What Is Dry Well Sizing (CFM & Volume Chart)
Dry-well sizing means matching a rainwater storage space to the runoff it receives and the soil’s ability to absorb that water within the time allowed by local rules. CFM is not a dry-well sizing unit; it measures airflow. Use rainfall, drainage area, tested soil infiltration, and site requirements instead.
If you are comparing waterproofing options because water collects near a building, a dry well may help manage runoff, but only if it is sized and placed for the site. A well that is too small, drains too slowly, or sends overflow toward a foundation can make water problems worse. The figures below are for early planning, not a final design.
What dry-well sizing means, and why CFM does not apply
A dry well is an underground space that temporarily holds stormwater and lets it soak into surrounding soil. Its size is based on water volume and drainage performance. CFM, or cubic feet per minute, measures airflow, so it does not describe how much rainwater a dry well can store or infiltrate.
People may see CFM in tool or equipment instructions and wonder if it applies to a drainage system. It does not. For a dry well, the useful units are gallons or cubic feet for storage, inches for rainfall, and a tested infiltration rate for soil.
Sizing also involves time. A well needs enough temporary storage for incoming runoff, and the soil needs to accept that water within the drawdown period required by local rules. A large excavation alone cannot solve soil that drains too slowly.
Site Assessment, Sizing Inputs & Safety Preparation
Before choosing a dry-well size, identify where the water comes from, how much rain the design must handle, and what the soil can absorb at the planned depth. Site limits matter too: groundwater, bedrock, utilities, nearby structures, and local rules can change whether a dry well is suitable at all.
Start by mapping the area that drains toward the proposed well. A roof downspout is a common source, but paved areas or other approved drainage areas may contribute too. Check local requirements for the design storm, calculation method, permits, setbacks, overflow routing, and required drawdown time.
How to estimate runoff volume
For roof runoff, a useful preliminary estimate is 0.623 gallon per square foot for each inch of rain. Multiply roof area by rainfall depth and 0.623. For other surfaces, local rules may require a runoff factor to account for water that soaks in or runs off differently.
Roof runoff estimate: area (ft²) × rainfall (inches) × 0.623 = gallons.
For example, a 1,000-square-foot roof in a 1-inch storm produces about 623 gallons before accounting for losses or other inflows. This is an estimate of incoming water, not a recommended well size. Use the design storm and calculation method required in your area.
Test the soil and check the site
Do not rely on labels such as “sand” or “clay” to predict how quickly a dry well will drain. Arrange the infiltration test required by your local authority, at the proposed bottom elevation. The test result helps determine how much water the site can absorb over time; the approved method and design rate may differ from a simple field observation.
Before excavation, locate utilities and check groundwater and bedrock depth. Confirm required separation distances from buildings, property lines, wells, septic systems, and other sensitive features. These setbacks are site- and jurisdiction-specific, so there is no safe universal distance to apply.
Dry-Well Storage Volume Chart and Sizing Calculation
A stone-filled excavation stores water in the open spaces between stones, not in the full excavation volume. The chart below estimates that storage using a 40% void fraction. It is a preliminary comparison only; it does not show whether the soil can absorb the stored water quickly enough.
For clean, washed, open-graded stone, a void fraction of 0.35 to 0.40 may be used for early estimates. Verify the actual aggregate or product data before design. To estimate gallons, multiply excavation volume in cubic feet by 7.4805 gallons per cubic foot and then by the void fraction.
| Excavation dimensions (L × W × depth) | Excavation volume | Estimated stone void storage at 40% |
|---|---|---|
| 4 × 4 × 3 ft | 48 ft³ | About 144 gal |
| 6 × 6 × 3 ft | 108 ft³ | About 323 gal |
| 8 × 8 × 3 ft | 192 ft³ | About 575 gal |
| 10 × 10 × 3 ft | 300 ft³ | About 898 gal |
Estimated gallons = length × width × depth (ft³) × 7.4805 × 0.40. These figures exclude any extra capacity from a manufactured chamber. They also do not account for site restrictions, usable free space, or required design factors.
Compare runoff with storage, then check drawdown
Consider the 1,000-square-foot roof example: a 1-inch storm produces about 623 gallons. An 8 × 8 × 3-foot stone excavation has an estimated 575 gallons of void storage, so that estimate alone is below the roof runoff estimate. A 10 × 10 × 3-foot excavation estimates about 898 gallons, but that does not prove it is the right size. Soil infiltration and local design requirements still control.
In practice, storage and infiltration work together. The local sizing method may account for water entering while water is already soaking into the soil. Do not assume the entire bottom and sides count as active infiltration area unless the approved method allows it. A designer or local authority can confirm how to use the test results and required drawdown limit.
| Planning question | What to use | What it tells you |
|---|---|---|
| How much water arrives? | Contributing area, design rainfall, and required runoff method | Estimated stormwater inflow |
| How much can the excavation hold? | Dimensions, stone void fraction, or product data | Temporary storage estimate |
| How fast can the site drain? | Required on-site infiltration test | Whether the soil can meet drawdown rules |
| Is the location acceptable? | Local setbacks, groundwater and bedrock checks, permits | Whether the well can be built there |
Dry-Well Installation and Runoff Routing
Installation must preserve the soil’s ability to absorb water and keep sediment out of the storage area. The exact construction details depend on the approved design and product instructions. Before digging, confirm the location, dimensions, materials, inlet, overflow route, access for inspection, and any required geotextile or pretreatment.
A basic sequence is:
- Mark the contributing drainage area and confirm the planned flow path.
- Locate underground utilities and verify permits and required setbacks.
- Excavate to the approved dimensions without smearing or compacting the infiltration surface.
- Install the specified chamber or clean stone, plus geotextile where required by the design.
- Add the designed inlet and accessible pretreatment for sediment.
- Provide an approved overflow route that will not threaten a foundation or send water onto neighboring property.
- Keep construction sediment and soil washout out of the well.
- Record the finished dimensions and check that the inlet and overflow work.
Worked planning examples, not universal designs
These examples show how to compare runoff and storage estimates; they are not field case studies or construction plans. In the first, a 500-square-foot roof in a 1-inch storm yields about 312 gallons by the roof formula. A 6 × 6 × 3-foot stone excavation estimates about 323 gallons of storage, leaving little margin in that simple comparison.
In the second, a 1,000-square-foot roof produces about 623 gallons in the same rainfall. The 8 × 8 × 3-foot chart estimate is about 575 gallons, while the 10 × 10 × 3-foot estimate is about 898 gallons. Neither chart result settles the design: tested infiltration, additional inflows, allowed drawdown, and local requirements must be checked.
If the soil drains poorly, making the excavation larger may only add storage without fixing the slow drawdown. Water could then back up, saturate nearby soil, or overflow toward a building or neighboring property. Redesign or an approved alternative may be needed.
Inspection, Sediment Control & Performance Checks
A dry well needs a clear path for water to enter and a functioning route for excess water to leave safely. Check the inlet, pretreatment, inspection access, and overflow after installation. Persistent ponding, backup, or water appearing near a structure can signal a blockage, sediment buildup, inadequate capacity, or poor infiltration.
At commissioning, verify that runoff reaches the inlet and that the overflow follows its intended route. Keep an as-built record of dimensions, materials, and any access points. After major storms, look for signs of backup or erosion, and inspect accessible pretreatment components for accumulated sediment.
Seasonal Maintenance and Winterization
Maintenance focuses on preventing sediment and debris from reducing flow. Inspect and clean accessible pretreatment components periodically and after major storms, following the product instructions. Avoid sending leaves, soil, or construction washout into the well. In freezing weather, follow the system’s design and local guidance rather than assuming a buried well is immune to winter issues.
If water remains on the surface longer than expected or repeatedly backs up, investigate promptly. Do not treat ongoing ponding as normal without checking the local design criteria. Sediment blockage, a damaged inlet, changed drainage patterns, or soil that cannot meet the required drawdown may need professional review.
Conclusion and FAQ
A dependable dry-well estimate starts with runoff volume, then checks storage, tested infiltration, drawdown, and site rules. There is no universal gallons-per-well chart, and CFM is not a capacity measure. Use the chart only for preliminary stone-storage estimates, then confirm the design with local requirements and site data.
What does CFM mean in dry-well sizing?
CFM means cubic feet per minute, a measure of airflow. It is not used to rate dry-well storage or soil infiltration. Dry-well planning uses water volume, rainfall, tested soil performance, and local design criteria.
How many gallons does one inch of rain produce on a roof?
A roof produces about 0.623 gallon per square foot for each inch of rain. A 1,000-square-foot roof therefore produces about 623 gallons in a 1-inch storm, before accounting for losses or other inflows.
How do I estimate stone dry-well capacity?
Multiply excavation length, width, and depth in feet to get cubic feet. Then multiply by 7.4805 and the verified stone void fraction. A 40% fraction is a preliminary estimate for suitable open-graded stone, not a guaranteed product value.
Is there a universal dry-well size chart?
No. A chart can estimate storage for given dimensions, but it cannot account for every site’s rainfall, infiltration, groundwater, setbacks, or local rules. Use it for early comparison, not as a final design.
Does a bigger dry well always drain better?
No. A larger well may hold more water, but it does not make slow-draining soil absorb water faster. The design must satisfy both storage needs and the site’s tested infiltration and drawdown requirements.
Can I size a dry well using roof area alone?
No. Roof area helps estimate runoff, but the design also needs the required storm rainfall, a site infiltration test, a storage calculation, and local requirements. Other surfaces or inflows may also change the total.
What soil test is needed for a dry well?
Use the on-site infiltration test required by the local authority at the proposed installation depth. Soil names alone do not establish drainage performance, and test methods or accepted design rates may vary by jurisdiction.
What should I check before digging?
Locate utilities, check groundwater and bedrock, and confirm permits and required setbacks from buildings, property lines, wells, septic systems, and other sensitive features. Distances and rules depend on the site and jurisdiction.
How can I tell if a dry well is failing?
Repeated ponding, water backing up at the inlet, or overflow near a building can indicate blockage, sediment, inadequate capacity, or poor infiltration. Check accessible pretreatment and seek a design review if the problem persists.
Does the chart include extra storage from a chamber?
No. The chart estimates stone void storage only. A chamber may provide additional capacity, but use the manufacturer’s data and the approved local sizing method rather than adding an assumed amount.
(This article was written by one of our staff writers, Evan Bradley. Visit our Meet the Team page.)
