Complete Drywell Calculator Guide: How to Size a Drywell Correctly
A drywell calculator helps property owners, contractors, and designers estimate how much underground storage and infiltration capacity is needed to manage stormwater runoff. If you route roof downspouts or paved surface drainage into a drywell, the two most important questions are simple: how much runoff arrives during a storm, and how quickly can that water infiltrate into native soil. This page answers both questions with a practical sizing calculator and a detailed design guide.
What is a drywell?
A drywell is an underground infiltration structure that temporarily stores runoff and releases it into surrounding soil. Drywells are commonly connected to roof downspouts, area drains, trench drains, or driveway inlets. A properly built system includes a pretreatment step such as a leaf screen, sump, filter basin, or sediment trap so soil pores around the drywell do not clog too quickly.
Drywells are used in residential and commercial drainage projects where site conditions permit infiltration. They are especially useful when municipal storm sewer connection is limited, expensive, or not available. A drywell system can reduce standing water, limit erosion, and decrease peak runoff leaving the property.
Why drywell sizing matters
Undersized drywells fill too quickly and overflow near foundations, walkways, or neighboring lots. Oversized systems can cost far more than necessary. The point of a drywell calculator is to balance performance and cost by estimating a realistic number of drywells and dimensions for each unit.
- Storage sizing checks whether the drywell can hold stormwater volume during a design event.
- Infiltration sizing checks whether stored water drains within the required drawdown window.
- Safety factor adds extra margin for construction variability, long-term clogging, and model uncertainty.
If local code requires a specific storm event and emptying time, these criteria should drive your final design.
Drywell calculator formula and method
This calculator uses a planning-level method with transparent inputs:
- Runoff volume (cubic feet) = Drainage Area × (Rainfall Depth / 12) × Impervious Fraction × Runoff Coefficient
- Storage per drywell = Cylinder Volume × Void Ratio
- Infiltration area per drywell = Side Area + Bottom Area
- Infiltration flow per drywell = Infiltration Area × (Soil Infiltration Rate / 12)
- Wells by storage = Runoff Volume / Storage per Drywell
- Wells by infiltration = Required Infiltration Area / Infiltration Area per Drywell
- Recommended number of wells = ceil(max(Storage Count, Infiltration Count) × Safety Factor)
Because infiltration and storage both matter, the larger count controls design. In some soils, storage dominates. In slow soils, infiltration area dominates.
How to choose drywell calculator inputs
Drainage area: Include all surfaces routed to the drywell. If only two downspouts are connected, do not enter the whole roof area. Measure accurately using plans, GIS, or field dimensions.
Impervious percentage: Use 100% when only impervious areas contribute runoff. If landscaped areas are included, reduce this value based on realistic contribution.
Runoff coefficient: Smooth roofs are typically high, while rough or partially pervious surfaces can be lower. The coefficient converts gross rainfall to expected runoff.
Design rainfall depth: This should match local standards or project goals. Short, intense storms may drive overflow risk, while long-duration rainfall may challenge drawdown.
Infiltration rate: Base this on site testing rather than assumptions whenever possible. Conservative values are better than optimistic values.
Drywell geometry and void ratio: Gravel-filled pits store less water than open chambers of the same dimensions because rock occupies volume. Void ratio reflects this storage fraction.
Drawdown time: Many standards require systems to drain within 24 to 72 hours. Shorter drawdown targets need more infiltration area.
Soil testing and infiltration rates
Soil performance determines whether a drywell is practical. A basic percolation test can help, but many jurisdictions require formal infiltration testing at the depth where the drywell sidewalls and base will contact native soil. Testing should consider seasonal high groundwater and restrictive layers such as clay lenses, hardpan, or shallow bedrock.
| Soil Texture (General) | Typical Infiltration Range (in/hr) | Design Note |
|---|---|---|
| Sand / Loamy Sand | 1.0 - 8.0+ | High potential, still verify setbacks and groundwater separation. |
| Sandy Loam | 0.5 - 2.0 | Commonly suitable with pretreatment and maintenance access. |
| Loam / Silt Loam | 0.2 - 1.0 | Can work with larger systems and conservative design. |
| Clay Loam / Silty Clay | 0.05 - 0.3 | Often infiltration-limited; overflow or alternate drainage may be needed. |
Use lower-end values for long-term reliability unless local engineering guidance states otherwise.
Selecting design rainfall for drywell sizing
A drywell calculator is only as accurate as the design rainfall depth selected. Local standards may require capture of a specific event such as a first-flush depth, a 10-year storm, or another return period. Residential best-practice projects often use a capture depth between 1 and 2 inches for roof runoff, but code rules vary widely by region.
If no code requirement exists, choose a value that reflects your risk tolerance and site sensitivity. Homes with basement seepage history, steep grades, or foundation concerns often justify a more conservative design. Always include a safe overflow route because any underground system can exceed capacity in extreme storms.
Drywell types, dimensions, and void ratio
Drywell systems are generally built as gravel pits, precast units, plastic chambers, or modular crates. The type you choose affects usable storage, installation speed, structural loading, and inspection access.
- Gravel drywell: durable and simple, but storage efficiency is lower due to rock displacement.
- Precast concrete drywell: robust and familiar to many crews, may require equipment for placement.
- Plastic chamber or crate: high storage efficiency and lighter handling, but verify load ratings and bedding requirements.
Void ratio in the calculator should match your chosen system and aggregate specifications. For gravel, values around 0.30 to 0.40 are common assumptions. Higher values can apply to open-void chambers.
Setbacks, separations, and code concerns
Before construction, confirm local requirements for:
- Minimum distance from building foundations and basement walls
- Separation from property lines and neighboring structures
- Distance from septic systems, wells, and utility corridors
- Vertical clearance above seasonal high groundwater and bedrock
- Pretreatment requirements and observation/cleanout access
Many failures are not math errors; they are siting errors. Place the system where infiltration is possible and where overflow can be safely controlled.
Installation best practices for long-term performance
Even a perfect drywell calculator result can underperform if installation quality is poor. Good field practice includes careful excavation, geotextile use where specified, clean stone handling, and strict control of sediment entry during construction.
- Install roof leader strainers, leaf guards, and sediment forebays.
- Avoid compacting native infiltration surfaces at the base and sidewalls.
- Do not allow muddy runoff to enter before landscape stabilization.
- Provide inspection ports and maintenance access points.
- Create a defined overflow path away from structures.
Drywell maintenance and service life
Drywells are not maintenance-free. Sediment and organic debris gradually reduce infiltration. A maintenance plan should include seasonal gutter cleaning, inlet inspection after major storms, and periodic removal of accumulated solids from pretreatment structures.
Warning signs include frequent surface ponding, standing water in observation ports long after rainfall, and overflow discharge during moderate storms. Early maintenance is usually less expensive than system replacement.
Drywell project cost drivers
Drywell cost depends on more than volume. Major cost factors include excavation access, depth, shoring requirements, soil export, imported clean aggregate, chamber type, pretreatment devices, and surface restoration. Utility conflicts and difficult groundwater conditions can significantly increase installation complexity.
Use calculator output as a first sizing target, then request site-specific quotes with clear scope and material specifications. Comparing bids is much easier when all contractors price the same design assumptions.
Worked example using the drywell calculator
Suppose a 2,000 sq ft roof drains to an infiltration system. You choose 1.5 inches of design rainfall, runoff coefficient 0.95, infiltration rate 0.5 in/hr, and 24-hour drawdown. Proposed drywell size is 4 ft diameter by 6 ft effective depth with 0.40 void ratio. The calculator returns runoff volume, storage per well, infiltration capacity per well, and a recommended number of units after safety factor. If planned count is entered, the tool also estimates drawdown time for that configuration.
This workflow gives a practical answer quickly: how many wells are needed and whether your preferred layout likely meets storage and drawdown objectives.
Common drywell sizing mistakes to avoid
- Using optimistic infiltration rates without verified testing
- Ignoring pretreatment and allowing sediment to clog the system
- Forgetting to multiply by impervious fraction or runoff coefficient
- Assuming gravel-filled pits have full geometric storage volume
- Skipping overflow design for storms larger than the target event
- Placing drywells too close to foundations or retaining walls
Drywell Calculator FAQ
How accurate is this drywell calculator?
It is a planning-level calculator that provides fast, transparent estimates. Final design should follow local code and engineering guidance.
What infiltration rate should I enter?
Use measured field data when possible. If uncertain, choose a conservative value to avoid under-sizing.
Can one large drywell replace multiple smaller wells?
Sometimes yes, but site layout, utility conflicts, and distribution piping often make multiple wells easier to place and maintain.
Why does the calculator report both storage-based and infiltration-based well counts?
Because a drywell must both hold runoff and drain it within target time. The larger requirement controls design.
Should I always include a safety factor?
Yes. Safety margin improves reliability against clogging, uncertain inputs, and construction variation.