How to Use This Calculator
- 1
Enter Your Property Details
Input your roof area in square feet, design storm depth in inches, and soil percolation rate in inches per hour. These three values determine how much stormwater your dry well must handle and how fast it can drain.
- 2
Set Well Depth and Calculate
Enter the planned excavation depth in feet, then click Calculate. The tool returns runoff volume, required well diameter, estimated drain time, and gravel fill needed -- everything for a complete dry well plan.
Example Calculation
A homeowner in 2026 wants to install a dry well for a 1,500 sq ft roof, designed for a 1-inch storm, with soil percolating at 0.5 in/hr and a 6-foot deep well.
Roof Area
1,500 sqft
Design Storm
1 in
Soil Percolation Rate
0.5 in/hr
Well Depth
6 ft
Results
Runoff Volume
125.0 cu ft
Volume in Gallons
935 gal
Required Well Volume
312.5 cu ft
Required Diameter
97.7 in
Estimated Drain Time
57.6 hrs
Tips
Run a Percolation Test First
Dig a 6-inch hole, pre-soak the soil, fill with 6 inches of water, and time the drop. Repeat three times and average the rate. Soil below 0.2 in/hr is generally unsuitable for dry wells -- you will need an alternative drainage strategy.
Split Oversized Wells
If the required diameter exceeds 6 feet, consider installing two or more smaller wells instead. Distributed wells drain faster, are easier to excavate, and reduce the risk of soil compaction around a single large pit.
Maintain 10-Foot Setbacks
Place dry wells at least 10 feet from foundations, property lines, and septic systems. This prevents water intrusion into basements and avoids contamination of neighboring drainage fields.
Verify Drain Time Meets Code
Most jurisdictions require a dry well to drain completely within 72 hours. If your estimated drain time exceeds this, increase well depth or diameter, or improve soil contact area by adding multiple wells.
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.
Frequently Asked Questions
What is a dry well and when do I need one?
A dry well is a buried chamber filled with gravel that collects stormwater runoff from roofs or driveways and slowly releases it into the surrounding soil. You need one when your property has poor surface drainage, local codes require on-site stormwater management, or you want to reduce erosion and recharge groundwater.
How does soil percolation rate affect dry well sizing?
Soil percolation rate determines how quickly water drains from the well. Sandy soils (around 2 in/hr) allow smaller wells because water exits quickly, while clay soils (around 0.2 in/hr) require much larger wells or multiple units to drain the same volume within the 72-hour code limit.
What is a design storm in dry well calculations?
A design storm is the rainfall event depth (for example, 1 inch or 2 inches) that your dry well is engineered to handle. Local building codes and EPA stormwater guidelines typically specify this value based on regional rainfall data. In 2026, many jurisdictions require managing at least the first 1 inch of rainfall on-site.
Why does the calculator use a 40% void ratio?
The 40% void ratio represents the empty space between gravel pieces inside the well. When you fill a dry well with washed 1-2 inch aggregate, roughly 40% of the total volume remains open for water storage. The actual ratio ranges from 35% to 45% depending on gravel size and shape.
Can I install a dry well myself or do I need a contractor?
For wells under about 150 cubic feet of total volume (roughly a 4-foot diameter at 6 feet deep), most homeowners can handle the excavation and installation. Larger wells often require heavy equipment and professional installation. Always check local permit requirements before starting.
How much gravel do I need for my dry well?
The required gravel volume equals the total well volume. For the example of a 1,500 sq ft roof with a 1-inch design storm, the required well volume is 312.5 cubic feet, or about 11.6 cubic yards of washed gravel. Order an extra 10% to account for settling and compaction during backfill.