely well-built tool with its own sizing calculator, a sizing map, and a soil drainage map, and it is worth using if you live in one of the 23 states it covers with a state-specific preset. This calculator exists for the other 27 states, including several of the largest gardening markets in the country, where that app falls back to a generic answer.