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Yard Drainage Fixes

Rain Garden Size Calculator

Most rain garden calculators divide your roof area by ten and stop. This one sizes the garden from the water it has to hold, then does the thing that decides whether it is a garden or a pond: it checks whether your soil can empty it in time, tells you what overtops in a bigger storm, and turns the area into a dig list. For how a rain garden works and what to plant, see rain gardens.

What drains into it?Square feet of each surface that will actually reach the garden. Leave what doesn't apply at 0.
Storm to hold without overflowing

A 1-inch storm covers roughly nine in ten rain events; bigger storms overtop into the overflow by design.

Ponding depth

Deeper holds more in a smaller footprint but takes longer to drain. Slow soil wants shallow.

Soil it sits in

Water to hold

590 gal ≈ 79.2 cu ft in a 1-inch storm

Garden size

160 sq ft

About 18 × 9 ft as a rounded oval, ponding 6 in deep. That's 16% of the hard surface feeding it. Any shape works if the flat bottom keeps this area.

Dig list

Dig 9 in deep

  • 6 in ponding + 3 in mulch — about 4.4 yd³ out
  • 1.5 yd³ of shredded hardwood mulch
  • Berm on the low side about 9 in high, with an overflow notch

The soil should empty it within a day

~12 hrs to empty at 0.5 in/hr

At this rate 6 inches of ponded water soaks away in about 12 hours — well inside the 24-to-48-hour window a rain garden needs to stay a garden rather than a pond. Confirm it with a percolation test in the actual spot before you dig.

Do the one-hour soil test →

Overflow: in a 2-inch storm about 590 gallons overtop this garden. That's normal — but it means the notch in the berm has to point somewhere safe, never back toward the house.

How it goes together

Rain garden cross-sectionCross-section of a rain garden. A house sits uphill on the left; a downspout pipe carries roof water at least 10 feet to a shallow, flat-bottomed bowl dug into the lawn. Water ponds about 6 inches deep over a layer of mulch, and arrows show it soaking downward into the native soil. A low berm on the downhill side holds the water in, with a notched overflow so a storm larger than the garden can hold spills toward a safe outlet rather than back toward the house.10 ft or moreponds 6 in, then soaks inberm + overflownative soil — must drain in 24–48 hdownspout

What this doesn't tell you

This is a planning size. It assumes the garden has a flat bottom, that all the water you entered actually reaches it, and rough infiltration rates for each soil — your real ground, a seasonal water table, and how wet the soil already is all shift the drain-down. It can't tell you the spot is at least 10 ft from the foundation and downhill of it, clear of a septic field and utilities, or that the overflow has somewhere to go. Test the soil in the actual spot, then plant it.

How to read your results

  • Garden size is the flat-bottom area that holds your chosen storm at your chosen ponding depth. The footprint suggestion is a rounded oval, because water spreads evenly across one; a long thin garden across a slope works the same way as long as the bottom is level.
  • The drain-down verdict is the part that matters. A rain garden has to be empty within 24 to 48 hours. Sandy and loamy soils clear that easily; clay usually can't without amended soil and a shallower bowl, and sometimes can't at all.
  • Overflow is not a failure — it's a design feature. Size for the common storm, and give the rare one a safe path through a notch in the berm.
  • The dig list is what to order: excavation volume, sandy mix if you are amending, and mulch. It is easy to underestimate — a 160 sq ft garden dug 21 inches deep is over 10 cubic yards of soil coming out.

The formula — check it yourself

runoff (cu ft) = area (sq ft) × runoff factor × storm (in) ÷ 12
garden area (sq ft) = runoff ÷ (ponding depth (in) ÷ 12)

Roofs shed about 95 percent of what lands on them and paving about 90 percent; a lawn sheds 15 to 35 percent depending on the soil. A 1,000 sq ft roof in a 1-inch storm makes about 79 cubic feet — roughly 590 gallons. Six inches of ponding is half a foot, so the garden needs about 160 sq ft. The drain-down time is simply the ponding depth divided by the soil's infiltration rate: 6 inches at half an inch an hour is 12 hours.

Sizing chart

Roof area draining to the garden, 1-inch storm, loam soil.

Roof areaWater to holdGarden at 6 in deepGarden at 4 in deep
250 sq ft150 gal40 sq ft60 sq ft
500 sq ft300 gal80 sq ft120 sq ft
750 sq ft440 gal120 sq ft180 sq ft
1,000 sq ft590 gal160 sq ft240 sq ft
1,500 sq ft890 gal240 sq ft360 sq ft
2,000 sq ft1,200 gal320 sq ft475 sq ft

The 4-inch column is the one to use in slow soil — more footprint, less standing time. Add any paving or lawn that also drains to the spot.

When a rain garden is the wrong answer

A rain garden is a soak-away, so it lives or dies by the soil. If the one-hour soil drainage test drops less than half an inch, stop: water this ground can't absorb needs to be moved, not stored. A swale or dry creek bed carries it across the yard on the surface; regrading stops it collecting in the first place. And if the problem is a single downspout rather than a whole roof, a dry well stores the same water underground in a fraction of the footprint — with the same soil caveat.

Frequently asked questions

How big should a rain garden be?
Size it to hold the runoff from a 1-inch storm: catchment area × runoff factor × 1 ÷ 12 gives cubic feet of water, and dividing that by the ponding depth in feet gives the garden area. A 1,000 square foot roof ponding 6 inches deep needs about 160 square feet; at 4 inches, about 240. The common rule of thumb — 20 to 30 percent of the area feeding it — lands in the same place for shallow gardens.
How deep should a rain garden be?
The ponding zone is usually 4 to 8 inches — shallow enough to empty in a day or two, deep enough to hold a storm. Dig deeper than that to fit 3 inches of mulch, and another 12 inches if you are replacing slow native soil with a sandy mix. Slow soil wants a shallower, wider garden; sandy soil can go deeper in a smaller footprint.
Will a rain garden work in clay soil?
Often not as-is. Heavy clay drains at a tenth of an inch an hour or less, so 6 inches of ponded water sits for days and the garden becomes a mosquito pond. Amending the top 12 inches with a sandy mix and keeping the ponding shallow sometimes gets it inside the 48-hour window. If a percolation test still reads under half an inch an hour, move the water with a swale or dry creek bed instead.
How far from the house should a rain garden be?
At least 10 feet from the foundation, and downhill of it — you are deliberately soaking water into the ground, so it must not be ground that drains toward the house. Keep it clear of septic drain fields, wells and buried utilities, and away from large tree roots you would have to cut through to dig it.
How long should a rain garden take to drain?
It should be empty within 24 to 48 hours of a storm. Faster is fine. Longer than 48 hours means the plants that tolerate wet-then-dry start dying, mosquitoes get a full breeding cycle, and the garden has no capacity left when the next storm arrives. The calculator estimates the drain-down from your soil rate and ponding depth.
What happens when a storm is bigger than the garden?
It overtops, by design. A garden sized for a 1-inch storm will spill in a 2-inch one, which is why the berm on the downhill side needs a notched overflow that points somewhere safe — a swale, a lawn that falls away, a drain — and never back toward the house. The calculator reports how many gallons overtop in a 2-inch storm so you can plan the path.
Do I need to replace the soil?
In sandy ground, no. In loam and clay, replacing the top 12 inches with a mix of roughly 60 percent sand, 20 percent compost and 20 percent topsoil is the usual way to get the drain-down inside the window and give plants a root zone that is not waterlogged. The calculator adds the mix and the extra excavation when you turn it on.

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