Free dry well calculator: water to hold, pit size and drain time
Get the size of a dry well and the hours it takes to empty in seconds, free and without an account. One dry well at a time: the roof or yard area that drains to it, the rain it must hold, the share of the pit the stone leaves to water, the depth you dig and the infiltration rate you measured. The calculation gives the water to hold, the pit that holds it, its footprint and the hours the soil takes to empty it. In imperial or metric.
Infiltration by soil (reference)
The minimum rates the Quebec guide gives by soil (table 11.13). Measure yours on site: New Hampshire's test fills a hole twice, reads the drop over 15 minutes and multiplies it by 4. Clay is not in the table, and below 15 mm/h the guide does not use an infiltration pit.
| Soil | mm/h | in/h (converted) |
|---|---|---|
| Sand | 210 | 8.3 |
| Silty sand | 60 | 2.4 |
| Sandy loam | 25 | 1.0 |
| Loam | 15 | 0.6 |
Sources: Quebec stormwater guide; New Hampshire homeowner's guide, dry well
Dry wells: what the guides say (reference)
The rules our template cites, as read. Your municipality and the site have the last word.
| What the sources say | |
|---|---|
| Design rain | Quebec guide: store at least 5 mm and at most 25 mm of rain from the roof. New Hampshire: a 1-inch storm, about 62 gallons per 100 sq ft |
| Stone voids | Quebec guide: 35 to 40% for clean stone. CVC and TRCA: 30 to 40%, typically 0.4 for 50 mm clear stone. New Hampshire: about 40% |
| Depth | Quebec guide: deeper than 1.5 m (5 ft) generally not recommended, the bottom at least 1 m above the water table or rock. New Hampshire: typically 3 ft. Oregon: 5 ft between the bottom and groundwater for roof runoff, 10 ft for other runoff |
| Time to empty | Quebec guide and CVC: 48 hours to design for; CVC: typically 72 hours at most. Oregon State: 24 to 36 hours, or 72 hours |
| Distance from the house | Quebec guide and CVC: at least 4 m (13 ft) from foundations; an overflow at least 2 m away (CVC). Portland: 10 ft |
| Size | Oregon State: one small drywell per 500 sq ft of impervious area |
Sources: Quebec stormwater guide; CVC and TRCA, LID guide (soakaways); New Hampshire homeowner's guide, dry well; Oregon State Extension, drywells
How it is calculated
Water to hold = area drained x design rain (in inches) / 12. Pit volume = water / (stone voids / 100): the stone fills the rest. Footprint = pit volume / depth. Hours to empty = depth x 12 x (voids / 100) / infiltration rate: the water standing in the full pit, over the rate the soil takes it in. The same formulas as the yard drainage survey template. The rain, the voids and the depth start at the typical values the sources print (New Hampshire's 1-inch storm, 40% voids, 3 ft); the infiltration rate is yours, measured where the pit goes.
Example: a back corner downspout
From our template page's demo: a back corner downspout drains 600 sq ft of roof. Designed for a 1-inch rain, with 40% voids in clean stone, 3 ft of depth and a soil that took in 1 in/h in the hole test, the well holds 50 cu ft of water in 125 cu ft of stone, 41.7 sq ft in plan, and empties in 14.4 hours, well under the 48 hours the Quebec guide designs for.
The whole job in MesuPro
Yard drainage survey
French drains and dry wells: trench, stone, geotextile, water to hold, pit volume and footprint, hours to empty.
You get: Water to hold
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