Dry Well Sizing Calculator for Garden Drainage

Calculator inputs

Use US customary units in this version. Enter either runoff inputs or a known water volume override.

Drainage area (ft²)

Design rainfall depth (in)

Runoff coefficient (0 to 1)

Known water volume override (gal)

Soil infiltration rate (in/hr)

Target drawdown time (hr)

Storage media void ratio (%)

Storage depth (ft)

Safety factor

Sidewall contribution factor (0 to 1)

Dry well shape

Length to width ratio

Formula used

This calculator sizes the minimum plan area that can store runoff and infiltrate water within the selected drawdown period.

1) Runoff volume

Runoff volume (ft³) = Drainage area (ft²) × Rainfall depth (ft) × Runoff coefficient

2) Design volume

Design volume (ft³) = Base inflow volume × Safety factor

3) Storage volume

Storage volume (ft³) = Plan area × Storage depth × Void ratio

4) Infiltration volume during drawdown

Infiltration volume (ft³) = Infiltration rate (ft/hr) × Drawdown hours × Effective infiltration area

5) Effective infiltration area

Effective area = Bottom area + (Sidewall area × Sidewall contribution factor)

6) Sizing rule
Minimum plan area is solved when:
Storage volume + Infiltration volume ≥ Design volume

How to use this calculator

Enter the drainage area that sends water toward the dry well.
Enter the design rainfall depth for the storm you want to manage.
Choose a realistic runoff coefficient for roof, paving, or mixed surfaces.
Add a known water volume only when you already measured or estimated inflow directly.
Enter the tested soil infiltration rate and your allowed drawdown time.
Set gravel or chamber void ratio, depth, safety factor, and sidewall contribution.
Select circular or rectangular geometry. For rectangular designs, set length-to-width ratio.
Press calculate to see required area, dimensions, excavation volume, and the Plotly capacity graph.

Example data table

Drainage Area (ft²)	Rainfall (in)	Runoff Coefficient	Infiltration (in/hr)	Depth (ft)	Void Ratio (%)	Safety Factor	Shape	Approx. Output
1200	1.50	0.90	1.10	4.00	40	1.20	Circular	Required area ≈ 22.96 ft², diameter ≈ 5.41 ft
1800	2.00	0.95	0.60	5.00	40	1.25	Rectangular	Larger footprint is needed because infiltration is slower

8 FAQs

1) What does this calculator size?

It estimates the plan area and suggested dimensions of a dry well that can both store stormwater and infiltrate it within your chosen drawdown time.

2) When should I use the known volume override?

Use it when you already know the inflow volume from a drainage study, roof capture estimate, or previous stormwater model. It replaces the area-and-rainfall runoff estimate.

3) Why does infiltration rate matter so much?

Higher infiltration lets the soil absorb more water during the drawdown window. That reduces the storage footprint needed. Slow soils usually require larger dry wells.

4) What is void ratio in a dry well?

Void ratio is the percentage of empty storage space inside gravel or chamber media. Washed stone often provides much less storage than prefabricated chamber systems.

5) Why include a safety factor?

A safety factor adds extra design volume for uncertainty in rainfall, runoff, clogging, partial sediment buildup, and real-world construction differences.

6) What does the sidewall contribution factor do?

It scales how much sidewall area helps infiltration. Use lower values when sidewalls may clog, soil layering is poor, or local guidance prefers conservative sizing.

7) Is circular or rectangular better?

Neither is always better. Circular wells are compact. Rectangular systems can fit narrow planting strips, property setbacks, or chamber layouts more easily.

8) Does this replace local engineering review?

No. Use it for planning and quick design checks. Final sizing should follow local stormwater criteria, setbacks, utility clearance rules, and site-specific soil testing.