Dry Well Calculator
Calculate Dry Well Capacity & Storm Runoff
Are you looking to capture and disperse stormwater on your property? A dry well is an excellent eco-friendly way to manage yard runoff. Enter your catchment roof area, storm size, and dry well dimensions to find your required storage volume.
What Is a Dry Well and Why Does It Matter?
A dry well is an underground structure designed to collect, temporarily store, and dissipate storm runoff back into the groundwater table. Unlike rain barrels that store water above ground, dry wells function as subterranean infiltration chambers, helping prevent yard erosion, localized flooding, and water damage to foundations.
Modern residential dry wells consist of a perforated plastic well chamber (like a Flo-Well) surrounded by a layer of coarse, washed aggregate wrapped in geotextile fabric. The fabric allows water to soak out into the soil while preventing fine sediment from washing in and filling the dry well voids.
Sizing a dry well depends on the catchment area (usually a roof segment redirected via downspouts) and the soil infiltration rate. Clay soils require larger gravel surrounds or longer absorption times, whereas sandy soils drain rapidly.
Dry Well Sizing & Capacity Recommendations
| Catchment Area (sq ft) | 1" Storm Runoff (Gal) | Standard Wells Needed | Ideal Soil Conditions |
|---|---|---|---|
| 500 sq ft | 311 Gallons | 1 dry well (with 6" wrap) | Loam or Sandy Loam |
| 1,000 sq ft | 623 Gallons | 1-2 dry wells | Sandy or Gravelly Soil |
| 1,500 sq ft | 935 Gallons | 2 dry wells | Sandy Loam / Fine Clay (slower) |
| 2,000 sq ft | 1,246 Gallons | 3 dry wells | Requires multiple drainage wells |
How to Use This Dry Well Calculator
Provide your roof area or yard drainage catchment size, the expected rainfall height from a heavy storm (e.g. 1 inch), and the structural diameter and depth of your well barrel. Include the gravel backfill thickness (minimum 6 inches recommended). Click Calculate to run the comparison.
Frequently Asked Questions
How long should it take for a dry well to drain?
Ideally, a dry well should drain within 24 to 72 hours after a storm event. If it takes longer, the soil is likely high in clay or the well is undersized for the catchment area.
Can I direct a French drain into a dry well?
Yes. French drains and dry wells work exceptionally well together. The French drain collects groundwater, and the dry well acts as the final gravity discharge point, preventing the water from flooding lower yard zones.
How far from a house foundation should a dry well be placed?
For safety reasons, dry wells should be installed at least 10 feet away from any building foundations, retaining walls, or property lines to avoid water creeping back into structural bases.
- U.S. Environmental Protection Agency (EPA) - Stormwater management best practices and underground injection controls.
- USDA Natural Resources Conservation Service - Soil texture maps and infiltration guidelines.