Dry Well Size Calculator

Enter Dry Well Data

Drainage Area

Area Unit

Design Rainfall Depth

Rainfall Unit

Runoff Coefficient

Gravel Void Ratio

Safety Factor

Soil Infiltration Rate Inches per hour

Target Drawdown Time Hours

Dry Well Shape

Proposed Depth Feet

Proposed Diameter Feet, used for cylinder

Pit Length Feet, used for rectangle

Pit Width Feet, used for rectangle

Example Data Table

Drainage Area	Rainfall	Runoff Coefficient	Void Ratio	Safety Factor	Soil Rate
1200 sq ft	2 in	0.95	0.40	1.15	0.50 in/hr
1800 sq ft	1.5 in	0.90	0.35	1.20	0.40 in/hr
900 sq ft	2.5 in	1.00	0.42	1.10	0.60 in/hr

Formula Used

Runoff volume = drainage area × rainfall depth × runoff coefficient.

Design storage = runoff volume × safety factor.

Required excavation volume = design storage ÷ gravel void ratio.

Cylinder volume = π × radius² × depth.

Rectangular pit volume = length × width × depth.

Infiltration drawdown time = design storage ÷ hourly infiltration volume.

The calculator converts rainfall depth to feet before volume calculations. Gallons are estimated by multiplying cubic feet by 7.48052.

How to Use This Calculator

Enter the impervious drainage area that flows into the dry well. Add the design storm depth from local drainage guidance. Choose inches or millimeters. Enter a runoff coefficient. Roofs and paving often use values near 0.90 to 1.00.

Set the gravel void ratio. Clean stone often stores water in the gaps between particles. Add a safety factor for sediment, construction tolerance, and unusual storms. Enter the soil infiltration rate from a test or local soil data.

Select the dry well shape. For a cylinder, enter proposed depth and diameter. For a rectangular pit, enter length, width, and depth. Submit the form. The result shows storage demand, excavation volume, proposed capacity, drawdown time, and pass checks.

Dry Well Size Planning Guide

What This Calculator Does

A dry well is a buried drainage structure. It receives runoff from roofs, yards, drives, or paved areas. Water enters the stone voids or chamber space. Then it slowly moves into the surrounding soil. Correct sizing matters because undersized wells can overflow. Oversized wells may cost more than needed.

Why Physics Matters

The design uses basic volume and infiltration ideas. Rainfall over an area creates a water volume. The runoff coefficient adjusts this volume for surface behavior. A roof sends nearly all rainfall to the well. A lawn sends less because some water wets soil and vegetation. The safety factor adds reserve capacity.

Storage and Void Space

Stone does not store water in its full excavation volume. Only the empty space between stones stores water. This empty space is called void ratio. A pit with forty percent voids needs more excavation than the water volume alone. That is why the calculator divides storage demand by void ratio.

Soil Drainage Check

A dry well also needs soil contact area. Bottom and side surfaces let water leave the structure. Faster soil drains quicker. Slow soil needs more surface area, more volume, or another outlet. The drawdown estimate compares stored water against hourly infiltration. A target period such as forty eight hours helps avoid standing water.

Practical Design Notes

Use local rules before final construction. Keep dry wells away from foundations, wells, septic areas, and steep slopes. Add pretreatment where sediment may enter. Use washed stone and filter fabric when suitable. Provide overflow routing for storms larger than the design event. Field testing is always better than guessing soil rates.

FAQs

1. What is a dry well size calculator?

It estimates the storage volume and excavation size needed for a dry well. It uses drainage area, rainfall depth, runoff coefficient, void ratio, safety factor, and soil infiltration rate.

2. What runoff coefficient should I use?

Use a higher value for roofs, concrete, and asphalt. Values near 0.90 to 1.00 are common for hard surfaces. Lawns and landscaped areas usually need lower values.

3. What does void ratio mean?

Void ratio is the share of excavation space that can hold water. Gravel may have about 0.35 to 0.45 void space, depending on stone size and compaction.

4. Why is the excavation volume larger than storage?

The stone itself occupies space. Only gaps between stones store water. The calculator divides required water storage by void ratio to find the needed excavation volume.

5. What is drawdown time?

Drawdown time is the estimated time needed for stored runoff to soak into soil. Many designs aim for drainage within one or two days.

6. Can this replace a soil test?

No. It is a planning tool. A field infiltration test gives better soil data. Local codes may require testing before installation.

7. Should I use cylinder or rectangular mode?

Use cylinder mode for round dry wells or vertical chambers. Use rectangular mode for stone trenches, box pits, or shallow storage beds.

8. Why add a safety factor?

A safety factor adds reserve for sediment, uncertain rainfall, soil variation, construction limits, and future site changes. It helps reduce overflow risk.