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.

Roof/Catchment Area (sq ft):
Projected Rainfall (in):
Well Core Diameter (ft):
Well Depth (ft):
Gravel Wrap Thickness (in):

What Is a Dry Well and Why Does It Matter?

A dry well is an underground structural chamber designed to collect, temporarily store, and dissipate storm runoff back into the soil and groundwater table. By acting as a subterranean collection basin, dry wells intercept stormwater from impervious surfaces like roofs, driveways, patios, and walkways. This intercepted runoff is then slowly discharged into the surrounding soil through natural percolation. Dry wells are an essential element of modern Low Impact Development (LID) and sustainable stormwater management systems. They mitigate localized flooding, soil erosion, lawn oversaturation, and structural water damage to residential foundations.

The primary benefit of a dry well is its ability to recharge local aquifers while filtering out suspended solids. As stormwater flows over roofs and hardscapes, it collects fine pollutants, sediment, and debris. Rather than directing this water to municipal storm sewers—where it can overwhelm public infrastructure and pollute natural waterways—a dry well manages runoff directly at the source. Once inside the well, gravity forces water through the perforated chamber walls and into a surrounding gravel base, which acts as a secondary filter. The biological processes in the soil then break down organic pollutants, returning clean water to the underlying water table.

Sizing a dry well is a critical engineering task. A well that is too small will overflow during moderate storm events, leading to surface flooding and soil washout around the well inlet. Conversely, an oversized dry well represents unnecessary excavation costs and labor. The efficiency of a dry well depends on three primary variables: the surface area of the catchment zone, the volume of rainfall during a design storm (e.g., a 10-year, 24-hour storm), and the infiltration capacity of the surrounding soil. Soils high in clay or silt have slow percolation rates, meaning they require larger gravel surrounds or multiple dry well units connected in a series to handle heavy rainfall. Sandy or gravelly soils absorb water rapidly, allowing for smaller, highly efficient designs.

How to Calculate Dry Well (Mathematical Formulas)

Determining the correct dimensions for a dry well requires calculating the expected stormwater runoff volume and comparing it to the physical storage capacity of the dry well chamber and its surrounding gravel bed. Below are the engineering formulas used to perform these calculations.

Formula for Stormwater Runoff Volume

To estimate the volume of water entering the dry well from a roof or other impervious surface, we use the Rational Method runoff volume calculation:

1. Catchment Area (sq ft): Area = Length (ft) × Width (ft) of the roof section draining to the downspout.
2. Rainfall Depth (ft): Rainfall (ft) = Rainfall Depth (in) / 12 (the depth of the design storm).
3. Runoff Volume in Cubic Feet (cu ft): Runoff Volume = Area (sq ft) × Rainfall (ft) × C (where C is the runoff coefficient; for clean shingle roofs, C ≈ 1.0).
4. Runoff Volume in Gallons (gal): Runoff Volume (gal) = Runoff Volume (cu ft) × 7.48 (since there are 7.48 gallons in one cubic foot).

Formula for Dry Well Chamber Capacity

The volume of the cylindrical dry well barrel is calculated using the formula for cylinder volume:

1. Chamber Radius (ft): Radius = Diameter (ft) / 2
2. Chamber Volume (cu ft): Chamber Volume = π × Radius² × Depth (ft)
3. Chamber Capacity in Gallons (gal): Chamber Capacity (gal) = Chamber Volume × 7.48

Formula for Gravel Surround Capacity

The gravel surrounding the well provides structural stability and extra water storage in its void spaces. Standard crushed gravel (1.5" washed stone) has a void ratio of approximately 40%. The formulas are:

1. Total Excavated Radius (ft): Total Radius = Chamber Radius (ft) + (Gravel Wrap Thickness (in) / 12)
2. Total Excavated Volume (cu ft): Total Volume = π × Total Radius² × Depth (ft)
3. Gravel Net Volume (cu ft): Gravel Volume = Total Volume - Chamber Volume
4. Gravel Weight (Tons): Gravel Tons = (Gravel Volume / 27) × 1.4 (assuming 1.4 tons per cubic yard).
5. Gravel Void Capacity in Gallons (gal): Gravel Void Capacity = Gravel Volume × 0.40 (Void Ratio) × 7.48

The Total Dry Well Storage Capacity is the sum of the chamber capacity and the gravel void capacity: Total Storage = Chamber Capacity (gal) + Gravel Void Capacity (gal). The percentage of storm water handled is calculated as: Percentage = (Total Storage / Runoff Volume) × 100.

Dry Well Sizing & Specifications Reference Chart

Choosing the correct number of dry well units depends on the runoff volume and soil infiltration rates. The reference chart below provides specifications for standard residential dry well configurations (assuming a 3-foot diameter by 4-foot deep plastic well core).

Catchment Area (Sq Ft)	Gravel Wrap Thickness	Total Storage (Gal)	Soil Percolation Rate	Required Wells (1" Rain)
500 sq ft	6 inches	311 Gallons	Fast (Sandy Loam)	1 Well
1,000 sq ft	12 inches	480 Gallons	Medium (Silt Loam)	1 to 2 Wells
1,500 sq ft	12 inches	480 Gallons (per unit)	Medium (Loam)	2 Wells (in series)
2,000 sq ft	18 inches	710 Gallons (per unit)	Slow (Clay Loam)	3 Wells (in series)
3,000 sq ft	18 inches	710 Gallons (per unit)	Slow (Heavy Clay)	4+ Wells (in series)

Step-by-Step Installation Guide & Professional Tips

Installing a dry well involves heavy excavation, careful handling of filter fabrics, and precise leveling. Follow this professional installation guide to ensure your system functions effectively and does not fail due to clogging or soil infiltration.

Step 1: Site Selection and Soil Testing

Choose a location at least 10 feet away from your home's foundation, retaining walls, and property lines. Ensure the dry well is placed downhill from any structures. Conduct a simple soil percolation test: dig a hole 12 inches deep and 12 inches wide at the installation depth, fill it with water, and measure how long it takes to drain. If the water drains in under 24 hours, the soil is suitable. If it takes longer than 48 hours, you must increase the gravel wrap thickness or add additional wells to compensate for the slow infiltration rate.

Step 2: Excavation

Dig a circular pit that is at least 12 to 24 inches wider than the diameter of your dry well chamber to accommodate the gravel wrap. The depth of the pit should match the height of the dry well chamber plus an additional 6 to 12 inches for a gravel base layer. For a 4-foot deep chamber, excavate the pit to a depth of 5 feet. Excavate a trench from the downspout or runoff source to the pit, sloping downward at a minimum of 1/8 inch per foot.

Step 3: Lining the Pit with Geotextile Fabric

Line the entire excavated pit with professional-grade non-woven geotextile landscape fabric. Ensure the fabric overlaps the edges of the pit by at least 12 to 18 inches. The geotextile fabric is a critical component; it allows water to pass through into the surrounding soil while preventing fine clay and silt particles from migrating into the gravel wrap and clogging the system over time.

Step 4: Installing the Gravel Base and Well Chamber

Fill the bottom of the fabric-lined pit with 6 to 12 inches of washed 1.5-inch or 2-inch crushed stone. Do not use pea gravel or stone dust, as they do not have sufficient void space. Compact the gravel lightly. Assemble the perforated plastic dry well chamber (such as a Flo-Well) and position it in the center of the pit. Knock out the pre-punched drainage holes on the chamber walls to allow water to exit. Connect the inlet pipe from your trench to the top of the well chamber.

Step 5: Backfilling with Gravel wrap

Fill the outer space between the plastic chamber walls and the fabric-lined pit with washed crushed stone, building it up in 12-inch layers. Continue backfilling until the gravel is level with the top of the dry well chamber. Place the lid securely on the well chamber and insert the cleanout/overflow pipe through the lid. Fold the excess geotextile fabric over the top of the gravel surround and the chamber lid, completely enclosing the gravel and well. This fabric wrap prevents soil backfill from entering the gravel layer.

Step 6: Backfill and Surface restoration

Cover the wrapped dry well with 6 to 12 inches of native soil. Compact the soil lightly to prevent settling. Lay sod or spread grass seed over the area. Ensure the cleanout/overflow pipe extends slightly above the finished grade and is capped with a pop-up emitter or a grate. In the event of an extraordinary storm that exceeds the dry well's capacity, the excess water will safely exit through the overflow emitter rather than backing up into your downspouts.

Frequently Asked Questions

How long does it take for a dry well to drain completely?

In well-drained sandy or gravelly soils, a dry well can empty in less than 12 to 24 hours. In standard silty loam soils, it typically takes between 24 and 48 hours. In heavy clay soils, percolation can take up to 72 hours. If a dry well takes longer than 72 hours to drain, it is considered failing, and standing water can lead to mosquito breeding and root rot in nearby plants.

Can I direct a French drain and a gutter downspout into the same dry well?

Yes, you can direct both into the same dry well. However, gutter downspouts carry a significant volume of water and debris (like asphalt shingle granules and leaves). You must install a debris filter (such as a downspout leaf separator or inline catch basin) before the water enters the dry well. If debris enters the well chamber, it will clog the perforated holes and reduce the storage capacity over time.

How far from a septic tank or water well should a dry well be placed?

A dry well should be located at least 50 feet away from any water wells, septic tanks, or absorption fields. Because dry wells concentrate stormwater runoff and inject it directly into the subgrade, placing them too close to septic systems can oversaturate the soil, interfering with septic drainage. Proximity to water wells can also present a risk of groundwater contamination.

What is the difference between a dry well and a rain garden?

A dry well is an engineered underground storage chamber that hides water beneath the surface in gravel voids and a plastic well barrel. A rain garden is a shallow, vegetated surface depression filled with soil amendments and water-tolerant plants. Rain gardens store water on the surface and allow it to filter down through plant roots and soil, whereas dry wells bypass surface soil layers entirely.

Can a dry well freeze in the winter and cause drainage back-ups?

Yes, if the dry well is installed above the frost line, the water inside can freeze, blocking the inlet pipe and causing gutters to overflow. In cold climates, ensure the inlet pipe enters the dry well chamber below the local frost line (typically 2 to 4 feet deep). Wrapping the top of the dry well with rigid foam insulation board before backfilling with soil can also prevent freezing.

All calculations are estimates. Always consult a professional contractor for accurate quotes. See our Disclaimer for details.