Dry Well Size Calculator
Estimate runoff storage, aggregate-pit volume, footprint and modular dry-well quantity from catchment area, rainfall, capture target and editable void ratio.
Default project preview
336.6 gal
Event runoff
673.2 gal
Aggregate pit volume
4.17 yd³
Pit footprint
28.1 ft²
Modular units
7
Calculator
Build the material order
Your order will appear here
Enter the project details and calculate a rounded material estimate.
Input and output guide
What each value means
The labels and units below are part of this calculator’s visible methodology. Replace every default with measurements or product data from the actual project.
Connected catchment area
Enter this project assumption in ft². The current imperial default is 1200 ft².
Design rainfall event
Enter this project assumption in in. The current imperial default is 1 in.
Runoff coefficient
Enter this project assumption in a unitless value. The current imperial default is 0.9. Minimum: 0.1. Maximum: 1.
Capture target
Enter this project assumption in %. The current imperial default is 50 %. Minimum: 1. Maximum: 100.
Aggregate void ratio
Enter this project assumption in a unitless value. The current imperial default is 0.4. Minimum: 0.1. Maximum: 0.9.
Usable pit depth
Enter this project assumption in ft. The current imperial default is 4 ft.
Modular unit capacity
Enter this project assumption in gal. The current imperial default is 50 gal.
Price per modular unit
Enter this project assumption in a unitless value. The current imperial default is 250. Minimum: 0.
How to read the outputs
Event runoff
coefficient applied. Default preview: 673.2 gal.
Aggregate pit volume
void ratio included. Default preview: 4.17 yd³.
Pit footprint
at entered usable depth. Default preview: 28.1 ft².
Modular units
rounded full units. Default preview: 7.
Purchased unit capacity
full modules. Default preview: 350 gal.
Modular-unit cost
units only. Default preview: $1,750.00.
Methodology
Transparent calculation
BuildMeter converts every input to a consistent internal unit, applies the selected allowance, and rounds products up to whole purchasable units.
Runoff = catchment area × rainfall depth × runoff coefficient. Target storage applies the capture percentage. Aggregate pit volume divides storage by void ratio; footprint divides by usable depth.
Reviewed project guidance
How to use this estimate in a real project
Last reviewed: August 19, 2026
Estimate runoff storage volume and convert it either to an aggregate-filled pit using void ratio or to a count of modular storage units.
Calculation steps
- 1Measure only the catchment actually routed to the dry well.
- 2Convert rainfall depth to runoff volume using the runoff coefficient.
- 3Apply the capture target to determine required storage.
- 4For stone systems divide water storage by verified void ratio; for chambers divide by rated unit storage.
Worked numerical example
For 1,200 ft² of connected roof, a 1 in storm and 0.90 runoff coefficient produce about 90 ft³ of runoff. Capturing 50% requires about 45 ft³ of storage. At a 0.40 aggregate void ratio, the excavation must provide roughly 112.5 ft³ of aggregate zone.
Purchasing and rounding
For aggregate systems, order washed stone by excavation volume rather than water-storage volume. For modular systems, use the manufacturer’s net storage capacity and required envelope dimensions.
Interpret the result
Storage volume is water space, not excavation volume. A stone-filled dry well must be larger because only the aggregate voids store water.
Page-specific assumptions
- Runoff coefficient represents the connected surface.
- Storage is evaluated for one design event.
- Entered stone void ratio or module capacity is representative.
Common mistakes
- Using total stone volume as water capacity.
- Ignoring infiltration rate and drawdown time.
- Routing roof area to the dry well that actually drains elsewhere.
Primary and technical references
BuildMeter provides planning estimates, not engineering, architectural, electrical, mechanical, plumbing, permit or code approval. Review the methodology and verify final decisions with product documentation and qualified professionals.
BuildMeter Guides
Understand the measurements and assumptions
Use these visible reference guides before changing project inputs or planning allowances.
FAQ
Dry Well Size Calculator questions
Why is the aggregate pit larger than the water volume?+
Stone occupies part of the excavation, so only the void spaces store water.
Does this account for infiltration during the storm?+
No. It conservatively compares static storage and does not model infiltration or routing.
Where should overflow go?+
Overflow needs a safe, project-specific route that does not damage buildings or neighboring property.