Dry Well Sizing Calculator
A dry well only works if it’s big enough to hold the water actually reaching it. Enter your catchment area, a design rainfall depth, and a runoff coefficient, and this calculator returns the total runoff volume and how many chambers to buy.
1 in is a typical moderate storm; check local rainfall data for your design storm.
Share of rain that actually runs off — about 90–95% for a hard roof, lower for permeable surfaces.
Everything is calculated in your browser. Nothing you type is sent anywhere or stored.
Sizing tips
—
Catchment & rainfall
System profile
Runoff volume in gallons equals catchment area (square feet) multiplied by rainfall depth (inches), multiplied by 0.623 (the standard gallons-per-square-foot-per-inch conversion), multiplied by your runoff coefficient. Divide that total by the capacity of the chamber or gravel unit you’re using, then round up to the next whole unit.
Runoff Coefficients by Surface Type
Runoff coefficient is the share of rainfall that actually runs off a surface instead of soaking in. Impervious surfaces send almost all their rainfall to the drain; permeable surfaces absorb more of it.
| Surface Type | Typical Runoff Coefficient | Notes |
|---|---|---|
| Roof (any pitch) | 90-95% | Nearly all rainfall runs off |
| Asphalt or concrete driveway | 85-95% | Slight loss to surface texture and cracks |
| Gravel driveway | 30-50% | Significant infiltration through base |
| Lawn or landscaped bed | 10-20% | Most rainfall infiltrates directly |
How to Size a Dry Well
- Measure the catchment area draining to the dry well, usually a roof section or paved surface.
- Pick a design rainfall depth in inches, often based on your area’s typical heavy storm.
- Apply a runoff coefficient based on the surface type feeding the well.
- Convert to gallons using the 0.623 gallons-per-square-foot-per-inch factor.
- Divide by chamber capacity and round up to the number of chambers needed.
Worked example
A 750 sq ft roof section (25 ft x 30 ft) with a 1 in design storm and 90% runoff works out like this.
- Catchment area750 sq ft
- Design rainfall1 in
- Runoff coefficient90%
- Total runoff volume420.5 gal
- Chamber selected100 gal plastic dry well chamber
- Chambers needed5
- Total cost5 x $75 = $375
Placement and Setback Rules
Most local codes require a dry well to sit at least 10 ft from a foundation wall and often further from a septic system, well, or property line, check your jurisdiction before digging. The well also needs to sit above the seasonal high water table so it can actually drain between storms; a percolation test tells you how fast your soil absorbs water and whether you need a bigger well or additional chambers than the base calculation suggests. In clay-heavy soil, plan on oversizing or pairing the dry well with a shallow infiltration trench, since slow-draining soil can leave a well full from the last storm when the next one hits.
Common mistakes
- Only accounting for the roofif a downspout also picks up runoff from an adjacent patio or driveway, the catchment area is larger than the roof footprint alone.
- Placing it too close to the foundationmost codes require a 10 ft minimum setback to avoid saturating soil near the footing.
- Ignoring soil percolation ratea fast-draining sandy soil and a slow clay soil need very different well sizes for the same runoff volume.
- No overflow pathstorms bigger than your design rainfall will overwhelm any dry well; grade the yard so overflow has somewhere safe to go.
What this calculator assumes
Adjust your result if your project differs from these.
- Catchment area is treated as a single impervious surface (roof or pavement) with a uniform runoff coefficient.
- The 0.623 gallons-per-square-foot-per-inch conversion is the standard rainfall-to-volume factor.
- Underlying soil is assumed to have adequate percolation for infiltration; this calculator doesn’t run a percolation test.
- Sizing targets the design rainfall you enter, not the largest storm your area could see.
- Cost reflects chamber or gravel unit price only, not excavation, gravel bedding, or fabric wrap.
Common questions
What is a dry well?
It’s an underground structure, typically a plastic chamber or gravel-filled pit, that collects roof or surface runoff and lets it infiltrate slowly into the surrounding soil.
How deep does a dry well need to be?
Typical residential dry wells sit 2-4 ft deep, but depth depends on your frost line, water table, and the chamber or pit design you use.
Do I need a permit for a dry well?
Many jurisdictions require one, especially near property lines, septic systems, or wells. Check with your local building department before digging.
Can one dry well handle multiple downspouts?
Yes, as long as you size it for the combined catchment area of everything draining into it.
What if my soil doesn't drain well?
Get a percolation test done. Slow-draining soil may need a larger well, additional chambers, or a different drainage solution like a French drain to daylight.
Embed this calculator
Related calculators
Dry Well Sizing Calculator Estimate
—
materialcalculate.com/tools/dry-well-sizing-calculator/
Catchment
| Dimensions | — |
| Area | — |
| System | — |
| Rainfall event | — |
| Runoff coefficient | — |
Result
| Water collected | — |
| Units needed | — |
| Estimated total cost | — |
Notes
Estimate generated by materialcalculate.com. Confirm local rainfall data and sizing with a professional for drainage-critical installations.