Dry Well Calculator — Storage, Pit Size & Drain Time
Size or check a residential dry well from roof runoff, rainfall, storage media, pit dimensions and a measured infiltration rate. Compare storage, drain-down, excavation, fabric, modules and editable cost.
8.80 yd³ gross storage volume · 7.71 ft square footprint at 4 ft deep.
Planned storage covers the eventRunoff event
Use the connected roof or hardscape area, the design rainfall required for the project and an appropriate runoff coefficient. Capture target lets an authority or project brief require less than the full event.
Storage system and proposed pit
The effective storage ratio is the water-holding fraction of the installed system. Stone-filled pits often use a tested or specified void fraction; chamber layouts require current manufacturer data.
Quantity allowances and optional current prices
Prices default to zero. Enter delivered local rates in one currency. Chamber embedment stone is product-specific and is deliberately not inferred from pit volume.
Sizing and capacity
- Event runoff before capture
- 711 gal
- Required gross pit volume
- 8.80 yd³
- Square side at entered depth
- 7.71 ft
- Proposed excavation
- 9.48 yd³
- Effective infiltration area
- 192 ft²
- Effective infiltration rate
- 0.25 in/hr
Material and cost audit
- Stone to order
- 10.43 yd³
- Filter fabric
- 282 ft²
- Modules to meet target
- not used
- Stone cost
- —
- Excavation cost
- —
- Fabric / modules
- —
- Total entered cost
- —
Planning screen only. A dry well needs a safe overflow route and a verified discharge/infiltration location. Confirm soil testing, groundwater and bedrock separation, setbacks from foundations, wells and property lines, utility clearance, pretreatment, permits and maintenance with the local authority and a qualified designer.
How runoff storage is calculated
Runoff volume is connected area × rainfall depth × runoff coefficient. The capture percentage is then applied. Required gross pit volume divides that water target by the entered effective storage ratio, so a stone-filled pit is not treated as an empty tank.
Check a proposed dry well
Planned storage is length × width × usable depth × storage ratio. The margin compares that capacity with the target event. A positive margin is only a volume check; it does not prove that the soil, inlet, overflow or separation distances are acceptable.
Drain-down screening
The screen applies the measured infiltration rate and safety factor to the pit bottom and four sides, then divides full planned storage by that steady rate. Real water levels and soil response vary, so use the local approved test and method rather than a guessed soil-type preset.
Stone, fabric and chamber quantities
A stone-filled option orders the gross excavation volume plus allowance. A chamber option rounds the target up to whole modules using the entered current usable capacity; its embedment stone must come from that product's layout. Fabric wraps the top, bottom and four sides plus allowance.
Primary guidance and design boundary
The Pennsylvania Housing Research Center's dry-well design handout demonstrates roof area × rainfall for runoff and dividing by 40% void space for a gravel-filled well. The Massachusetts residential stormwater guide likewise shows stormwater volume divided by depth and a 0.4 void ratio for area. Those examples explain the arithmetic but do not replace current local criteria. This calculator does not design groundwater protection, structural loading, pretreatment, emergency overflow or a lawful discharge point.
Frequently asked questions
How large should a dry well be?
Start with the required runoff volume, then divide by the verified water-storage fraction of the stone or chamber system. Use allowable depth to convert gross volume into footprint, and check a proposed geometry separately.
Why does stone storage use less than the excavated volume?
Stone occupies most of the pit. Only the connected voids can temporarily hold water, so the storage ratio must come from the selected aggregate specification or approved local assumption.
Does a passing volume margin approve the installation?
No. Soil infiltration, groundwater, bedrock, setbacks, overflow, utilities, frost, maintenance and permits remain site-specific.
