Example Data Table
| Surface |
Area |
Rainfall |
Coefficient |
Suggested Starting Units |
| Small roof section |
500 sq ft |
1 in |
0.95 |
1 to 2 |
| Patio and walkway |
750 sq ft |
1 in |
0.85 |
2 |
| Large roof plane |
1,200 sq ft |
1.5 in |
0.95 |
4 or more |
Formula Used
Runoff gallons = Drainage area × Rainfall inches ÷ 12 × 7.48052 × Runoff coefficient.
Required storage = Runoff gallons × Safety factor.
Chamber storage = Number of units × Storage gallons per unit.
Aggregate void storage = Open aggregate volume × Gravel void ratio × 7.48052.
Total usable storage = Chamber storage + Aggregate void storage.
Infiltration gallons per hour = Soil rate ÷ 12 × Effective seepage area × 7.48052 × Port factor.
Drawdown time = Total usable storage ÷ Infiltration gallons per hour.
How To Use This Calculator
- Measure the surface area that sends water to the dry well.
- Enter the design rainfall depth for your planning storm.
- Select a runoff coefficient that matches the surface type.
- Enter planned chamber count, storage, excavation, and gravel values.
- Add soil infiltration details from a field test when available.
- Press calculate and review storage, overflow, drawdown, and cost.
- Download CSV or PDF results for project notes.
NDS Flo Well Planning Basics
A dry well is a storage chamber for stormwater. It receives runoff from roofs, patios, driveways, or low yard areas. The chamber holds water during a storm. Then the surrounding soil accepts that water over time. A Flo Well layout is useful when surface discharge is not convenient. It can also reduce standing water near foundations.
Why Sizing Matters
Sizing is not only about the chamber volume. The total design depends on runoff volume, extra gravel voids, soil absorption, and a safety margin. A small storm may fit inside one chamber. A larger design storm may need several chambers, more aggregate, or another outlet. Soil is the controlling limit. Sandy soil can empty a system quickly. Clay soil may hold water for many hours.
Important Field Inputs
Start with the drainage area. Measure the roof plane or paved surface that sends water to the pipe. Choose a rainfall depth for the design event. Many homeowners test one inch first. Use a runoff coefficient that matches the surface. Roofs and concrete often use high values. Lawns use lower values because some water soaks in before reaching the inlet.
Storage And Drawdown
The calculator estimates runoff gallons and compares them with chamber storage plus aggregate void storage. Aggregate void storage is not the full rock volume. It is only the open space between stones. The tool also estimates drawdown time from the soil infiltration rate and available seepage area. Short drawdown times leave room for the next storm.
Practical Design Notes
Place the dry well away from weak foundations, septic fields, and steep slopes. Use clean pipe, proper slope, and accessible inlets. Filter fabric can help keep fines from entering the chamber and rock envelope. A cleanout or catch basin is helpful when leaves or grit are common. Local codes may require setbacks, overflow routes, or professional review. Use the result as planning guidance. Confirm final sizing with site tests and local rules.
Maintenance Tips
Inspect inlets after heavy rain. Remove leaves, sediment, and mulch from grates. Check that overflow paths remain open. Watch the area for soft soil or long ponding. Regular checks protect capacity and reduce hidden drainage failures. They also extend service life safely.
FAQs
What does this calculator estimate?
It estimates runoff volume, required storage, chamber capacity, aggregate void storage, overflow risk, drawdown time, gravel volume, and basic project cost.
Can I change the chamber storage value?
Yes. The storage per unit field is editable. Use the value that matches the exact product, configuration, or local design requirement.
What runoff coefficient should I use?
Use high values for roofs, concrete, and asphalt. Use lower values for lawns or landscaped areas. When unsure, choose a conservative higher value.
Why is gravel void ratio included?
Rock around the chamber can hold water in open spaces. The void ratio estimates that usable space, not the entire rock volume.
Why does soil infiltration matter?
Storage handles the storm peak, but soil infiltration empties the system. Slow soil may require more storage, more seepage area, or another outlet.
What is drawdown time?
Drawdown time estimates how long stored water may take to leave the dry well through surrounding soil and open seepage areas.
Is this a final engineering design?
No. It is a planning calculator. Site soil tests, setbacks, codes, overflow routing, and professional review may still be required.
When should I add more units?
Add more units when required storage exceeds usable storage, overflow is high, or local rules require larger stormwater detention volume.