Drywell Size Calculator
Enter the roof area that drains to the dry well, the design rainfall, the type of pit and how deep it will be. The calculator gives the storm water to hold, the pit volume and its width.
- Dry well volume
- Storm water
- Pit size
Design rainfall is the storm you want to hold, often 1 to 2 in.
Each inch of rain on 1,000 sq ft of roof is about 623 gallons. A gravel pit stores water only in the gaps between stones, about 40%, so it needs about 208 cu ft. At 4 ft deep that is a round pit about 8 ft across. A hollow dry well needs far less.
Dry well size chart
1 in of rain. Search for a roof.
| Roof (sq ft) | Water (gal) | Gravel pit (cu ft) | Hollow well (cu ft) |
|---|---|---|---|
| 250 | 156 | 52 | 22 |
| 500 | 312 | 104 | 44 |
| 750 | 467 | 156 | 66 |
| 1000 | 623 | 208 | 88 |
| 1500 | 934 | 312 | 132 |
| 2000 | 1246 | 416 | 175 |
| 2500 | 1558 | 521 | 219 |
| 3000 | 1869 | 625 | 263 |
Cubic feet.
Roof and dry well
1 in of rain, gravel pit. Slide the roof area.
The pit grows with the roof.
How to size a dry well
Roof to pit. Step through it.
Roof
Area.
Wrap the gravel in fabric so soil does not clog it.
Gravel pit, plastic or concrete
Dry well types. Compare them.
Simple.
Modular.
Heavy duty.
Hollow dry wells hold more water in a smaller hole.
Before you dig
Dry well checks. Tick each item.
Tick each item as you go.
Call before you dig to find buried services.
What you get
Dry well sizing. Each tile explains one result.
Gallons.
Cubic feet.
Width and depth.
Plastic.
Area.
Clay soils drain too slowly for most dry wells.
Frequently Asked Questions
No question matches that search.
What size dry well do I need?
Multiply the roof area by the rainfall in inches and by point six two three for gallons, then allow for the pit type.
How big is a dry well for a thousand square foot roof?
About two hundred cubic feet as a gravel pit for an inch of rain.
How deep should a dry well be?
Usually three to five feet, above the water table.
How far should a dry well be from the house?
At least ten feet from foundations.
Does a dry well work in clay?
Poorly. Test how fast the soil drains first.
Is a plastic dry well better than gravel?
It stores more water in a smaller hole.
What is a drywell?
A pit that lets roof or yard water soak into the ground.
Rainwater that soaks away
Size the dry well to your roof and storms and water disappears into the ground instead of pooling.
More size calculators
Each one has its own calculator, worked examples and answers to common questions.