Storage logic
Water volume is calculated from drainage area and rainfall. Stone-bed volume is water volume divided by void space.
RainYard Tools
Underground storage
Dry Well Size Calculator
Estimate stone-bed volume and footprint for a dry well or small infiltration pit.
Water to store296 galRunoff volume for the selected storm.
Stone volume98.9 cu ftTotal gravel-bed volume before excavation extras.
Footprint33.0 sq ftAt the selected stone depth.
Example
A 500 sq ft roof section, 1 inch of rain, and 95% runoff creates about 296 gallons. With 40% stone voids, the stone bed needs about 99 cubic feet.
Dry wells are site-sensitive
Soil, groundwater, utilities, basements, septic fields, and local rules can make a dry well unsuitable even when the volume looks reasonable.
Always plan overflow
A dry well can fill during larger storms or slow soil conditions. The overflow route should be visible and safe.
Questions
Common checks.
Can a dry well fix basement water?
Do not assume that. Basement water problems need careful grading, waterproofing, and discharge planning.
What void space should I use?
Clean open-graded stone is often screened around 35-45% void space, but material varies.
Do I need a permit?
Many places regulate dry wells. Check local rules before digging.
Helpful referencesCall 811 before diggingEPA National Stormwater Calculator