Dry Well / Soakaway Calculator

How the dry well calculator sizes the pit

A dry well, called a soakaway in the UK and a soakwell in Australia, is a pit that takes roof water and lets it soak into the ground. This calculator works out how big the pit needs to be to hold a short, heavy storm. It takes the roof area, multiplies it by the rain that falls in 15 minutes at your chosen intensity, and divides by the void ratio of whatever fills the pit, because stone or rubble leaves only a fraction of the hole free for water.

With the US defaults, 860 ft² of roof in a 2 in/hr storm collects 2 ÷ 12 × 0.25 = 0.042 ft of rain in 15 minutes, which is 35.8 ft³ of water (1.33 yd³, about 268 gallons). A pit filled with rubble at a void ratio of 0.30 needs 35.8 ÷ 0.30 = 119.4 ft³ of gross volume, which the calculator shows as 4.42 yd³ of excavation. The metric default is 80 m² at 50 mm/hr: 12.5 mm of rain, 1.0 m³ of water and a 3.33 m³ pit.

That is a storage figure, and the biggest limitation belongs up front. The calculator assumes nothing soaks away during the storm and looks only at a 15-minute event. Both simplifications cut in different directions, so the result is a first sizing, not a design.

What a full soakaway design adds

England's Approved Document H sets out the standard method. Soakaways are designed for a storm that happens once in ten years on average, and the design is repeated for storms of different lengths to find the one that needs the most storage. For each storm, the pit has to hold the difference between the water flowing in and the water soaking out through its sides, and that outflow depends on the soil's infiltration rate measured in a percolation test. For small soakaways serving 25 m² or less, the document allows a design rainfall of 10 mm in 5 minutes as the worst case; larger ones follow BRE Digest 365 or BS EN 752.

Shropshire Council's worked BRE Digest 365 example shows why storm length matters. For its 125 m² catchment, the critical storm turned out to be about an hour long, not ten minutes, and the finished design also had to pass a check that the pit drains to half empty within 24 hours. In slow-draining soil a long storm can need more storage than this calculator's 15-minute figure. In fast-draining sand the outflow credit can make the real pit smaller.

Choosing the inputs

Use the roof area on plan that actually drains to the pit, not the sloping roof surface. For rainfall, use a local design figure. In the US, NOAA's Precipitation Frequency Data Server gives NOAA Atlas 14 rainfall frequency estimates for your location, including short durations and a range of return periods; elsewhere, the national weather service or local authority publishes equivalent data. The calculator stores intensity × 0.25 hours of rain, so to reproduce Approved Document H's 10 mm small-soakaway storm, enter 40 mm/hr.

The void ratio is the share of the pit that is open space once it's filled. The Shropshire example uses 30% free volume for granular fill, which is where the 0.30 default comes from; the council also notes that the void space of any fill has to be known before the design is done. Plastic infiltration crates hold far more water for their size, so use the maker's stated void ratio for them. A crate system quoted at 95% voids would need about 37.7 ft³ (1.40 yd³) or 1.05 m³ for the default storms, instead of 4.42 yd³ or 3.33 m³.

Turning volume into a hole

Any shape with the right volume stores the same water, but the standard method credits only the sides of the pit with soaking water away, not the base, so a deeper, narrower pit usually drains better than a wide, shallow one. The metric default of 3.33 m³ fits a pit 1.5 m square and 1.5 m deep (3.375 m³). The US default of 119.4 ft³ (4.42 yd³) fits a pit 4 ft square and 8 ft deep (128 ft³, or 4.74 yd³).

In the US, the shape also matters legally. The EPA treats stormwater drainage wells, including dry wells, as Class V injection wells, and defines a well as a hole whose depth is greater than its largest surface dimension. They are usually allowed without an individual permit, but the owner has to send basic inventory details to the permitting authority, and some states set stricter rules.

Where not to put one

Approved Document H says infiltration devices should not be built within 5 m of a building or road, in unstable ground, where the water table reaches the bottom of the pit at any time of year, where they would overload the ground's soakage alongside other soakaways or drainage fields, or where contaminated runoff could pollute groundwater. Local rules elsewhere cover the same ground, so check yours before digging. And whatever the size, a percolation test on the actual site is what tells you whether the soil can take the water at all.