Calculator Inputs
Use measured site data when possible. For final design, confirm with local engineering and code requirements.
Estimate dry well dimensions for roof runoff and light residential stormwater management. Enter drainage area, storm depth, soil infiltration rate, and design preferences to calculate storage volume, infiltration area, and suggested drywell diameter.
Use measured site data when possible. For final design, confirm with local engineering and code requirements.
A drywell is an underground stormwater infiltration structure that collects runoff from roofs, gutters, and other impervious surfaces, then slowly disperses that water into surrounding soil. Unlike a detention basin that stores water on the surface, a drywell keeps most of the system below grade, helping reduce puddling, prevent foundation moisture issues, and limit peak flow to downstream drains.
Most residential drywells are built as either:
The key challenge is sizing: too small and the system overflows in moderate storms; too large and project cost increases unnecessarily. A drywell size calculator helps estimate practical dimensions before detailed engineering.
This calculator estimates drywell dimensions using two core checks:
To support conservative planning, the tool applies a user-defined safety factor. It then computes:
In many real projects, drywell sizing is also affected by groundwater depth, frost depth, setbacks, utility conflicts, pretreatment requirements, and municipal stormwater rules. Use calculator output as a planning baseline, then refine with field conditions.
This is the total drainage area routed to the drywell. For roof-only systems, measure plan-view roof area draining to selected downspouts. If driveways or patios are included, add those surfaces too.
The runoff coefficient represents how much rain becomes runoff. Metal or shingle roofs often approach 0.9 to 0.98. Landscaped zones are much lower because water infiltrates before reaching drains.
Many jurisdictions use local design storms such as first-flush capture or specific return-period rainfall. Choosing a higher storm depth increases required drywell size. For simple residential planning, 1 to 2 inches is common, but local criteria should always take priority.
Infiltration rate has one of the largest impacts on drywell sizing. Slow soil requires larger infiltration area and may require multiple drywells. Avoid guessing when possible—field test data provides far better reliability.
Drawdown is how long the system takes to empty after a storm. Faster drawdown improves recovery between storm events and reduces standing water risk. Typical targets are 24 to 48 hours depending on local guidance.
Depth affects both storage and sidewall infiltration area. Deeper systems can reduce diameter needs, but excavation constraints, groundwater elevation, and safety considerations may limit practical depth.
Void ratio is the percentage of excavation volume that can hold water. A stone-filled pit may be around 35 to 45 percent void space. Chamber systems can be much higher.
This multiplier increases design volume to account for uncertainty, future clogging, compaction, seasonal soil variability, or underestimation of contributing area. A factor of 1.1 to 1.5 is common depending on risk tolerance.
Suppose a home has:
The raw runoff is calculated as area × rainfall depth (converted to feet) × runoff coefficient:
1,800 × (1.5/12) × 0.95 = 213.75 cu ft
After safety factor:
213.75 × 1.2 = 256.5 cu ft
With 40% voids, required excavation-equivalent storage volume is:
256.5 / 0.40 = 641.25 cu ft
Now infiltration capacity per square foot over 24 hours at 0.5 in/hr is:
(0.5/12) × 24 = 1.0 cu ft per sq ft
So required infiltration area is approximately 256.5 sq ft. The calculator then solves a cylindrical geometry using bottom + sidewall area to estimate diameter at 6 ft depth, and cross-checks whether storage volume is also satisfied. The larger requirement controls final diameter.
When people search for a “drywell size calculator,” they often focus on roof size and rainfall while overlooking soil behavior. In real performance, soil governs success or failure. Key factors include:
Because of these variables, conservative infiltration assumptions and good pretreatment (screens, leaf guards, sump structures) significantly improve long-term system reliability.
| Mistake | Why It Causes Problems | Better Approach |
|---|---|---|
| Using guessed soil rate | Can drastically under- or oversize the system | Use field infiltration/percolation data |
| Ignoring void ratio | Storage volume appears larger than it truly is | Account for material-specific void space |
| No safety factor | Little margin for clogging or unusual storms | Apply conservative factor (e.g., 1.2+) |
| No overflow plan | Water can pond near structures during large storms | Provide emergency overflow route |
| No maintenance plan | Gradual clogging reduces infiltration over years | Schedule inspections and cleaning |
A correctly sized drywell still needs routine maintenance to perform as designed:
Good maintenance protects infiltration capacity and extends service life. Many underperforming systems fail due to neglect, not initial sizing math.
Local stormwater regulations vary. Some areas require engineered drawings, infiltration testing, setback verification, or permit review before installation. Even for small residential projects, code compliance protects you from drainage disputes and retrofit costs later.
Use this drywell size calculator for planning and concept design, then validate with:
It depends on roof area, storm depth, runoff coefficient, and soil infiltration rate. The same roof can require very different drywell sizes in sandy vs. clay soils.
Higher rates reduce required size. Practical design should be based on measured field rates and local guidance, not generic assumptions.
Yes, if total contributing area and piping are designed accordingly. In many homes, distributing flow to multiple units improves reliability.
Void ratio determines how much of the excavation volume actually stores water. Ignoring this can understate required excavation size.
Use it for planning and budgeting. Final design should follow local codes and be reviewed with site-specific data.
Disclaimer: This tool provides preliminary estimates and does not replace licensed engineering design, local code review, or site-specific geotechnical evaluation.