How to Do Dry Well Calculations Correctly
Dry well calculations are used to size underground infiltration systems that receive stormwater from roofs, driveways, and other impervious surfaces. The core objective is simple: temporarily store runoff and allow it to infiltrate into surrounding soil within an acceptable drawdown period. In practice, however, good dry well design depends on reliable rainfall assumptions, realistic runoff coefficients, conservative soil infiltration rates, and code-compliant setbacks and details. If any of those inputs are too optimistic, a dry well can surcharge, overflow in the wrong place, or remain saturated too long.
A properly sized system starts with volume. You estimate runoff generated by a design storm over the contributing drainage area. Next, you convert that runoff volume into required underground storage. Then you verify that the dry well can dewater quickly enough, based on tested soil infiltration and available infiltration surface area. This two-step approach is what makes dry well calculations dependable: storage handles the event peak and infiltration governs recovery time before the next storm.
1) Determine Contributing Drainage Area
Measure the hard surfaces that actually discharge to the dry well. For residential projects, the biggest source is usually roof area tied to downspouts. For commercial or site retrofits, areas might include sections of paving or courtyards, if pretreated and allowed by local standards. Be specific about what is connected. A common sizing mistake is to include total roof area when only one downspout branch drains to the proposed location.
When multiple surfaces feed a shared system, calculate each area and use an appropriate runoff coefficient. Roofs and asphalt typically produce high runoff; landscaped areas generally do not belong in a dry well tributary calculation unless intentionally routed and modeled accordingly.
2) Select Design Rainfall Depth
The design rainfall depth is usually set by local stormwater criteria. Some jurisdictions specify a water quality event (for example, first-flush capture), while others require detention/infiltration based on return-period storms and local rainfall frequency data. If your project is regulated, use the required value from your code path. If your project is voluntary or conceptual, use a conservative design depth that reflects local climate risk rather than long-term averages.
Rainfall depth is entered in inches and converted to feet for volume calculations. Because runoff volume scales directly with rainfall depth, even small changes can significantly alter required storage.
3) Apply Runoff Coefficient
Runoff coefficient accounts for losses and is typically close to 1.0 for hard roof surfaces. For many roof-only dry well calculations, values between 0.9 and 1.0 are used. Use local guidance if published. If you are combining different surfaces, calculate weighted runoff as needed to avoid overestimating or underestimating generated flow volume.
4) Calculate Runoff and Required Storage Volume
The baseline runoff volume equation in cubic feet is:
Runoff volume = Area (sq ft) × Rainfall (in / 12) × Runoff coefficient
Once runoff is estimated, apply a safety factor to account for uncertainty in field conditions, sediment accumulation, compaction impacts, or future partial clogging:
Required storage = Runoff volume × (1 + safety factor)
This adjusted number is often the key target for preliminary sizing. It is also useful to convert cubic feet to gallons for owner communication and O&M planning.
5) Convert Physical Geometry to Usable Storage
A dry well filled with stone does not store water in its full geometric volume; only void space stores water. For clean uniformly graded aggregate, void ratio often ranges near 35% to 45%. That means only a portion of the excavated volume is active storage. For a cylindrical system:
Storage volume = π × (diameter/2)² × stone depth × void ratio × number of wells
If your system uses chambers or proprietary units, storage is based on manufacturer-rated effective volume, not stone void assumptions alone.
6) Check Infiltration and Drawdown Time
After sizing for storage, confirm the system can drain in an acceptable time window. Many standards require complete or near-complete drawdown within 24 to 72 hours. Drain time estimation uses field infiltration rate and active infiltration area (typically sidewall plus bottom where permitted):
Drain time = Required storage / (Infiltration rate in ft/hr × infiltration area)
This check protects performance between storms. A system that stores enough water but drains too slowly can lose capacity before the next event.
7) Use Tested Soil Data, Not Assumptions
Soil infiltration rate is one of the most sensitive dry well calculation inputs. Field testing at or near proposed invert elevation is critical. Depending on local standards, percolation tests, double-ring infiltrometer tests, or geotechnical recommendations may be required. Design rates are often derated from measured values for long-term reliability. If regulations require a reduction factor, apply it consistently.
Avoid using generic infiltration values from internet tables as a final design basis. Texture class alone is not enough. Urban soils can be heterogeneous, layered, or compacted, and groundwater conditions can vary seasonally.
8) Essential Siting and Code Checks
- Minimum horizontal setbacks from foundations, property lines, utilities, septic components, and drinking water wells.
- Required vertical separation to seasonal high groundwater, bedrock, or restrictive layers.
- Overflow path routing for storms larger than design conditions.
- Pretreatment measures such as leaf screens, cleanouts, sump features, or sediment forebays.
- Construction sequencing to prevent sediment loading before site stabilization.
Even perfect dry well calculations can fail if siting and detailing are ignored. Geotextile use, stone cleanliness, inlet protection, and accessible maintenance points all influence long-term function.
9) Example Dry Well Calculation Workflow
Suppose a 2,000 sq ft roof drains to one location. Design rainfall is 1.5 inches, runoff coefficient is 0.95, and safety factor is 15%. Estimated runoff is about 237.5 cubic feet. With safety factor, required storage is roughly 273 cubic feet. If a single 6-foot diameter, 8-foot stone depth well with 40% voids is proposed, available storage is about 452 cubic feet, which exceeds required storage. Next, using tested infiltration of 0.5 in/hr and side+bottom infiltration area, estimated drain time can be checked against a 48-hour target. If drawdown is too long, increase infiltration area, split into multiple wells, or revise design configuration.
10) Common Mistakes in Dry Well Sizing
- Using total parcel area instead of connected impervious area.
- Ignoring stone void ratio and assuming all excavation volume stores water.
- Skipping infiltration testing or using unadjusted optimistic values.
- No overflow route for storms exceeding design capacity.
- Placing the system too close to structures or in poorly drained soils.
- Omitting maintenance access and pretreatment at inlets.
Each of these errors can cause performance problems that are much more expensive to fix after installation than to prevent during design.
11) Maintenance Planning for Long-Term Performance
Dry wells are not install-and-forget systems. Long-term performance depends on reducing sediment inflow and maintaining pretreatment and conveyance components. Establish a simple maintenance plan with seasonal inspection, downspout screen cleaning, debris removal, and verification that overflow structures are clear. If cleanouts are present, inspect for standing water duration after storm events. Persistent ponding indicates possible clogging or infiltration decline.
Maintenance records are also useful for compliance documentation in regulated projects and can extend service life significantly.
12) Residential vs Commercial Dry Well Design Considerations
Residential dry well calculations are often straightforward: one roof area, one design storm depth, and a compact footprint. Commercial systems can be more complex due to larger tributary areas, loading restrictions, utility conflicts, pretreatment requirements, and formal review processes. The underlying math remains the same, but assumptions and documentation rigor typically increase with project scale and regulatory oversight.
For larger projects, involve civil and geotechnical professionals early. Iterative layout and utility coordination often reduce redesign risk and improve final constructability.
13) Why Conservative Inputs Usually Save Money
It may feel efficient to minimize volume or rely on high infiltration assumptions, but undersized systems typically generate recurring problems: nuisance discharge, saturated soils, landscaping damage, or structural concerns near foundations. Conservative dry well calculations, combined with accessible maintenance details, usually reduce lifecycle cost by avoiding early rehabilitation or replacement.
Frequently Asked Questions About Dry Well Calculations
What rainfall event should I use for dry well sizing? Use the event required by your local code path. If no standard applies, choose a conservative design depth aligned with site risk tolerance and storm history.
Can I size from roof area only? Yes, if only roof runoff is connected. Include only surfaces that physically drain to the dry well.
What void ratio should be used for aggregate? Many clean stone installations use roughly 35% to 45% effective voids. Use material-specific data when available.
How fast should a dry well drain? Local standards commonly target 24 to 72 hours drawdown, but requirements vary.
Do I need a professional engineer? Many jurisdictions require professional design for regulated or larger systems. Even when not required, professional review can improve reliability and code compliance.
Final Planning Reminder
This page is designed to make dry well calculations fast, transparent, and practical for concept design. Final implementation should always be checked against local regulations, utility constraints, and site-specific soil and groundwater conditions. A dry well that is correctly sized, properly sited, and maintained routinely can provide effective stormwater management for many years.