Calculators / Rainwater and drainage
How big should your dry well be?
Put in the roof area on the downspout and the size of storm to hold. You get the pit size, the stone and the digging, and a check that your soil empties it in time.
Updated · How we check our numbers
Dig it this size
5.9 × 5.9 ft, 36 in of stone
Holds a 1 in rain off 500 sq ft of roof: 312 gallons in 3.9 cubic yards of stone.
- Water to hold
- 312 gal
- Stone
- 3.9 cu yd
- Pit floor
- 34.7 sq ft
- Dig out
- 5.1 cu yd
Estimate for planning only, not professional advice. Check the manufacturer’s instructions, your local code and a licensed professional before you buy, build or install. Use at your own risk. Terms.
Or a rain gardenSame roof, planted instead of buried
runoff = 500 sq ft × 1 in ÷ 12 = 41.7 cu ft (312 gal)
stone = 41.7 ÷ 40% voids = 104.2 cu ft (3.9 cu yd)
pit floor = 104.2 cu ft ÷ 36 in of stone = 34.7 sq ft, about 5.9 × 5.9 ft
dig = 34.7 sq ft × (36 in stone + 12 in soil) = 5.1 cu yd
Dig a test hole as deep as the pit floor, fill it with water, and time how fast it drops. Put the inches an hour in above to check it drains within 72 hours.
At least 10 ft from the house, downhill from it, and away from septic systems and wells. Call 811 before you dig.
Line the pit with nonwoven landscape fabric, fill with clean washed stone (Pennsylvania specifies AASHTO No. 3), fold the fabric over the top, and cover with at least 12 in of soil.
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A big storm will overfill it. Give the water somewhere safe to go, like a pop-up emitter in the lawn.
See pop-up emitters on Amazon (opens Amazon in a new tab)How we work it out
First, the water. One inch of rain on one square foot is 1/12 of a cubic foot, or about 0.62 gallons:
runoff (cu ft) = roof sq ft × inches of rain ÷ 12
A stone-filled pit only holds water in the gaps between stones. The Pennsylvania Stormwater BMP Manual counts 40% of the pit as water:
stone (cu ft) = runoff ÷ 0.40
pit floor (sq ft) = stone ÷ stone depth (ft)
Pennsylvania's design uses 18 to 48 in of clean washed stone (AASHTO No. 3) wrapped in nonwoven geotextile, with at least 12 in of soil on top. It keeps dry wells 10 ft from building foundations, gives every one an overflow, and wants it empty within 72 hours.
hours to drain = stone depth (in) ÷ soil drain rate (in/hr)
We only count water soaking out the bottom. The sides help too, so this is on the safe side. Test the soil with a hole dug to the pit's floor.
Where the numbers come from
Also
Rainwater and drainage
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