The Dry Well Size Calculator helps homeowners and contractors determine the optimal dimensions for a stormwater dry well in 2026. By entering your roof area, design storm depth, soil percolation rate, and planned well depth, you get precise runoff volumes, required well diameter, and estimated drain time -- everything needed to plan an effective installation that meets modern stormwater codes.
How Dry Well Sizing Formulas Work
The calculator uses standard civil engineering formulas to convert rainfall on impervious surfaces into required well dimensions. The two core equations are:
Runoff Volume (cu ft) = Roof Area (sq ft) x (Design Storm (in) / 12)
Required Well Volume (cu ft) = Runoff Volume / Void Ratio (0.40)
From the required volume and your chosen depth, the calculator derives the well diameter using cylindrical geometry. It then estimates drain time by dividing runoff volume by the product of your soil percolation rate (converted to ft/hr) and the bottom area of the well.
| Parameter | Example Value | Result |
|---|---|---|
| Roof Area | 1,500 sq ft | 125.0 cu ft runoff |
| Design Storm | 1 in | 935 gal total |
| Soil Percolation Rate | 0.5 in/hr | 57.6 hrs drain time |
| Well Depth | 6 ft | 97.7 in diameter needed |
| Well Volume (at 40% void) | -- | 312.5 cu ft |
Worked Example: 1,500 Sq Ft Roof
Using the default inputs (1,500 sq ft roof, 1-inch storm, 0.5 in/hr soil, 6 ft depth):
- Runoff volume: 1,500 x (1/12) = 125.0 cu ft, or 935 gallons
- Required well volume: 125.0 / 0.40 = 312.5 cu ft (accounting for 40% gravel void ratio)
- Required diameter: Cross-sectional area = 312.5 / 6 = 52.08 sq ft, radius = 4.07 ft, diameter = 8.14 ft (97.7 inches)
- Drain time: 125.0 / (0.0417 ft/hr x 52.08 sq ft) = 57.6 hours
At 57.6 hours, this design meets the common 72-hour drain requirement but leaves limited headroom. Increasing the well depth to 8 feet or splitting into two 4-foot-diameter wells would improve drain performance.
2026 Industry Benchmarks and Best Practices
Modern stormwater codes in 2026 continue to tighten requirements for on-site management. Key benchmarks include:
- Drain time: Most jurisdictions require full drainage within 24 to 72 hours. Wells draining under 24 hours are rated excellent.
- Soil suitability: Percolation rates below 0.2 in/hr are generally considered unsuitable for dry wells. Alternative systems like rain gardens or French drains may be needed.
- Void ratio: Standard gravel fill provides 35-45% void space, with 40% being the industry default for sizing calculations.
- Setback distances: Maintain at least 10 feet from building foundations, property lines, and septic systems per EPA stormwater guidelines.
- Design storm: EPA and most local codes recommend managing the first 1 inch of rainfall from impervious surfaces on-site. Some regions now require 1.5 inches for new construction.